| ↓ Period | ||||
|---|---|---|---|---|
| 4 |  25 Transition metal | |||
| 5 |  43 Transition metal | |||
| 6 |  75 Transition metal | |||
| 7 | Bohrium (Bh) 107 Transition metal | |||
Legend
| ||||
Group 7, numbered byIUPAC nomenclature, is agroup of elements in theperiodic table. It containsmanganese (Mn),technetium (Tc),rhenium (Re) andbohrium (Bh). This group lies in thed-block of the periodic table, and are hencetransition metals. This group is sometimes called themanganese group ormanganese family after its lightest member; however, the group itself has not acquired atrivial name because it belongs to the broader grouping of the transition metals.
The group 7 elements tend to have a major groupoxidation state (+7), although this trend is markedly less coherent than the previous groups. Like other groups, the members of this family show patterns in theirelectron configurations, especially the outermost shells resulting in trends in chemical behavior. In nature, manganese is a fairly common element, whereas rhenium is rare, technetium only occurs in trace quantities, and bohrium is entirelysynthetic.
The trends in group 7 follow, although less noticeably, those of the other early d-block groups and reflect the addition of a filled f-shell into the core in passing from the fifth to the sixth period. All group 7 elements crystallize in thehexagonal close packed (hcp) structure except manganese, which crystallizes in thebody centered cubic (bcc) structure. Bohrium is also expected to crystallize in the hcp structure.[1]
The table below is a summary of the key physical properties of the group 7 elements. The properties of bohrium are either unknown or predicted, as they have not been measured.[2]
| Name | Mn,manganese | Tc,technetium | Re,rhenium | Bh,bohrium |
|---|---|---|---|---|
| Melting point | 1519 K (1246 °C) | 2430 K (2157 °C) | 3459 K (3186 °C) | Unknown |
| Boiling point | 2334 K (2061 °C) | 4538 K (4265 °C) | 5903 K (5630 °C) | Unknown |
| Density | 7.21 g·cm−3 | 11 g·cm−3 | 21.02 g·cm−3 | 26-27 g·cm−3?[3][4] |
| Appearance | silvery metallic | silvery-gray | silvery-gray | Unknown |
| Atomic radius | 127 pm | 136 pm | 137 pm | 128 pm?[2] |
Like other groups, the members of this family show patterns in itselectron configuration, especially the outermost shells:
| Z | Element | Electrons pershell |
|---|---|---|
| 25 | manganese | 2, 8, 13, 2 |
| 43 | technetium | 2, 8, 18, 13, 2 |
| 75 | rhenium | 2, 8, 18, 32, 13, 2 |
| 107 | bohrium | 2, 8, 18, 32, 32, 13, 2 |
All the members of the group readily portray their group oxidation state of +7 and the state becomes more stable as the group is descended. Technetium also shows a stable +4 state whilst rhenium exhibits stable +4 and +3 states.
Bohrium may therefore also show these lower states as well. The higher +7 oxidation state is more likely to exist in oxyanions, such as perbohrate, BhO4−, analogous to the lighterpermanganate,pertechnetate, andperrhenate. Nevertheless, bohrium(VII) is likely to be unstable in aqueous solution, and would probably be easily reduced to the more stable bohrium(IV).[5]
_oxide.jpg)
Manganese forms a variety of oxides:MnO,Mn3O4,Mn2O3,MnO2, MnO3 andMn2O7. Manganese(II) oxide is an inorganic compound that forms green crystals. Like many monoxides, MnO adopts therock salt structure, where cations and anions are both octahedrally coordinated. Also like many oxides, manganese(II) oxide is oftennonstoichiometric: its composition can vary from MnO to MnO1.045.[6]Manganese(II,III) oxide is formed when any manganese oxide is heated in air above 1000 °C.[6] Considerable research has centred on producing nanocrystalline Mn3O4 and various syntheses that involve oxidation of MnII or reduction of MnVI.[7][8][9] Manganese(III) oxide is unlike many other transition metal oxides in that it does not adopt thecorundum (Al2O3) structure.[6] Two forms are generally recognized, α-Mn2O3 and γ-Mn2O3,[10] although a high pressure form with the CaIrO3 structure has been reported too.[11] Manganese(IV) oxide is a blackish or brown solid occurs naturally as the mineralpyrolusite, which is the main ore of manganese and a component ofmanganese nodules. The principal use for MnO2 is for dry-cellbatteries, such as thealkaline battery and thezinc–carbon battery.[6] Manganese(VII) oxide is dark green in itscrystalline form. The liquid is green by reflected light and red by transmitted light.[12] It is soluble incarbon tetrachloride, and decomposes when in contact with water.
