Hypothetical charge of an atom if all its bonds to different atoms were fully ionic
Inchemistry, theoxidation state, oroxidation number, is the hypotheticalcharge of an atom if all of itsbonds to other atoms are fullyionic. It describes the degree ofoxidation (loss ofelectrons) of anatom in achemical compound. Conceptually, the oxidation state may be positive, negative or zero. Beside nearly-pureionic bonding, manycovalent bonds exhibit a strong ionicity, making oxidation state a useful predictor of charge.
The oxidation state of an atom does not represent the "real" charge on that atom, or any other actual atomic property. This is particularly true of high oxidation states, where theionization energy required to produce a multiply positive ion is far greater than the energies available in chemical reactions. Additionally, the oxidation states of atoms in a given compound may vary depending onthe choice ofelectronegativity scale used in their calculation. Thus, the oxidation state of an atom in a compound is purely a formalism. It is nevertheless important in understanding the nomenclature conventions ofinorganic compounds. Also, several observations regarding chemical reactions may be explained at a basic level in terms of oxidation states.
Oxidation states are typically represented byintegers which may be positive, zero, or negative. In some cases, the average oxidation state of an element is a fraction, such as8/3 foriron inmagnetiteFe3O4 (see below). The highest known oxidation state is reported to be +9, displayed byiridium in thetetroxoiridium(IX) cation (IrO+4).[1] It is predicted that even a +10 oxidation state may be achieved byplatinum in tetroxoplatinum(X),PtO2+4.[2] The lowest oxidation state is −5, as forboron inAl3BC[3] andgallium inpentamagnesium digallide (Mg5Ga2).
InStock nomenclature, which is commonly used for inorganic compounds, the oxidation state is represented by aRoman numeral placed after the element name inside parentheses or as a superscript after the element symbol, e.g.Iron(III) oxide. The termoxidation was first used byAntoine Lavoisier to signify the reaction of a substance withoxygen. Much later, it was realized that the substance, upon being oxidized, loses electrons, and the meaning was extended to include otherreactions in which electrons are lost, regardless of whether oxygen was involved.The increase in the oxidation state of an atom, through a chemical reaction, is known as oxidation; a decrease in oxidation state is known as areduction. Such reactions involve the formal transfer of electrons: a net gain in electrons being a reduction, and a net loss of electrons being oxidation. For pure elements, the oxidation state is zero.
Oxidation numbers are assigned to elements in a molecule such that the overall sum is zero in a neutral molecule. The number indicates the degree of oxidation of each element caused by molecular bonding. In ionic compounds, the oxidation numbers are the same as the element's ionic charge. Thus for KCl, potassium is assigned +1 and chlorine is assigned −1.[4] The complete set of rules for assigning oxidation numbers are discussed in the following sections.
Oxidation numbers are fundamental to thechemical nomenclature of ionic compounds. For example, Cu compounds with Cu oxidation state +2 are calledcupric and those with state +1 arecuprous.[4]: 172 The oxidation numbers of elements allow predictions of chemical formula and reactions, especiallyoxidation-reduction reactions.The oxidation numbers of the most stable chemical compounds follow trends in the periodic table.[5]: 140
International Union of Pure and Applied Chemistry (IUPAC) has published a "Comprehensive definition of oxidation state (IUPAC Recommendations 2016)".[6] It is a distillation of an IUPAC technical report: "Toward a comprehensive definition of oxidation state".[7] According to the IUPACGold Book: "The oxidation state of an atom is the charge of this atom after ionic approximation of its heteronuclear bonds."[8] The termoxidation number is nearly synonymous.[9]
The ionic approximation means extrapolating bonds to ionic. Several criteria[10] were considered for the ionic approximation:
Extrapolation of the bond's polarity;
from the electronegativity difference,
from the dipole moment, and
from quantum‐chemical calculations of charges.
Assignment of electrons according to the atom's contribution to the bondingMolecular orbital (MO)[10][11] or the electron's allegiance in aLCAO–MO model.[12]
In a bond between two different elements, the bond's electrons are assigned to its main atomic contributor typically of higher electronegativity; in a bond between two atoms of the same element, the electrons are divided equally. Most electronegativity scales depend on the atom's bonding state, which makes the assignment of the oxidation state a somewhat circular argument. For example, some scales may turn out unusual oxidation states, such as −6 forplatinum inPtH2−4, forPauling andMulliken scales.[7] The dipole moments would sometimes also turn out abnormal oxidation numbers, such as inCO andNO, whichare oriented with their positive end towards oxygen. Therefore, this leaves the atom's contribution to thebonding MO, the atomic-orbital energy, and from quantum-chemical calculations of charges, as the only viable criteria with cogent values for ionic approximation. However, for a simple estimate for the ionic approximation, we can useAllen electronegativities,[7] as only that electronegativity scale is truly independent of the oxidation state, as it relates to the average valence‐electron energy of the free atom:
Introductory chemistry uses postulates: the oxidation state for an element in a chemical formula is calculated from the overall charge and postulated oxidation states for all the other atoms.
where OS stands for oxidation state. This approach yields correct oxidation states in oxides and hydroxides of any single element, and in acids such assulfuric acid (H2SO4) ordichromic acid (H2Cr2O7). Its coverage can be extended either by a list of exceptions or by assigning priority to the postulates. The latter works forhydrogen peroxide (H2O2) where the priority of rule 1 leaves both oxygens with oxidation state −1.
Additional postulates and their ranking may expand the range of compounds to fit a textbook's scope. As an example, one postulatory algorithm from many possible; in a sequence of decreasing priority:
An element in a free form has OS = 0.
In a compound or ion, the sum of the oxidation states equals the total charge of the compound or ion.
Fluorine in compounds has OS = −1; this extends tochlorine andbromine only when not bonded to a lighter halogen, oxygen or nitrogen.
Group 1 andgroup 2 metals in compounds have OS = +1 and +2, respectively.
Hydrogen has OS = +1 but adopts −1 when bonded as ahydride to metals or metalloids.
Oxygen in compounds has OS = −2 but only when not bonded to oxygen (e.g. in peroxides) or fluorine.
This set of postulates covers oxidation states of fluorides, chlorides, bromides, oxides, hydroxides, and hydrides of any single element. It covers alloxoacids of any central atom (and all their fluoro-, chloro-, and bromo-relatives), as well assalts of such acids with group 1 and 2 metals. It also coversiodides,sulfides, and similar simple salts of these metals.
where each "—" represents an electron pair (either shared between two atoms or solely on one atom), and "OS" is the oxidation state as a numerical variable.
After the electrons have been assigned according to the vertical red lines on the formula, the total number of valence electrons that now "belong" to each atom is subtracted from the numberN of valence electrons of the neutral atom (such as 5 for nitrogen ingroup 15) to yield that atom's oxidation state.
This example shows the importance of describing the bonding. Its summary formula,HNO3, corresponds to twostructural isomers; theperoxynitrous acid in the above figure and the more stablenitric acid. With the formulaHNO3, thesimple approach without bonding considerations yields −2 for all three oxygens and +5 for nitrogen, which is correct for nitric acid. For the peroxynitrous acid, however, both oxygens in the O–O bond have OS = −1, and the nitrogen has OS = +3, which requires a structure to understand.
Analogously fortransition-metal compounds;CrO(O2)2 on the left has a total of 36 valence electrons (18 pairs to be distributed), andhexacarbonylchromium (Cr(CO)6) on the right has 66 valence electrons (33 pairs):
A key step is drawing the Lewis structure of the molecule (neutral, cationic, anionic): Atom symbols are arranged so that pairs of atoms can be joined by single two-electron bonds as in the molecule (a sort of "skeletal" structure), and the remaining valence electrons are distributed such that sp atoms obtain anoctet (duet for hydrogen) with a priority that increases in proportion with electronegativity. In some cases, this leads to alternative formulae that differ in bond orders (the full set of which is called theresonance formulas). Consider thesulfate anion (SO2−4) with 32 valence electrons; 24 from oxygens, 6 from sulfur, 2 of the anion charge obtained from the implied cation. Thebond orders to the terminal oxygens do not affect the oxidation state so long as the oxygens have octets. Already the skeletal structure, top left, yields the correct oxidation states, as does the Lewis structure, top right (one of the resonance formulas):
The bond-order formula at the bottom is closest to the reality of four equivalent oxygens each having a total bond order of 2. That total includes the bond of order1/2 to the implied cation and follows the 8 − N rule[7] requiring that the main-group atom's bond-order total equals 8 − N valence electrons of the neutral atom, enforced with a priority that proportionately increases with electronegativity.