_oxide.png)
Technetium's main oxides aretechnetium(IV) oxide andtechnetium(VII) oxide. Technetium(IV) oxide was first produced in 1949 by electrolyzing a solution ofammonium pertechnetate underammonium hydroxide. It has often been used to separate technetium frommolybdenum and rhenium.[13][14][15] More efficient ways are the reduction of ammonium pertechnetate byzinc metal andhydrochloric acid,stannous chloride,hydrazine,hydroxylamine,ascorbic acid,[14] by the hydrolysis ofpotassium hexachlorotechnetate[16] or by the decomposition of ammonium pertechnetate at 700 °C under an inert atmosphere.[13][17][18] It reacts with oxygen to produce technetium(VII) oxide at 450 °C.
Technetium(VII) oxide can be prepared directly by the oxidation of technetium at 450-500 °C.[19] It is a rare example of a molecular binary metal oxide. Other examples areruthenium(VIII) oxide andosmium(VIII) oxide. It adopts acentrosymmetric corner-shared bi-tetrahedral structure in which the terminal and bridging Tc−O bonds are 167pm and 184 pm respectively and the Tc−O−Tc angle is 180°.[20]
Rhenium's main oxides arerhenium(IV) oxide andrhenium(VII) oxide. Rhenium(IV) oxide is a gray to black crystalline solid that can be formed bycomproportionation.[21] At high temperatures it undergoesdisproportionation. It is a laboratory reagent that can be used as acatalyst. It adopts therutile structure. It formsperrhenates with alkalinehydrogen peroxide andoxidizing acids.[22] In molten sodium hydroxide it forms sodium rhenate:[23]
Rhenium(VII) oxide can be formed when rhenium or its oxides or sulfides are oxidized a 500-700 °C in air.[24] It dissolves in water to giveperrhenic acid. Heating Re2O7 gives rhenium(IV) oxide, signalled by the appearance of the dark blue coloration.[25] In its solid form, Re2O7 consists of alternating octahedral and tetrahedral Re centres. It is the raw material for all rhenium compounds, being the volatile fraction obtained upon roasting the host ore.[26]
Rhenium, in addition to the +4 and +7 oxidation states, also forms atrioxide. It can be formed by reducing rhenium(VII) oxide withcarbon monoxide at 200 C or elemental rhenium at 4000 C.[27] It can also be reduced withdioxane.[28] It is a red solid with a metallic lustre that resemblescopper in appearance, and is the only stabletrioxide of the group 7 elements.
Manganese can form compounds in the +2, +3 and +4 oxidation states. The manganese(II) compounds are often light pink solids. Like some other metal difluorides, MnF2 crystallizes in therutile structure, which features octahedral Mn centers.[29] and it is used in the manufacture of special kinds ofglass andlasers.[30] Scacchite is the natural, anhydrous form of manganese(II) chloride.[31] The only other currently known mineral systematized as manganese chloride is kempite - a representative of the atacamite group, a group of hydroxide-chlorides.[32] It can be used in place ofpalladium in theStille reaction, which couples two carbon atoms using anorganotin compound.[33] It can be used as a pink pigment or as a source of the manganese ion or iodide ion. It is often used in the lighting industry.[33]
The following binary (containing only two elements) technetium halides are known:TcF6, TcF5,TcCl4, TcBr4, TcBr3, α-TcCl3, β-TcCl3, TcI3, α-TcCl2, and β-TcCl2. Theoxidation states range from Tc(VI) to Tc(II). Technetium halides exhibit different structure types, such as molecular octahedral complexes, extended chains, layered sheets, and metal clusters arranged in a three-dimensional network.[34][35] These compounds are produced by combining the metal and halogen or by less direct reactions.