This algorithm works equally for molecular cations composed of several atoms. An example is theammonium cation of 8 valence electrons (5 from nitrogen, 4 from hydrogens, minus 1 electron for the cation's positive charge):
Drawing Lewis structures with electron pairs as dashes emphasizes the essential equivalence of bond pairs and lone pairs when counting electrons and moving bonds onto atoms. Structures drawn with electron dot pairs are of course identical in every way:
The algorithm contains a caveat, which concerns rare cases oftransition-metalcomplexes with a type ofligand that is reversibly bonded as aLewis acid (as an acceptor of the electron pair from the transition metal); termed a "Z-type" ligand in Green'scovalent bond classification method. The caveat originates from the simplifying use of electronegativity instead of theMO-based electron allegiance to decide the ionic sign.[6] One early example is theO2S−RhCl(CO)(PPh3)2 complex[13] withsulfur dioxide (SO2) as the reversibly-bonded acceptor ligand (released upon heating). The Rh−S bond is therefore extrapolated ionic against Allen electronegativities ofrhodium and sulfur, yielding oxidation state +1 for rhodium:
This algorithm works on Lewis structures and bond graphs of extended (non-molecular) solids:
Oxidation state is obtained by summing the heteronuclear-bond orders at the atom as positive if that atom is the electropositive partner in a particular bond and as negative if not, and the atom’s formal charge (if any) is added to that sum. The same caveat as above applies.
An example of a Lewis structure with no formal charge,
illustrates that, in this algorithm, homonuclear bonds are simply ignored (the bond orders are in blue).
Carbon monoxide exemplifies a Lewis structure withformal charges:
To obtain the oxidation states, the formal charges are summed with the bond-order value taken positively at the carbon and negatively at the oxygen.
Applied to molecular ions, this algorithm considers the actual location of the formal (ionic) charge, as drawn in the Lewis structure. As an example, summing bond orders in theammonium cation yields −4 at the nitrogen of formal charge +1, with the two numbers adding to the oxidation state of −3:
The sum of oxidation states in the ion equals its charge (as it equals zero for a neutral molecule).
Also in anions, the formal (ionic) charges have to be considered when nonzero. For sulfate this is exemplified with the skeletal or Lewis structures (top), compared with the bond-order formula of all oxygens equivalent and fulfilling the octet and 8 − N rules (bottom):
Abond graph insolid-state chemistry is a chemical formula of an extended structure, in which direct bonding connectivities are shown. An example is theAuORb3perovskite, the unit cell of which is drawn on the left and the bond graph (with added numerical values) on the right:
We see that the oxygen atom bonds to the six nearestrubidium cations, each of which has 4 bonds to theauride anion. The bond graph summarizes these connectivities. The bond orders (also calledbond valences) sum up to oxidation states according to the attached sign of the bond's ionic approximation (there are no formal charges in bond graphs).
Determination of oxidation states from a bond graph can be illustrated onilmenite,FeTiO3. We may ask whether the mineral containsFe2+ andTi4+, orFe3+ andTi3+. Its crystal structure has each metal atom bonded to six oxygens and each of the equivalent oxygens to twoirons and twotitaniums, as in the bond graph below. Experimental data show that three metal-oxygen bonds in the octahedron are short and three are long (the metals are off-center). The bond orders (valences), obtained from the bond lengths by thebond valence method, sum up to 2.01 at Fe and 3.99 at Ti; which can be rounded off to oxidation states +2 and +4, respectively:
Oxidation states can be useful for balancing chemical equations for oxidation-reduction (orredox) reactions, because the changes in the oxidized atoms have to be balanced by the changes in the reduced atoms. For example, in the reaction ofacetaldehyde withTollens' reagent to formacetic acid (shown below), thecarbonyl carbon atom changes its oxidation state from +1 to +3 (loses two electrons). This oxidation is balanced by reducing twoAg+ cations toAg0 (gaining two electrons in total).
An inorganic example is the Bettendorf reaction usingtin dichloride (SnCl2) to prove the presence ofarsenite ions in a concentratedHCl extract. When arsenic(III) is present, a brown coloration appears forming a dark precipitate ofarsenic, according to the following simplified reaction:
2 As3+ + 3 Sn2+ → 2 As0 + 3 Sn4+
Here threetin atoms are oxidized from oxidation state +2 to +4, yielding six electrons that reduce two arsenic atoms from oxidation state +3 to 0. The simple one-line balancing goes as follows: the two redox couples are written down as they react;
As3+ + Sn2+ ⇌ As0 + Sn4+
One tin is oxidized from oxidation state +2 to +4, a two-electron step, hence 2 is written in front of the two arsenic partners. One arsenic is reduced from +3 to 0, a three-electron step, hence 3 goes in front of the two tin partners. An alternative three-line procedure is to write separately thehalf-reactions for oxidation and reduction, each balanced with electrons, and then to sum them up such that the electrons cross out. In general, these redox balances (the one-line balance or each half-reaction) need to be checked for the ionic and electron charge sums on both sides of the equation being indeed equal. If they are not equal, suitable ions are added to balance the charges and the non-redox elemental balance.
A nominal oxidation state is a general term with two different definitions:
Electrochemical oxidation state[7]: 1060 represents a molecule or ion in theLatimer diagram orFrost diagram for its redox-active element. An example is the Latimer diagram forsulfur at pH 0 where the electrochemical oxidation state +2 for sulfur putsHS 2O− 3 between S andH2SO3:
Systematic oxidation state is chosen from close alternatives as a pedagogical description. An example is the oxidation state of phosphorus inH3PO3 (structurallydiprotic HPO(OH)2) taken nominally as +3, whileAllen electronegativities ofphosphorus andhydrogen suggest +5 by a narrow margin that makes the two alternatives almost equivalent:
Both alternative oxidation numbers for phosphorus make chemical sense, depending on which chemical property or reaction is emphasized. By contrast, a calculated alternative, such as the average (+4) does not.
Lewis formulae are rule-based approximations of chemical reality, as areAllen electronegativities. Still, oxidation states may seem ambiguous when their determination is not straightforward. If only an experiment can determine the oxidation state, the rule-based determination is ambiguous (insufficient). There are also trulydichotomous values that are decided arbitrarily.
Oxidation-state determination from resonance formulas
Seemingly ambiguous oxidation states are derived from a set ofresonance formulas of equal weights for a molecule having heteronuclear bonds where the atom connectivity does not correspond to the number of two-electron bonds dictated by the 8 − N rule.[7]: 1027 An example isS2N2 where four resonance formulas featuring one S=N double bond have oxidation states +2 and +4 for the two sulfur atoms, which average to +3 because the two sulfur atoms are equivalent in this square-shaped molecule.
A physical measurement is needed to determine oxidation state
when anon-innocentligand is present, of hidden or unexpected redox properties that could otherwise be assigned to the central atom. An example is thenickeldithiolate complex,Ni(S 2C 2H 2)2− 2.[7]: 1056–1057
when the redox ambiguity of a central atom and ligand yields dichotomous oxidation states of close stability, thermally inducedtautomerism may result, as exemplified bymanganesecatecholate,Mn(C6H4O2)3.[7]: 1057–1058 Assignment of such oxidation states requires spectroscopic,[14] magnetic or structural data.
when the bond order has to be ascertained along with an isolated tandem of a heteronuclear and a homonuclear bond. An example isthiosulfateS 2O2− 3 having two possible oxidation states (bond orders are in blue and formal charges in green):
The S–S distance measurement inthiosulfate is needed to reveal that this bond order is very close to 1, as in the formula on the left.
when the electronegativity difference between two bonded atoms is very small (as inH3PO3). Two almost equivalent pairs of oxidation states, arbitrarily chosen, are obtained for these atoms.
when an electronegativep-block atom forms solely homonuclear bonds, the number of which differs from the number of two-electron bonds suggested byrules. Examples are homonuclear finite chains likeN− 3 (the central nitrogen connects two atoms with four two-electron bonds while only three two-electron bonds[15] are required by the 8 − N rule[7]: 1027 ) orI− 3 (the central iodine connects two atoms with two two-electron bonds while only one two-electron bond fulfills the 8 − N rule). A sensible approach is to distribute the ionic charge over the two outer atoms.[7] Such a placement of charges in apolysulfideS2− n (where all inner sulfurs form two bonds, fulfilling the 8 − N rule) follows already from its Lewis structure.[7]
when the isolated tandem of a heteronuclear and a homonuclear bond leads to a bonding compromise in between two Lewis structures of limiting bond orders. An example isN2O:
The typical oxidation state of nitrogen in N2O is +1, which also obtains for both nitrogens by a molecular orbital approach.[10] The formal charges on the right comply with electronegativities, which implies an added ionic bonding contribution. Indeed, the estimated N−N and N−O bond orders are 2.76 and 1.9, respectively,[7] approaching the formula of integer bond orders that would include the ionic contribution explicitly as a bond (in green):
Conversely, formal charges against electronegativities in a Lewis structure decrease the bond order of the corresponding bond. An example iscarbon monoxide with a bond-order estimate of 2.6.[16]
Fractional oxidation states are often used to represent the average oxidation state of several atoms of the same element in a structure. For example, the formula ofmagnetite isFe 3O 4, implying an average oxidation state for iron of +8/3.[17]: 81–82 However, this average value may not be representative if the atoms are not equivalent. In aFe 3O 4 crystal below 120 K (−153 °C), two-thirds of the cations areFe3+ and one-third areFe2+ , and the formula may be more clearly represented as FeO·Fe 2O 3.[18]
Likewise,propane,C 3H 8, has been described as having a carbon oxidation state of −8/3.[19] Again, this is an average value since the structure of the molecule isH 3C−CH 2−CH 3, with the first and third carbon atoms each having an oxidation state of −3 and the central one −2.
An example with true fractional oxidation states for equivalent atoms is potassiumsuperoxide,KO 2. The diatomic superoxide ionO− 2 has an overall charge of −1, so each of its two equivalent oxygen atoms is assigned an oxidation state of −1/2. This ion can be described as aresonance hybrid of two Lewis structures, where each oxygen has an oxidation state of 0 in one structure and −1 in the other.
For thecyclopentadienyl anionC 5H− 5, the oxidation state of C is −1 + −1/5 = −6/5. The −1 occurs because each carbon is bonded to one hydrogen atom (a less electronegative element), and the −1/5 because the total ionic charge of −1 is divided among five equivalent carbons. Again this can be described as a resonance hybrid of five equivalent structures, each having four carbons with oxidation state −1 and one with −2.
Examples of fractional oxidation states for carbon
Finally, fractional oxidation numbersare not used in the chemical nomenclature.[20]: 66 For example the red leadPb 3O 4 is represented as lead(II,IV) oxide, showing the oxidation states of the two nonequivalentlead atoms.
Many compounds withluster andelectrical conductivity maintain a simplestoichiometric formula, such as the goldenTiO, blue-blackRuO2 or copperyReO3, all of obvious oxidation state. Ultimately, assigning the free metallic electrons to one of the bonded atoms is not comprehensive and can yield unusual oxidation states. Examples are the LiPb andCu 3Au orderedalloys, the composition and structure of which are largely determined byatomic size andpacking factors. Should oxidation state be needed for redox balancing, it is best set to 0 for all atoms of such an alloy.
This is a list of known oxidation states of thechemical elements, excludingnonintegral values. The most common states appear in bold. The table is based on that of Greenwood and Earnshaw,[21] with additions noted. Every element exists in oxidation state 0 when it is the pure non-ionized element in any phase, whether monatomic or polyatomicallotrope. The column for oxidation state 0 only shows elements known to exist in oxidation state 0 in compounds.
A figure with a similar format was used byIrving Langmuir in 1919 in one of the early papers about theoctet rule.[210] The periodicity of the oxidation states was one of the pieces of evidence that led Langmuir to adopt the rule.
The oxidation state in compound naming fortransition metals andlanthanides andactinides is placed either as a right superscript to the element symbol in a chemical formula, such as FeIII or in parentheses after the name of the element in chemical names, such as iron(III). For example,Fe 2(SO 4) 3 is namediron(III) sulfate and its formula can be shown as FeIII 2(SO 4) 3. This is because asulfate ion has a charge of −2, so each iron atom takes a charge of +3.
Oxidation itself was first studied byAntoine Lavoisier, who defined it as the result of reactions withoxygen (hence the name).[211][212] The term has since been generalized to imply aformal loss of electrons. Oxidation states, calledoxidation grades byFriedrich Wöhler in 1835,[213] were one of the intellectual stepping stones thatDmitri Mendeleev used to derive theperiodic table.[214]William B. Jensen[215] gives an overview of the history up to 1938.
When it was realized that some metals form two different binary compounds with the same nonmetal, the two compounds were often distinguished by using the ending-ic for the higher metal oxidation state and the ending-ous for the lower. For example, FeCl3 isferric chloride and FeCl2 isferrous chloride. This system is not very satisfactory (although sometimes still used) because different metals have different oxidation states which have to be learned: ferric and ferrous are +3 and +2 respectively, but cupric and cuprous are +2 and +1, and stannic and stannous are +4 and +2. Also, there was no allowance for metals with more than two oxidation states, such asvanadium with oxidation states +2, +3, +4, and +5.[17]: 84
This system has been largely replaced by one suggested byAlfred Stock in 1919[216] and adopted[217] byIUPAC in 1940. Thus, FeCl2 was written asiron(II) chloride rather than ferrous chloride. The Roman numeral II at the central atom came to be called the "Stock number" (now an obsolete term), and its value was obtained as a charge at the central atom after removing its ligands along with theelectron pairs they shared with it.[20]: 147
The term "oxidation state" in English chemical literature was popularized byWendell Mitchell Latimer in his 1938 book about electrochemical potentials.[218] He used it for the value (synonymous with the German termWertigkeit) previously termed "valence", "polar valence" or "polar number"[219] in English, or "oxidation stage" or indeed[220][221] the "state of oxidation". Since 1938, the term "oxidation state" has been connected withelectrochemical potentials and electrons exchanged inredox couples participating in redox reactions. By 1948, IUPAC used the 1940 nomenclature rules with the term "oxidation state",[222][223] instead of the original[217]valency. In 1948Linus Pauling proposed that oxidation number could be determined by extrapolating bonds to being completely ionic in the direction ofelectronegativity.[224] A full acceptance of this suggestion was complicated by the fact that thePauling electronegativities as such depend on the oxidation state and that they may lead to unusual values of oxidation states for some transition metals. In 1990 IUPAC resorted to a postulatory (rule-based) method to determine the oxidation state.[225] This was complemented by the synonymous term oxidation number as a descendant of the Stock number introduced in 1940 into the nomenclature. However, the terminology using "ligands"[20]: 147 gave the impression that oxidation number might be something specific tocoordination complexes. This situation and the lack of a real single definition generated numerous debates about the meaning of oxidation state, suggestions about methods to obtain it and definitions of it. To resolve the issue, an IUPAC project (2008-040-1-200) was started in 2008 on the "Comprehensive Definition of Oxidation State", and was concluded by two reports[7][6] and by the revised entries "Oxidation State"[8] and "Oxidation Number"[9] in theIUPAC Gold Book. The outcomes were a single definition of oxidation state and two algorithms to calculate it in molecular and extended-solid compounds, guided byAllen electronegativities that are independent of oxidation state.
^Wang, G.; Zhou, M.; Goettel, G. T.; Schrobilgen, G. J.; Su, J.; Li, J.; Schlöder, T.; Riedel, S. (2014). "Identification of an iridium-containing compound with a formal oxidation state of IX".Nature.514 (7523):475–477.Bibcode:2014Natur.514..475W.doi:10.1038/nature13795.PMID25341786.S2CID4463905.
^Muir, K. W.; Ibers, J. A. (1969). "The structure of chlorocarbonyl(sulfur dioxide)bis(triphenylphosphine)rhodium, (RhCl(CO)(SO2)(P(C6H5)3 2)".Inorg. Chem.8 (9):1921–1928.doi:10.1021/ic50079a024.
^Jørgensen, C. K. (1966). "Electric Polarizability, Innocent Ligands and Spectroscopic Oxidation States".Structure and Bonding. Vol. 1. Berlin: Springer-Verlag. pp. 234–248.
^"The Two-Electron Bond".Chemistry LibreTexts. June 25, 2016.Archived from the original on February 9, 2021. RetrievedSeptember 1, 2020.
^Martinie, R. J.; Bultema, J. J.; Wal, M. N. V.; Burkhart, B. J.; Griend, D. A. V.; DeCock, R. L. (2011). "Bond order and chemical properties of BF, CO, and N2".J. Chem. Educ.88 (8):1094–1097.Bibcode:2011JChEd..88.1094M.doi:10.1021/ed100758t.
^Li(–1) has been observed in the gas phase; seeR. H. Sloane; H. M. Love (1947). "Surface Formation of Lithium Negative Ions".Nature.159 (4035):302–303.Bibcode:1947Natur.159..302S.doi:10.1038/159302a0.