TcCl4 is obtained by chlorination of Tc metal or Tc2O7 Upon heating, TcCl4 gives the corresponding Tc(III) and Tc(II) chlorides.[35]
The structure of TcCl4 is composed of infinite zigzag chains of edge-sharing TcCl6 octahedra. It is isomorphous to transition metal tetrachlorides ofzirconium,hafnium, andplatinum.[35]
.jpg)
Two polymorphs oftechnetium trichloride exist, α- and β-TcCl3. The α polymorph is also denoted as Tc3Cl9. It adopts a confacialbioctahedral structure.[36] It is prepared by treating the chloro-acetate Tc2(O2CCH3)4Cl2 with HCl. LikeRe3Cl9, the structure of the α-polymorph consists of triangles with short M-M distances. β-TcCl3 features octahedral Tc centers, which are organized in pairs, as seen also formolybdenum trichloride. TcBr3 does not adopt the structure of either trichloride phase. Instead it has the structure ofmolybdenum tribromide, consisting of chains of confacial octahedra with alternating short and long Tc—Tc contacts. TcI3 has the same structure as the high temperature phase ofTiI3, featuring chains of confacial octahedra with equal Tc—Tc contacts.[35]
Several anionic technetium halides are known. The binary tetrahalides can be converted to the hexahalides [TcX6]2− (X = F, Cl, Br, I), which adoptoctahedral molecular geometry.[37] More reduced halides form anionic clusters with Tc–Tc bonds. The situation is similar for the related elements of Mo, W, Re. These clusters have the nuclearity Tc4, Tc6, Tc8, and Tc13. The more stable Tc6 and Tc8 clusters have prism shapes where vertical pairs of Tc atoms are connected by triple bonds and the planar atoms by single bonds. Every technetium atom makes six bonds, and the remaining valence electrons can be saturated by one axial and twobridging ligand halogen atoms such aschlorine orbromine.[38]
The most common rhenium chlorides areReCl6,ReCl5,ReCl4, andReCl3.[6] The structures of these compounds often feature extensive Re-Re bonding, which is characteristic of this metal in oxidation states lower than VII. Salts of [Re2Cl8]2− feature aquadruple metal-metal bond. Although the highest rhenium chloride features Re(VI), fluorine gives the d0 Re(VII) derivativerhenium heptafluoride. Bromides and iodides of rhenium are also well known.
Like tungsten and molybdenum, with which it shares chemical similarities, rhenium forms a variety ofoxyhalides. The oxychlorides are most common, and include ReOCl4, ReOCl3.
Organomanganese compounds were first reported in 1937 by Gilman and Bailee who described the reaction ofphenyllithium andmanganese(II) iodide to form phenylmanganese iodide (PhMnI) and diphenylmanganese (Ph2Mn).[39]
Following this precedent, other organomanganese halides can be obtained by alkylation ofmanganese(II) chloride,manganese(II) bromide, andmanganese(II) iodide. Manganese iodide is attractive because the anhydrous compound can be prepared in situ from manganese andiodine inether. Typical alkylating agents areorganolithium ororganomagnesium compounds.
The chemistry of organometallic compounds of Mn(II) are unusual among thetransition metals due to the high ionic character of the Mn(II)-C bond.[40] The reactivity of organomanganese compounds can be compared to that oforganomagnesium andorganozinc compounds. Theelectronegativity of Mn (1.55) is comparable to that of Mg (1.31) and Zn (1.65), making the carbon atom (EN = 2.55)nucleophilic. Thereduction potential of Mn is also intermediate between Mg and Zn.
_6Cation.png)
Technetium forms a variety ofcoordination complexes with organic ligands. Many have been well-investigated because of their relevance tonuclear medicine.[41]
Technetium forms a variety of compounds with Tc–C bonds, i.e. organotechnetium complexes. Prominent members of this class are complexes with CO, arene, and cyclopentadienyl ligands.[42] The binary carbonyl Tc2(CO)10 is a white volatile solid.[43] In this molecule, two technetium atoms are bound to each other; each atom is surrounded byoctahedra of five carbonyl ligands. The bond length between technetium atoms, 303 pm,[44][45] is significantly larger than the distance between two atoms in metallic technetium (272 pm). Similarcarbonyls are formed by technetium'scongeners, manganese and rhenium.[46] Interest in organotechnetium compounds has also been motivated by applications innuclear medicine.[42] Unusual for other metal carbonyls, Tc forms aquo-carbonyl complexes, prominent being [Tc(CO)3(H2O)3]+.[42]
Dirhenium decacarbonyl is the most common entry to organorhenium chemistry. Its reduction with sodiumamalgam gives Na[Re(CO)5] with rhenium in the formal oxidation state −1.[47] Dirhenium decacarbonyl can be oxidised withbromine tobromopentacarbonylrhenium(I):[48]
Reduction of this pentacarbonyl withzinc andacetic acid givespentacarbonylhydridorhenium:[49]
Methylrhenium trioxide ("MTO"), CH3ReO3 is a volatile, colourless solid has been used as acatalyst in some laboratory experiments. It can be prepared by many routes, a typical method is the reaction of Re2O7 andtetramethyltin:
Analogous alkyl and aryl derivatives are known. MTO catalyses for the oxidations withhydrogen peroxide. Terminalalkynes yield the corresponding acid or ester, internal alkynes yield diketones, andalkenes give epoxides. MTO also catalyses the conversion ofaldehydes anddiazoalkanes into an alkene.[50]
The polyoxotechnetate (polyoxometalate of technetium) contains both Tc(V) and Tc(VII) in ratio 4: 16 and is obtained as thehydronium salt [H7O3]4[Tc20O68]·4H2O by concentrating an HTcO4 solution.[51] The first empirically isolated polyoxorhenate was the white [Re4O15]2− and contained Re(VII) in both octahedral and tetrahedral coordination.[52]
Manganese dioxide, which is abundant in nature, has long been used as a pigment. The cave paintings inGargas that are 30,000 to 24,000 years old are made from the mineral form of MnO2 pigments.[53] Manganese compounds were used by Egyptian and Roman glassmakers, either to add to, or remove, color from glass.[54] Use as "glassmakers soap" continued through theMiddle Ages until modern times and is evident in 14th-century glass fromVenice.[55]
Rhenium (Latin:Rhenus meaning: "Rhine")[56] was the last-discovered of the elements that have a stable isotope (other new elements discovered in nature since then, such asfrancium, are radioactive).[57] The existence of a yet-undiscovered element at this position in theperiodic table had been first predicted byDmitri Mendeleev. Other calculated information was obtained byHenry Moseley in 1914.[58] In 1908,Japanese chemistMasataka Ogawa announced that he had discovered the 43rd element and named itnipponium (Np) afterJapan (Nippon in Japanese). In fact, what he had was rhenium (element 75), nottechnetium.[59][60] The symbol Np was later used for the elementneptunium, and the name "nihonium", alsonamed after Japan, along with symbol Nh, was later used forelement 113. Element 113 was also discovered by a team of Japanese scientists and was named in respectful homage to Ogawa's work.[61]
Rhenium was rediscovered byWalter Noddack,Ida Noddack, andOtto Berg inGermany. In 1925 they reported that they had detected the element in platinum ore and in the mineralcolumbite. They also found rhenium ingadolinite andmolybdenite.[62] In 1928 they were able to extract 1 g of the element by processing 660 kg of molybdenite.[63] It was estimated in 1968 that 75% of the rhenium metal in theUnited States was used for research and the development ofrefractory metal alloys. It took several years from that point before the superalloys became widely used.[64][65]
Thediscovery of element 43 was finally confirmed in a 1937 experiment at theUniversity of Palermo in Sicily byCarlo Perrier andEmilio Segrè.[66] In mid-1936, Segrè visited the United States, firstColumbia University in New York and then theLawrence Berkeley National Laboratory in California. He persuadedcyclotron inventorErnest Lawrence to let him take back some discarded cyclotron parts that had becomeradioactive. Lawrence mailed him amolybdenum foil that had been part of the deflector in the cyclotron.[67]