^Berthold, Chantsalmaa; Maurer, Johannes; Klerner, Lukas; Harder, Sjoerd; Buchner, Magnus R. (2024-05-31). "Formation, Structure and Reactivity of a Beryllium(0) Complex with Mgδ+−Beδ− Bond Polarization".Angewandte Chemie International Edition.63 (35) e202408422.doi:10.1002/anie.202408422.
^Beryllium(0) is present in LMgBeCp* (L = a complex diimide ligand, Cp* = pentamethylcyclopentadienyl) with a magnesium-beryllium polar bond.[24]
^Tetrazoles contain a pair of double-bonded nitrogen atoms with oxidation state 0 in the ring. A Synthesis of the parent 1H-tetrazole,CH2N4 (two atoms N(0)) is given inHenry, Ronald A.; Finnegan, William G. (1954). "An Improved Procedure for the Deamination of 5-Aminotetrazole".Journal of the American Chemical Society.76 (1):290–291.Bibcode:1954JAChS..76..290H.doi:10.1021/ja01630a086.ISSN0002-7863.
^Gold heptafluoride, synthesized at low temperature, is calculated to be a complex of molecular fluorine with gold pentafluoride, with F-F bonding in the F2 evidenced by IR spectroscopy; seeHimmel, Daniel; Riedel, Sebastian (2007-05-31). "After 20 Years, Theoretical Evidence That "AuF7" Is Actually AuF5·F2".Inorganic Chemistry.46 (13):5338–5342.doi:10.1021/ic700431s.PMID17511450.
^Ne(0) has been observed in Cr(CO)5Ne; seePerutz, Robin N.; Turner, James J. (August 1975). "Photochemistry of the Group 6 hexacarbonyls in low-temperature matrices. III. Interaction of the pentacarbonyls with noble gases and other matrices".Journal of the American Chemical Society.97 (17):4791–4800.Bibcode:1975JAChS..97.4791P.doi:10.1021/ja00850a001.
^Mg(0) has been synthesized in a compound containing a Na2Mg22+ cluster coordinated to a bulky organic ligand; seeRösch, B.; Gentner, T. X.; Eyselein, J.; Langer, J.; Elsen, H.; Li, W.; Harder, S. (2021). "Strongly reducing magnesium(0) complexes".Nature.592 (7856):717–721.Bibcode:2021Natur.592..717R.doi:10.1038/s41586-021-03401-w.PMID33911274.S2CID233447380
^Al(−2) has been observed in Sr14[Al4]2[Ge]3, seeWemdorff, Marco; Röhr, Caroline (2007). "Sr14[Al4]2[Ge]3: Eine Zintl-Phase mit isolierten [Ge]4–- und [Al4]8–-Anionen / Sr14[Al4]2[Ge]3: A Zintl Phase with Isolated [Ge]4–- and [Al4]8– Anions".Zeitschrift für Naturforschung B (in German).62 (10): 1227.doi:10.1515/znb-2007-1001.S2CID94972243.
^Al(–1) has been reported in Na5Al5; seeHaopeng Wang; Xinxing Zhang; Yeon Jae Ko; Andrej Grubisic; Xiang Li; Gerd Ganteför; Hansgeorg Schnöckel; Bryan W. Eichhorn; Mal-Soon Lee; P. Jena; Anil K. Kandalam; Boggavarapu Kiran; Kit H. Bowen (2014). "Aluminum Zintl anion moieties within sodium aluminum clusters".The Journal of Chemical Physics.140 (5).Bibcode:2014JChPh.140e4301W.doi:10.1063/1.4862989.
^Dohmeier, C.; Loos, D.; Schnöckel, H. (1996). "Aluminum(I) and Gallium(I) Compounds: Syntheses, Structures, and Reactions".Angewandte Chemie International Edition.35 (2):129–149.doi:10.1002/anie.199601291.
^Wang, Yuzhong; Xie, Yaoming; Wei, Pingrong; King, R. Bruce; Schaefer, Iii; Schleyer, Paul v. R.; Robinson, Gregory H. (2008). "Carbene-Stabilized Diphosphorus".Journal of the American Chemical Society.130 (45):14970–1.Bibcode:2008JAChS.13014970W.doi:10.1021/ja807828t.PMID18937460.
^Ellis, Bobby D.; MacDonald, Charles L. B. (2006). "Phosphorus(I) Iodide: A Versatile Metathesis Reagent for the Synthesis of Low Oxidation State Phosphorus Compounds".Inorganic Chemistry.45 (17):6864–74.doi:10.1021/ic060186o.PMID16903744.
^John E. Ellis (2006). "Adventures with Substances Containing Metals in Negative Oxidation States".Inorganic Chemistry.45 (8):3167–3186.doi:10.1021/ic052110i.
^ Calcium(I) has been obtained as a dinuclear organometallic complex with an arene dianion, seeKrieck, Sven; Görls, Helmar; Yu, Lian; Reiher, Markus; Westerhausen, Matthias (2009). "Stable "Inverse" Sandwich Complex with Unprecedented Organocalcium(I): Crystal Structures of [(thf)2Mg(Br)-C6H2-2,4,6-Ph3] and [(thf)3Ca{μ-C6H3-1,3,5-Ph3}Ca(thf)3]".Journal of the American Chemical Society.131 (8):2977–2985.doi:10.1021/ja808524y..
^Cloke, F. Geoffrey N.; Khan, Karl & Perutz, Robin N. (1991). "η-Arene complexes of scandium(0) and scandium(II)".J. Chem. Soc., Chem. Commun. (19):1372–1373.doi:10.1039/C39910001372.
^Smith, R. E. (1973). "Diatomic Hydride and Deuteride Spectra of the Second Row Transition Metals".Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences.332 (1588):113–127.Bibcode:1973RSPSA.332..113S.doi:10.1098/rspa.1973.0015.S2CID96908213.
^McGuire, Joseph C.; Kempter, Charles P. (1960). "Preparation and Properties of Scandium Dihydride".Journal of Chemical Physics.33 (5):1584–1585.Bibcode:1960JChPh..33.1584M.doi:10.1063/1.1731452.
^Ti(-2) is known inTi(CO)2−6; seeJohn E. Ellis (2006). "Adventures with Substances Containing Metals in Negative Oxidation States".Inorganic Chemistry.45 (8):3167–3186.doi:10.1021/ic052110i.
^Jilek, Robert E.; Tripepi, Giovanna; Urnezius, Eugenijus; Brennessel, William W.; Young, Victor G. Jr.; Ellis, John E. (2007). "Zerovalent titanium–sulfur complexes. Novel dithiocarbamato derivatives ofTi(CO)6:[Ti(CO)4(S2CNR2)]−".Chem. Commun. (25):2639–2641.doi:10.1039/B700808B.PMID17579764.
^V(–3) is known inV(CO)3−5; seeJohn E. Ellis (2006). "Adventures with Substances Containing Metals in Negative Oxidation States".Inorganic Chemistry.45 (8):3167–3186.doi:10.1021/ic052110i.
^V(0) is known inV(CO)6; seeJohn E. Ellis (2006). "Adventures with Substances Containing Metals in Negative Oxidation States".Inorganic Chemistry.45 (8):3167–3186.doi:10.1021/ic052110i.
^Cr(–4) is known inNa4Cr(CO)4; seeJohn E. Ellis (2006). "Adventures with Substances Containing Metals in Negative Oxidation States".Inorganic Chemistry.45 (8):3167–3186.doi:10.1021/ic052110i.
^Cr(0) is known inCr(CO)6; seeJohn E. Ellis (2006). "Adventures with Substances Containing Metals in Negative Oxidation States".Inorganic Chemistry.45 (8):3167–3186.doi:10.1021/ic052110i.
^Mn(–2) is known inMn(cod)2−2; seeJohn E. Ellis (2006). "Adventures with Substances Containing Metals in Negative Oxidation States".Inorganic Chemistry.45 (8):3167–3186.doi:10.1021/ic052110i.
^Demazeau, G.; Buffat, B.; Pouchard, M.; Hagenmuller, P. (1982). "Recent developments in the field of high oxidation states of transition elements in oxides stabilization of six-coordinated Iron(V)".Zeitschrift für anorganische und allgemeine Chemie.491:60–66.doi:10.1002/zaac.19824910109.
^Lu, J.; Jian, J.; Huang, W.; Lin, H.; Li, J; Zhou, M. (2016). "Experimental and theoretical identification of the Fe(VII) oxidation state in FeO4−".Physical Chemistry Chemical Physics.18 (45):31125–31131.Bibcode:2016PCCP...1831125L.doi:10.1039/C6CP06753K.PMID27812577.
^Co(–3) is known inNa3Co(CO)3; seeJohn E. Ellis (2006). "Adventures with Substances Containing Metals in Negative Oxidation States".Inorganic Chemistry.45 (8):3167–3186.doi:10.1021/ic052110i.
^Ni(–2) is known inNi(COD)2−2; seeJohn E. Ellis (2006). "Adventures with Substances Containing Metals in Negative Oxidation States".Inorganic Chemistry.45 (8):3167–3186.doi:10.1021/ic052110i.
^Ni(0) is known inNi(CO)4; seeJohn E. Ellis (2006). "Adventures with Substances Containing Metals in Negative Oxidation States".Inorganic Chemistry.45 (8):3167–3186.doi:10.1021/ic052110i.
^Pfirrmann, Stefan; Limberg, Christian; Herwig, Christian; Stößer, Reinhard; Ziemer, Burkhard (2009). "A Dinuclear Nickel(I) Dinitrogen Complex and its Reduction in Single-Electron Steps".Angewandte Chemie International Edition.48 (18):3357–61.doi:10.1002/anie.200805862.PMID19322853.
^Carnes, Matthew; Buccella, Daniela; Chen, Judy Y.-C.; Ramirez, Arthur P.; Turro, Nicholas J.; Nuckolls, Colin; Steigerwald, Michael (2009). "A Stable Tetraalkyl Complex of Nickel(IV)".Angewandte Chemie International Edition.48 (2):290–4.doi:10.1002/anie.200804435.PMID19021174.
^ Zn(−2) have been observed (as dimeric and monomeric anions; dimeric ions were initially reported to be [T–T]2−, but later shown to be [T–T]4− for all these elements) in Ca5Zn3 (structure (AE2+)5(T–T)4−T2−⋅4e−); seeChanghoon Lee; Myung-Hwan Whangbo (2008). "Late transition metal anions acting as p-metal elements".Solid State Sciences.10 (4):444–449.Bibcode:2008SSSci..10..444K.doi:10.1016/j.solidstatesciences.2007.12.001. andChanghoon Lee; Myung-Hwan Whangbo; Jürgen Köhler (2010). "Analysis of Electronic Structures and Chemical Bonding of Metal-rich Compounds. 2. Presence of Dimer (T–T)4– and Isolated T2– Anions in the Polar Intermetallic Cr5B3-Type Compounds AE5T3 (AE = Ca, Sr; T = Au, Ag, Hg, Cd, Zn)".Zeitschrift für Anorganische und Allgemeine Chemie.636 (1):36–40.doi:10.1002/zaac.200900421.
^Zn(0) has been observed; seeSingh, Amit Pratap; Samuel, Prinson P.; Roesky, Herbert W.; Schwarzer, Martin C.; Frenking, Gernot; Sidhu, Navdeep S.; Dittrich, Birger (2013). "A Singlet Biradicaloid Zinc Compound and Its Nonradical Counterpart".J. Am. Chem. Soc.135 (19):7324–9.doi:10.1021/ja402351x.PMID23600486. andSoleilhavoup, Michèle; Bertrand, Guy (2015). "Cyclic (Alkyl)(Amino)Carbenes (CAACs): Stable Carbenes on the Rise".Acc. Chem. Res.48 (2):256–266.doi:10.1021/ar5003494.PMID25515548.
^Ga(−1) has been observed in LiGa; seeHolleman, Arnold F.; Wiberg, Egon; Wiberg, Nils (2008).Lehrbuch der Anorganischen Chemie (in German) (102 ed.). Walter de Gruyter. p. 1185.ISBN978-3-11-020684-5.
^abcGe(−1), Ge(−2), Ge(−3), and Ge(–4) have been observed ingermanides; seeHolleman, Arnold F.; Wiberg, Egon; Wiberg, Nils (1995). "Germanium".Lehrbuch der Anorganischen Chemie (in German) (101 ed.). Walter de Gruyter. pp. 953–959.ISBN978-3-11-012641-9.
^As(−2) has been observed in CaAs; seeHolleman, Arnold F.; Wiberg, Egon; Wiberg, Nils (2008).Lehrbuch der Anorganischen Chemie (in German) (102 ed.). Walter de Gruyter. p. 829.ISBN978-3-11-020684-5.
^As(−1) has been observed in LiAs; seeReinhard Nesper (1990). "Structure and chemical bonding in zintl-phases containing lithium".Progress in Solid State Chemistry.20 (1):1–45.doi:10.1016/0079-6786(90)90006-2.
^Abraham, Mariham Y.; Wang, Yuzhong; Xie, Yaoming; Wei, Pingrong; Shaefer III, Henry F.; Schleyer, P. von R.; Robinson, Gregory H. (2010). "Carbene Stabilization of Diarsenic: From Hypervalency to Allotropy".Chemistry: A European Journal.16 (2):432–5.doi:10.1002/chem.200902840.PMID19937872.
^Ellis, Bobby D.; MacDonald, Charles L. B. (2004). "Stabilized Arsenic(I) Iodide: A Ready Source of Arsenic Iodide Fragments and a Useful Reagent for the Generation of Clusters".Inorganic Chemistry.43 (19):5981–6.doi:10.1021/ic049281s.PMID15360247.
^As(IV) has been observed inarsenic(IV) hydroxide (As(OH)4) andHAsO−; seeKläning, Ulrik K.; Bielski, Benon H. J.; Sehested, K. (1989). "Arsenic(IV). A pulse-radiolysis study".Inorganic Chemistry.28 (14):2717–24.doi:10.1021/ic00313a007.
^Se(−1) has been observed in diselenides(Se2−2, such asdisodium diselenide (Na2Se2); seeHolleman, Arnold F.; Wiberg, Egon; Wiberg, Nils (2008).Lehrbuch der Anorganischen Chemie (in German) (102 ed.). Walter de Gruyter. p. 829.ISBN978-3-11-020684-5. andH. Föppl; E. Busmann; F.-K. Frorath (1962). "Die Kristallstrukturen von α-Na2S2 und K2S2, β-Na2S2 und Na2Se2".Zeitschrift für anorganische und allgemeine Chemie (in German).314 (1):12–20.doi:10.1002/zaac.19623140104.
^A Se(0) atom has been identified using DFT in [ReOSe(2-pySe)3]; seeCargnelutti, Roberta; Lang, Ernesto S.; Piquini, Paulo; Abram, Ulrich (2014). "Synthesis and structure of [ReOSe(2-Se-py)3]: A rhenium(V) complex with selenium(0) as a ligand".Inorganic Chemistry Communications.45:48–50.doi:10.1016/j.inoche.2014.04.003.ISSN1387-7003.
^Se(III) has been observed in Se2NBr3; seeLau, Carsten; Neumüller, Bernhard; Vyboishchikov, Sergei F.; Frenking, Gernot; Dehnicke, Kurt; Hiller, Wolfgang; Herker, Martin (1996). "Se2NBr3, Se2NCl5, Se2NCl−6: New Nitride Halides of Selenium(III) and Selenium(IV)".Chemistry: A European Journal.2 (11):1393–1396.doi:10.1002/chem.19960021108.
^Rb(–1) is known inrubidides; seeJohn E. Ellis (2006). "Adventures with Substances Containing Metals in Negative Oxidation States".Inorganic Chemistry.45 (8):3167–3186.doi:10.1021/ic052110i.
^abcdefghijklmnYttrium and all lanthanides except Ce and Pm have been observed in the oxidation state 0 in bis(1,3,5-tri-t-butylbenzene) complexes, seeCloke, F. Geoffrey N. (1993). "Zero Oxidation State Compounds of Scandium, Yttrium, and the Lanthanides".Chem. Soc. Rev.22:17–24.doi:10.1039/CS9932200017. andArnold, Polly L.; Petrukhina, Marina A.; Bochenkov, Vladimir E.; Shabatina, Tatyana I.; Zagorskii, Vyacheslav V.; Cloke (2003-12-15). "Arene complexation of Sm, Eu, Tm and Yb atoms: a variable temperature spectroscopic investigation".Journal of Organometallic Chemistry.688 (1–2):49–55.doi:10.1016/j.jorganchem.2003.08.028.
^Zr(–2) is known inZr(CO)2−6; seeJohn E. Ellis (2006). "Adventures with Substances Containing Metals in Negative Oxidation States".Inorganic Chemistry.45 (8):3167–3186.doi:10.1021/ic052110i.