Two groups claimeddiscovery of the element bohrium. Evidence of bohrium was first reported in 1976 by a Soviet research team led byYuri Oganessian, in which targets ofbismuth-209 andlead-208 were bombarded with accelerated nuclei ofchromium-54 andmanganese-55 respectively.[68] Two activities, one with a half-life of one to two milliseconds, and the other with an approximately five-second half-life, were seen. Since the ratio of the intensities of these two activities was constant throughout the experiment, it was proposed that the first was from theisotope bohrium-261 and that the second was from its daughterdubnium-257. Later, the dubnium isotope was corrected to dubnium-258, which indeed has a five-second half-life (dubnium-257 has a one-second half-life); however, the half-life observed for its parent is much shorter than the half-lives later observed in the definitive discovery of bohrium atDarmstadt in 1981. TheIUPAC/IUPAP Transfermium Working Group (TWG) concluded that while dubnium-258 was probably seen in this experiment, the evidence for the production of its parent bohrium-262 was not convincing enough.[69]
In 1981, a German research team led byPeter Armbruster andGottfried Münzenberg at theGSI Helmholtz Centre for Heavy Ion Research (GSI Helmholtzzentrum für Schwerionenforschung) in Darmstadt bombarded a target of bismuth-209 with accelerated nuclei of chromium-54 to produce five atoms of the isotope bohrium-262:[70]
This discovery was further substantiated by their detailed measurements of the alpha decay chain of the produced bohrium atoms to previously known isotopes offermium andcalifornium. TheIUPAC/IUPAP Transfermium Working Group (TWG) recognised the GSI collaboration as official discoverers in their 1992 report.[69]
Manganese comprises about 1000 ppm (0.1%) of theEarth's crust and is the12th most abundant element.[71] Soil contains 7–9000 ppm of manganese with an average of 440 ppm.[71] The atmosphere contains 0.01 μg/m3.[71] Manganese occurs principally aspyrolusite (MnO2),braunite (Mn2+Mn3+6)(SiO12),[72]psilomelane(Ba,H2O)2Mn5O10, and to a lesser extent asrhodochrosite (MnCO3).
The most important manganese ore is pyrolusite (MnO2). Other economically important manganese ores usually show a close spatial relation to the iron ores, such assphalerite.[74][75] Land-based resources are large but irregularly distributed. About 80% of the known world manganese resources are in South Africa; other important manganese deposits are in Ukraine, Australia, India, China,Gabon and Brazil.[73] According to 1978 estimate, theocean floor has 500 billion tons ofmanganese nodules.[76] Attempts to find economically viable methods of harvesting manganese nodules were abandoned in the 1970s.[77]
In South Africa, most identified deposits are located nearHotazel in theNorthern Cape Province, with a 2011 estimate of 15 billion tons. In 2011 South Africa produced 3.4 million tons, topping all other nations.[78]
Manganese is mainly mined in South Africa, Australia, China, Gabon, Brazil, India, Kazakhstan, Ghana, Ukraine and Malaysia.[79]
For the production offerromanganese, the manganese ore is mixed with iron ore and carbon, and then reduced either in a blast furnace or in an electric arc furnace.[80] The resultingferromanganese has a manganese content of 30 to 80%.[74] Pure manganese used for the production of iron-free alloys is produced byleaching manganese ore withsulfuric acid and a subsequentelectrowinning process.[81]
A more progressive extraction process involves directly reducing (a low grade) manganese ore in a heap leach. This is done bypercolating natural gas through the bottom of the heap; the natural gas provides the heat (needs to be at least 850 °C) and the reducing agent (carbon monoxide). This reduces all of the manganese ore to manganese oxide (MnO), which is a leachable form. The ore then travels through agrinding circuit to reduce the particle size of the ore to between 150 and 250 μm, increasing the surface area to aid leaching. The ore is then added to a leach tank ofsulfuric acid andferrous iron (Fe2+) in a 1.6:1 ratio. The iron reacts with themanganese dioxide (MnO2) to formiron(III) oxide-hydroxide (FeO(OH)) and elemental manganese (Mn):
This process yields approximately 92% recovery of the manganese. For further purification, the manganese can then be sent to an electrowinning facility.[82]