^Zr(0) occur in (η6-(1,3,5-tBu)3C6H3)2Zr and [(η5-C5R5Zr(CO)4]−, seeChirik, P. J.; Bradley, C. A. (2007). "4.06 - Complexes of Zirconium and Hafnium in Oxidation States 0 to ii".Comprehensive Organometallic Chemistry III. From Fundamentals to Applications. Vol. 4. Elsevier Ltd. pp. 697–739.doi:10.1016/B0-08-045047-4/00062-5.ISBN978-0-08-045047-6.
^Nb(–3) occurs inCs3Nb(CO)5; seeJohn E. Ellis (2003). "Metal Carbonyl Anions: from [Fe(CO)4]2− to [Hf(CO)6]2− and Beyond†".Organometallics.22 (17):3322–3338.doi:10.1021/om030105l.
^abNb(0) and Nb(I) has been observed in Nb(bpy)3 and CpNb(CO)4, respectively; seeHolleman, Arnold F.; Wiberg, Egon; Wiberg, Nils (2008).Lehrbuch der Anorganischen Chemie (in German) (102 ed.). Walter de Gruyter. p. 1554.ISBN978-3-11-020684-5.
^Mo(–4) occurs inNa4Mo(CO)4; seeJohn E. Ellis (2003). "Metal Carbonyl Anions: from [Fe(CO)4]2– to [Hf(CO)6]2– and Beyond†".Organometallics.22 (17):3322–3338.doi:10.1021/om030105l.
^Mo(0) occurs inmolybdenum hexacarbonyl; seeJohn E. Ellis (2003). "Metal Carbonyl Anions: from [Fe(CO)4]2– to [Hf(CO)6]2– and Beyond†".Organometallics.22 (17):3322–3338.doi:10.1021/om030105l.
^Marcia F. Bailey; Lawrence F. Dahl (1965). "The Crystal Structure of Ditechnetium Decacarbonyl".Inorg. Chem.4 (8):1140–1145.doi:10.1021/ic50030a011.
^Ru(–1) occurs in [Ru2(CO)8]2–; seeXuenian Chen; Hima Kumar Lingam; Edward A. Meyers; Sheldon G. Shore (2012). "Structures of DMF solvated potassium and sodium salts of [Fe(CO)4]2– and [M2(CO)8]2– (M = Fe, Ru)".Journal of Organometallic Chemistry.721–722:137–143.doi:10.1016/j.jorganchem.2012.07.040.
^Ru(0) is present in the carbonyl-phosphine complex Ru(CO)2(PPh3)3, seeStephane Sentets; Maria del Carmen Rodriguez Martinez; Laure Vendier; Bruno Donnadieu; Vincent Huc; Noël Lugan; Guy Lavigne (2005). "Instant "Base-Promoted" Generation of Roper's-type Ru(0) Complexes Ru(CO)2(PR3)3 from a Simple Carbonylchlororuthenium(II) Precursor".J. Am. Chem. Soc.127 (42):14554–14555.doi:10.1021/ja055066e.PMID16231891.
^Ellis J E. Highly Reduced Metal Carbonyl Anions: Synthesis, Characterization, and Chemical Properties. Adv. Organomet. Chem, 1990, 31: 1-51.
^Ag(−2) have been observed as dimeric and monomeric anions in Ca5Ag3, (structure (Ca2+)5(Ag–Ag)4−Ag2−⋅4e−); seeChanghoon Lee; Myung-Hwan Whangbo; Jürgen Köhler (2010). "Analysis of Electronic Structures and Chemical Bonding of Metal-rich Compounds. 2. Presence of Dimer (T–T)4– and Isolated T2– Anions in the Polar Intermetallic Cr5B3-Type Compounds AE5T3 (AE = Ca, Sr; T = Au, Ag, Hg, Cd, Zn)".Zeitschrift für Anorganische und Allgemeine Chemie.636 (1):36–40.doi:10.1002/zaac.200900421.
^The Ag− ion has been observed in metal ammonia solutions: seeTran, N. E.; Lagowski, J. J. (2001). "Metal Ammonia Solutions: Solutions Containing Argentide Ions".Inorganic Chemistry.40 (5):1067–68.doi:10.1021/ic000333x.
^Ag(0) has been observed in carbonyl complexes in low-temperature matrices: seeMcIntosh, D.; Ozin, G. A. (1976). "Synthesis using metal vapors. Silver carbonyls. Matrix infrared, ultraviolet-visible, and electron spin resonance spectra, structures, and bonding of silver tricarbonyl, silver dicarbonyl, silver monocarbonyl, and disilver hexacarbonyl".J. Am. Chem. Soc.98 (11):3167–75.Bibcode:1976JAChS..98.3167M.doi:10.1021/ja00427a018.
^Cd(−2) have been observed (as dimeric and monomeric anions; dimeric ions were initially reported to be [T–T]2−, but later shown to be [T–T]4−) in Sr5Cd3; seeChanghoon Lee; Myung-Hwan Whangbo; Jürgen Köhler (2010). "Analysis of Electronic Structures and Chemical Bonding of Metal-rich Compounds. 2. Presence of Dimer (T–T)4– and Isolated T2– Anions in the Polar Intermetallic Cr5B3-Type Compounds AE5T3 (AE = Ca, Sr; T = Au, Ag, Hg, Cd, Zn)".Zeitschrift für Anorganische und Allgemeine Chemie.636 (1):36–40.doi:10.1002/zaac.200900421.
^A cubic Cd98+ cluster with the central atom as Cd(0) has been identified in zeolite A; seeJennifer E. Readman; Peter D. Barker; Ian Gameson; Joe Hriljac; Wuzong Zhou; Peter Edwards; Paul Anderson (2004). "An ordered array of cadmium clusters assembled in zeolite A".Chemical communications (Cambridge, England).10:736–7.doi:10.1039/b400166d.
^Cd(I) has been observed incadmium(I) tetrachloroaluminate (Cd2(AlCl4)2); seeHolleman, Arnold F.; Wiberg, Egon; Wiberg, Nils (1985). "Cadmium".Lehrbuch der Anorganischen Chemie (in German) (91–100 ed.). Walter de Gruyter. pp. 1056–1057.ISBN978-3-11-007511-3.
^Guloy, A. M.; Corbett, J. D. (1996). "Synthesis, Structure, and Bonding of Two Lanthanum Indium Germanides with Novel Structures and Properties".Inorganic Chemistry.35 (9):2616–22.doi:10.1021/ic951378e.PMID11666477.
^In(−2) has been observed in Na2In, see[1], p. 69.
^In(−1) has been observed in NaIn; seeHolleman, Arnold F.; Wiberg, Egon; Wiberg, Nils (2008).Lehrbuch der Anorganischen Chemie (in German) (102 ed.). Walter de Gruyter. p. 1185.ISBN978-3-11-020684-5.
^Unstable In(0) carbonyls and clusters have been detected, see[2], p. 6.
^Sn(−2) has been observed in SrSn; seeHolleman, Arnold F.; Wiberg, Egon; Wiberg, Nils (2008).Lehrbuch der Anorganischen Chemie (in German) (102 ed.). Walter de Gruyter. p. 1007.ISBN978-3-11-020684-5.
^Sn(−1) has been observed in CsSn; seeHolleman, Arnold F.; Wiberg, Egon; Wiberg, Nils (2008).Lehrbuch der Anorganischen Chemie (in German) (102 ed.). Walter de Gruyter. p. 1007.ISBN978-3-11-020684-5.
^"HSn".NIST Chemistry WebBook. National Institute of Standards and Technology. Retrieved23 January 2013.
^"SnH3".NIST Chemistry WebBook. National Institure of Standards and Technology. Retrieved23 January 2013.
^abSb(−2) and Sb(−1) has been observed in [Sb2]4− and1∞[Sbn]n−, respectively; seeBoss, Michael; Petri, Denis; Pickhard, Frank; Zönnchen, Peter; Röhr, Caroline (2005). "Neue Barium-Antimonid-Oxide mit den Zintl-Ionen [Sb]3−, [Sb2]4− und1∞[Sbn]n− / New Barium Antimonide Oxides containing Zintl Ions [Sb]3−, [Sb2]4− and1∞[Sbn]n−".Zeitschrift für Anorganische und Allgemeine Chemie (in German).631 (6–7):1181–1190.doi:10.1002/zaac.200400546.
^Sb(I) has been observed inorganoantimony compounds; seeŠimon, Petr; de Proft, Frank; Jambor, Roman; Růžička, Aleš; Dostál, Libor (2010). "Monomeric Organoantimony(I) and Organobismuth(I) Compounds Stabilized by an NCN Chelating Ligand: Syntheses and Structures".Angewandte Chemie International Edition.49 (32):5468–5471.doi:10.1002/anie.201002209.PMID20602393.
^Sb(+2) has been observed in distibines, seePatai, Saul, ed. (1994).The Chemistry of Organic Arsenic, Antimony, and Bismuth Compounds. Chemistry of Functional Groups. Chichester, UK: Wiley. p. 442.doi:10.1002/0470023473.ISBN0-471-93044-X.
^Sb(IV) has been observed in[SbCl6]2−, seeNobuyoshi Shinohara; Masaaki Ohsima (2000). "Production of Sb(IV) Chloro Complex by Flash Photolysis of the Corresponding Sb(III) and Sb(V) Complexes in CH3CN and CHCl3".Bulletin of the Chemical Society of Japan.73 (7):1599–1604.doi:10.1246/bcsj.73.1599.
^Te(V) is mentioned by Greenwood and Earnshaw, but they do not give any example of a Te(V) compound. What was long thought to beditellurium decafluoride (Te2F10) is actually bis(pentafluorotelluryl) oxide, F5TeOTeF5: seeWatkins, P. M. (1974). "Ditellurium decafluoride - A Continuing Myth".Journal of Chemical Education.51 (9):520–521.Bibcode:1974JChEd..51..520W.doi:10.1021/ed051p520. However, Te(V) has been observed inHTeO−,TeO−,HTeO−2, andTeO−3; seeKläning, Ulrik K.; Sehested, K. (2001)."Tellurium(V). A Pulse Radiolysis Study".The Journal of Physical Chemistry A.105 (27):6637–45.Bibcode:2001JPCA..105.6637K.doi:10.1021/jp010577i.
^I(IV) has been observed iniodine dioxide (IO2); seePauling, Linus (1988). "Oxygen Compounds of Nonmetallic Elements".General Chemistry (3rd ed.). Dover Publications, Inc. p. 259.ISBN978-0-486-65622-9.
^I(VI) has been observed in IO3, IO42−, H5IO6−, H2IO52−, H4IO62−, and HIO53−; seeKläning, Ulrik K.; Sehested, Knud; Wolff, Thomas (1981). "Laser flash photolysis and pulse radiolysis of iodate and periodate in aqueous solution. Properties of iodine(VI)".J. Chem. Soc., Faraday Trans. 1.77 (7):1707–18.doi:10.1039/F19817701707.
^Xu, Wei; Lerner, Michael M. (2018). "A New and Facile Route Using Electride Solutions to Intercalate Alkaline Earth Ions into Graphite".Chemistry of Materials.30 (19):6930–6935.doi:10.1021/acs.chemmater.8b03421.S2CID105295721.
^abcdeAll thelanthanides, except Pm, in the +2 oxidation state have been observed in organometallic molecular complexes, seeLanthanides Topple Assumptions andMeyer, G. (2014). "All the Lanthanides Do It and Even Uranium Does Oxidation State +2".Angewandte Chemie International Edition.53 (14):3550–51.doi:10.1002/anie.201311325.PMID24616202.. Additionally, all thelanthanides (La–Lu) form dihydrides (LnH2), dicarbides (LnC2), monosulfides (LnS), monoselenides (LnSe), and monotellurides (LnTe), but for most elements these compounds have Ln3+ ions with electrons delocalized into conduction bands, e. g. Ln3+(H−)2(e−).
^Hf(–2) occurs inHf(CO)62−; seeJohn E. Ellis (2003). "Metal Carbonyl Anions: from [Fe(CO)4]2− to [Hf(CO)6]2− and Beyond†".Organometallics.22 (17):3322–3338.doi:10.1021/om030105l.
^Hf(0) occur in (η6-(1,3,5-tBu)3C6H3)2Hf and [(η5-C5R5Hf(CO)4]−, seeChirik, P. J.; Bradley, C. A. (2007). "4.06 - Complexes of Zirconium and Hafnium in Oxidation States 0 to ii".Comprehensive Organometallic Chemistry III. From Fundamentals to Applications. Vol. 4. Elsevier Ltd. pp. 697–739.doi:10.1016/B0-08-045047-4/00062-5.ISBN978-0-08-045047-6.
^Ta(–3) occurs inTa(CO)53−; seeJohn E. Ellis (2003). "Metal Carbonyl Anions: from [Fe(CO)4]2− to [Hf(CO)6]2− and Beyond†".Organometallics.22 (17):3322–3338.doi:10.1021/om030105l.
^Ta(0) is known in Ta(CNDipp)6; seeKhetpakorn Chakarawet; Zachary W. Davis-Gilbert; Stephanie R. Harstad; Victor G. Young Jr.; Jeffrey R. Long; John E. Ellis (2017). "Ta(CNDipp)6: An Isocyanide Analogue of Hexacarbonyltantalum(0)".Angewandte Chemie International Edition.56 (35):10577–10581.doi:10.1002/anie.201706323.PMID28697283. Additionally, Ta(0) has also been previously reported in Ta(bipy)3, but this has been proven to contain Ta(V).
^Ta(I) has been observed in CpTa(CO)4; seeHolleman, Arnold F.; Wiberg, Egon; Wiberg, Nils (2008).Lehrbuch der Anorganischen Chemie (in German) (102 ed.). Walter de Gruyter. p. 1554.ISBN978-3-11-020684-5.
^W(−4) is known inW(CO)4−4; seeJohn E. Ellis (2006). "Adventures with Substances Containing Metals in Negative Oxidation States".Inorganic Chemistry.45 (8):3167–3186.doi:10.1021/ic052110i.
^W(0) is known inW(CO)6; seeJohn E. Ellis (2006). "Adventures with Substances Containing Metals in Negative Oxidation States".Inorganic Chemistry.45 (8):3167–3186.doi:10.1021/ic052110i.
^Re(0) is known inRe2(CO)10; seeJohn E. Ellis (2006). "Adventures with Substances Containing Metals in Negative Oxidation States".Inorganic Chemistry.45 (8):3167–3186.doi:10.1021/ic052110i.
^Os(–1) occurs in [Os2(CO)8]2–; seeLeh Yeh Hsu; Nripendra Bhattacharyya; Sheldon G. Shore (1985). "Binuclear carbonylates [Ru2(CO)8]2– and [Os2(CO)8]2–: syntheses and crystal structures".Organometallics.4 (8):1483–1485.doi:10.1021/om00127a041.
^Os(0) is known inOs(CO)5; seeRushman, Paul; Van Buuren, Gilbert N.; Shiralian, Mahmoud; Pomeroy, Roland K. (1983). "Properties of the Pentacarbonyls of Ruthenium and Osmium".Organometallics.2 (5):693–694.doi:10.1021/om00077a026.
^Ir(–2) has been observed in IrVO2–; seeLe-Shi Chen; Yun-Zhu Liu; Jiao-Jiao Chen; Si-Dun Wang; Tong-Mei Ma; Xiao-Na Li; Sheng-Gui He (2022). "Water–Gas Shift Catalyzed by Iridium–Vanadium Oxide Clusters IrVO2– with Iridium in a Rare Oxidation State of −II".The Journal of Physical Chemistry A.126 (32):5294–5301.doi:10.1021/acs.jpca.2c03974.
^Ir(VIII) has been observed iniridium tetroxide (IrO4); seeGong, Yu; Zhou, Mingfei; Kaupp, Martin; Riedel, Sebastian (2009). "Formation and Characterization of the Iridium Tetroxide Molecule with Iridium in the Oxidation State +VIII".Angewandte Chemie International Edition.48 (42):7879–7883.doi:10.1002/anie.200902733.PMID19593837.
^Wang, Guanjun; Zhou, Mingfei; Goettel, James T.; Schrobilgen, Gary G.; Su, Jing; Li, Jun; Schlöder, Tobias; Riedel, Sebastian (2014). "Identification of an iridium-containing compound with a formal oxidation state of IX".Nature.514 (7523):475–477.Bibcode:2014Natur.514..475W.doi:10.1038/nature13795.PMID25341786.S2CID4463905.
^Pt(−2) has been observed in Cs2Pt, seeKarpov, Andrey; Nuss, Jürgen; Wedig, Ulrich; Jansen, Martin (2003-10-13). "Cs2Pt: A Platinide(-II) Exhibiting Complete Charge Separation".Angewandte Chemie International Edition.42 (39). Wiley:4818–4821.doi:10.1002/anie.200352314.ISSN1433-7851..
^Pt(0) has been observed inorganoplatinum compounds, e. g. (PPh3)2PtC2H4; seeCheng, P.-T.; Nyburg, S. C. (1972). "The Crystal and Molecular Structure of bis(triphenylphosphine)-(ethylene)platinum, (PPh3)2PtC2H4".Canadian Journal of Chemistry.50 (6):912–916.doi:10.1139/v72-142.