In 1972 theCIA'sProject Azorian, through billionaireHoward Hughes, commissioned the shipHughes Glomar Explorer with the cover story of harvesting manganese nodules from the sea floor.[83] That triggered a rush of activity to collect manganese nodules, which was not actually practical. The real mission ofHughes Glomar Explorer was to raise a sunkenSoviet submarine, theK-129, with the goal of retrieving Soviet code books.[84]
An abundant resource of manganese in the form ofMn nodules found on the ocean floor.[85][86] These nodules, which are composed of 29% manganese,[87] are located along theocean floor and the potential impact of mining these nodules is being researched. Physical, chemical, and biological environmental impacts can occur due to this nodule mining disturbing the seafloor and causing sediment plumes to form. This suspension includes metals and inorganic nutrients, which can lead to contamination of the near-bottom waters from dissolved toxic compounds. Mn nodules are also the grazing grounds, living space, and protection for endo- and epifaunal systems. When theses nodules are removed, these systems are directly affected. Overall, this can cause species to leave the area or completely die off.[88] Prior to the commencement of the mining itself, research is being conducted byUnited Nations affiliated bodies and state-sponsored companies in an attempt to fully understandenvironmental impacts in the hopes of mitigating these impacts.[89]
Technetium was created by bombardingmolybdenum atoms withdeuterons that had been accelerated by a device called acyclotron. Technetium occurs naturally in the Earth'scrust in minute concentrations of about 0.003 parts per trillion. Technetium is so rare because thehalf-lives of97Tc and98Tc are only 4.2 million years. More than a thousand of such periods have passed since the formation of theEarth, so the probability of survival of even one atom ofprimordial technetium is effectively zero. However, small amounts exist as spontaneousfission products inuranium ores. A kilogram of uranium contains an estimated 1 nanogram (10−9 g) equivalent to ten trillion atoms of technetium.[90][91][92] Somered giant stars with the spectral types S-, M-, and N contain a spectral absorption line indicating the presence of technetium.[93][94] These red giants are known informally astechnetium stars.
Rhenium is one of the rarest elements inEarth's crust with an average concentration of 1 ppb;[6][95] other sources quote the number of 0.5 ppb making it the 77th most abundant element in Earth's crust.[96] Rhenium is probably not found free in nature (its possible natural occurrence is uncertain), but occurs in amounts up to 0.2%[6] in the mineralmolybdenite (which is primarilymolybdenum disulfide), the major commercial source, although single molybdenite samples with up to 1.88% have been found.[97]Chile has the world's largest rhenium reserves, part of the copper ore deposits, and was the leading producer as of 2005.[98] It was only recently that the first rheniummineral was found and described (in 1994), a rheniumsulfide mineral (ReS2) condensing from afumarole onKudriavy volcano,Iturup island, in theKuril Islands.[99] Kudriavy discharges up to 20–60 kg rhenium per year mostly in the form of rhenium disulfide.[100][101] Namedrheniite, this rare mineral commands high prices among collectors.[102]
Most of the rhenium extracted comes fromporphyrymolybdenum deposits.[103] These ores typically contain 0.001% to 0.2% rhenium.[6] Roasting the ore volatilizes rhenium oxides.[97]Rhenium(VII) oxide andperrhenic acid readily dissolve in water; they are leached from flue dusts and gasses and extracted by precipitating withpotassium orammonium chloride as theperrhenate salts, and purified byrecrystallization.[6] Total world production is between 40 and 50 tons/year; the main producers are in Chile, the United States, Peru, and Poland.[104] Recycling of used Pt-Re catalyst and special alloys allow the recovery of another 10 tons per year. Prices for the metal rose rapidly in early 2008, from $1000–$2000 perkg in 2003–2006 to over $10,000 in February 2008.[105][106] The metal form is prepared by reducingammonium perrhenate withhydrogen at high temperatures:[25]
Bohrium is a synthetic element that does not occur in nature. Very few atoms have been synthesized, and also due to its radioactivity, only limited research has been conducted. Bohrium is only produced in nuclear reactors and has never been isolated in pure form.