^Pt(I) has been observed in [Pt2(CO)6]2+; seeXu, Qiang; Heaton, Brian T.; Jacob, Chacko; Mogi, Koichi; Ichihashi, Yuichi; Souma, Yoshie; Kanamori, Kan; Eguchi, Taro (2000). "Hexacarbonyldiplatinum(I). Synthesis, spectroscopy, and density functional calculation of the first homoleptic, dinuclear platinum(I) carbonyl cation, [{Pt(CO)3}2]2+, formed in concentrated sulfuric acid".Journal of the American Chemical Society.122 (29):6862–6870.doi:10.1021/ja000716u.
^Pt(III) has been observed; seeO'Halloran, Thomas V.; Lippard, Stephen J. (1985). "The Chemistry of Platinum in the +3 Oxidation State".Israel Journal of Chemistry.25 (2):130–137.doi:10.1002/ijch.198500021.
^Mézaille, Nicolas; Avarvari, Narcis; Maigrot, Nicole; Ricard, Louis; Mathey, François; Le Floch, Pascal; Cataldo, Laurent; Berclaz, Théo; Geoffroy, Michel (1999). "Gold(I) and Gold(0) Complexes of Phosphinine-Based Macrocycles".Angewandte Chemie International Edition.38 (21):3194–3197.doi:10.1002/(SICI)1521-3773(19991102)38:21<3194::AID-ANIE3194>3.0.CO;2-O.PMID10556900.
^Tl(+2) has been observed intetrakis(hypersilyl)dithallium ([(Me3Si)Si]2Tl—Tl[Si(SiMe3)]2), seeSonja Henkel; Dr. Karl Wilhelm Klinkhammer; Dr. Wolfgang Schwarz (1994). "Tetrakis(hypersilyl)dithallium(Tl—Tl): A Divalent Thallium Compound".Angew. Chem. Int. Ed.33 (6):681–683.doi:10.1002/anie.199406811.
^Pb(−2) has been observed in BaPb, seeFerro, Riccardo (2008). Nicholas C. Norman (ed.).Intermetallic Chemistry. Elsevier. p. 505.ISBN978-0-08-044099-6. andTodorov, Iliya; Sevov, Slavi C. (2004). "Heavy-Metal Aromatic Rings: Cyclopentadienyl Anion Analogues Sn56− and Pb56− in the Zintl Phases Na8BaPb6, Na8BaSn6, and Na8EuSn6".Inorganic Chemistry.43 (20):6490–94.doi:10.1021/ic000333x.
^Pb(−1) has been observed in CsPb, seeHewaidy, I. F.; Busmann, E.; Klemm, W. (1964). "Die Struktur der AB-Verbindungen der schweren Alkalimetalle mit Zinn und Blei".Zeitschrift für anorganische und allgemeine Chemie (in German).328 (5–6):283–293.doi:10.1002/zaac.19643280511.
^Bi(0) state exists in aN-heterocyclic carbene complex of dibismuthene; seeDeka, Rajesh; Orthaber, Andreas (May 9, 2022). "Carbene chemistry of arsenic, antimony, and bismuth: origin, evolution and future prospects".Royal Society of Chemistry.51 (22):8540–8556.doi:10.1039/d2dt00755j.PMID35578901.S2CID248675805.
^Bi(I) has been observed inorganobismuth compounds; seeŠimon, Petr; de Proft, Frank; Jambor, Roman; Růžička, Aleš; Dostál, Libor (2010). "Monomeric Organoantimony(I) and Organobismuth(I) Compounds Stabilized by an NCN Chelating Ligand: Syntheses and Structures".Angewandte Chemie International Edition.49 (32):5468–5471.doi:10.1002/anie.201002209.PMID20602393.
^Bi(IV) has been observed; seeA. I. Aleksandrov, I. E. Makarov (1987). "Formation of Bi(II) and Bi(IV) in aqueous hydrochloric solutions of Bi(III)".Bulletin of the Academy of Sciences of the USSR, Division of Chemical Science.36 (2):217–220.doi:10.1007/BF00959349.S2CID94865394.
^Thayer, John S. (2010). "Relativistic Effects and the Chemistry of the Heavier Main Group Elements".Relativistic Methods for Chemists. Challenges and Advances in Computational Chemistry and Physics. Vol. 10. p. 78.doi:10.1007/978-1-4020-9975-5_2.ISBN978-1-4020-9974-8.
^abPore, Jennifer L.; Gates, Jacklyn M.; Dixon, David A.; Garcia, Fatima H.; Gibson, John K.; Gooding, John A.; McCarthy, Mallory; Orford, Rodney; Shafi, Ziad; Shuh, David K.; Sprouse, Sarah (2025). "Direct identification of Ac and No molecules with an atom-at-a-time technique".Nature.644. Springer Science and Business Media LLC:376–380.doi:10.1038/s41586-025-09342-y.ISSN0028-0836.
^U(II) has been observed in [K(2.2.2-Cryptand)][(C5H4SiMe3)3U], seeMacDonald, Matthew R.; Fieser, Megan E.; Bates, Jefferson E.; Ziller, Joseph W.; Furche, Filipp; Evans, William J. (2013). "Identification of the +2 Oxidation State for Uranium in a Crystalline Molecular Complex, [K(2.2.2-Cryptand)][(C5H4SiMe3)3U]".J. Am. Chem. Soc.135 (36):13310–13313.doi:10.1021/ja406791t.PMID23984753.
^Windorff, Cory J.; Chen, Guo P; Cross, Justin N; Evans, William J.; Furche, Filipp; Gaunt, Andrew J.; Janicke, Michael T.; Kozimor, Stosh A.; Scott, Brian L. (2017). "Identification of the Formal +2 Oxidation State of Plutonium: Synthesis and Characterization of {PuII[C5H3(SiMe3)2]3}−".J. Am. Chem. Soc.139 (11):3970–3973.doi:10.1021/jacs.7b00706.PMID28235179.
^Preparation of plutonium(VIII) compounds has been claimed, seeZaitsevskii, Andréi; Mosyagin, Nikolai S.; Titov, Anatoly V.; Kiselev, Yuri M. (July 21, 2013). "Relativistic density functional theory modeling of plutonium and americium higher oxide molecules".The Journal of Chemical Physics.139 (3) 034307.Bibcode:2013JChPh.139c4307Z.doi:10.1063/1.4813284.PMID23883027. andKiselev, Yu. M.; Nikonov, M. V.; Dolzhenko, V. D.; Ermilov, A. Yu.; Tananaev, I. G.; Myasoedov, B. F. (January 17, 2014). "On existence and properties of plutonium(VIII) derivatives".Radiochimica Acta.102 (3):227–237.doi:10.1515/ract-2014-2146.S2CID100915090. But their existence has been partially disproven, seeFedosseev, Alexander M.; Bessonov, Alexi A.; Shilov, Vladimir P. (September 5, 2022). "Is octavalent plutonium really formed during oxidation in alkaline aqueous solutions?".Radiochimica Acta.110 (12):955–959.doi:10.1515/ract-2022-0056.
^Sullivan, Jim C.; Schmidt, K. H.; Morss, L. R.; Pippin, C. G.; Williams, C. (1988). "Pulse radiolysis studies of berkelium(III): preparation and identification of berkelium(II) in aqueous perchlorate media".Inorganic Chemistry.27 (4): 597.doi:10.1021/ic00277a005.
^Hs(VIII) has been observed in hassium tetroxide (HsO4); see"Chemistry of Hassium"(PDF).Gesellschaft für Schwerionenforschung mbH. 2002. Retrieved2007-01-31.
^Noyes, A. A.; Pitzer, K. S.; Dunn, C. L. (1935). "Argentic salts in acid solution, I. The oxidation and reduction reactions".J. Am. Chem. Soc.57 (7):1221–1229.Bibcode:1935JAChS..57.1221N.doi:10.1021/ja01310a018.
^Noyes, A. A.; Pitzer, K. S.; Dunn, C. L. (1935). "Argentic salts in acid solution, II. The oxidation state of argentic salts".J. Am. Chem. Soc.57 (7):1229–1237.Bibcode:1935JAChS..57.1229N.doi:10.1021/ja01310a019.
^Fernelius, W. C. (1948). "Some problems of inorganic nomenclature".Chem. Eng. News.26:161–163.doi:10.1021/cen-v026n003.p161.
^Fernelius, W. C.; Larsen, E. M.; Marchi, L. E.; Rollinson, C. L. (1948). "Nomenclature of coördination compounds".Chem. Eng. News.26 (8):520–523.doi:10.1021/cen-v026n008.p520.