Thefacial isomer of both rhenium and manganese 2,2'-bipyridyl tricarbonyl halide complexes have been extensively researched as catalysts forelectrochemical carbon dioxide reduction due to their high selectivity and stability. They are commonly abbreviated as M(R-bpy)(CO)3X where M = Mn, Re; R-bpy = 4,4'-disubstituted2,2'-bipyridine; and X = Cl, Br.
The rarity of rhenium has shifted research toward the manganese version of these catalysts as a more sustainable alternative.[108] The first reports of catalytic activity of Mn(R-bpy)(CO)3Br towards CO2 reduction came from Chardon-Noblat and coworkers in 2011.[109] Compared to Re analogs, Mn(R-bpy)(CO)3Br shows catalytic activity at lower overpotentials.[110]
The catalytic mechanism for Mn(R-bpy)(CO)3X is complex and depends on the steric profile of the bipyridine ligand. When R is not bulky, the catalyst dimerizes to form [Mn(R-bpy)(CO)3]2 before forming the active species. When R is bulky, however, the complex forms the active species without dimerizing, reducing the overpotential of CO2 reduction by 200-300 mV. Unlike Re(R-bpy)(CO)3X, Mn(R-bpy)(CO)3X only reduces CO2 in the presence of an acid.[110]
Technetium-99m ("m" indicates that this is ametastable nuclear isomer) is used in radioactive isotopemedical tests. For example, Technetium-99m is aradioactive tracer that medical imaging equipment tracks in the human body.[90][111][112] It is well suited to the role because it emits readily detectable 140 keVgamma rays, and its half-life is 6.01 hours (meaning that about 94% of it decays to technetium-99 in 24 hours).[113] The chemistry of technetium allows it to be bound to a variety of biochemical compounds, each of which determines how it is metabolized and deposited in the body, and this single isotope can be used for a multitude of diagnostic tests. More than 50 commonradiopharmaceuticals are based on technetium-99m for imaging and functional studies of thebrain, heart muscle,thyroid,lungs,liver,gall bladder,kidneys,skeleton,blood, andtumors.[114] Technetium-99m is also used in radioimaging.[115]
The longer-lived isotope, technetium-95m with a half-life of 61 days, is used as aradioactive tracer to study the movement of technetium in the environment and in plant and animal systems.[116]
Technetium-99 decays almost entirely by beta decay, emitting beta particles with consistent low energies and no accompanying gamma rays. Moreover, its long half-life means that this emission decreases very slowly with time. It can also be extracted to a high chemical and isotopic purity from radioactive waste. For these reasons, it is aNational Institute of Standards and Technology (NIST) standard beta emitter, and is used for equipment calibration.[117] Technetium-99 has also been proposed for optoelectronic devices andnanoscalenuclear batteries.[118]
Likerhenium andpalladium, technetium can serve as acatalyst. In processes such as thedehydrogenation ofisopropyl alcohol, it is a far more effective catalyst than either rhenium or palladium. However, its radioactivity is a major problem in safe catalytic applications.[119]
When steel is immersed in water, adding a small concentration (55 ppm) of potassium pertechnetate(VII) to the water protects thesteel from corrosion, even if the temperature is raised to 250 °C (523 K).[120] For this reason, pertechnetate has been used as an anodiccorrosion inhibitor for steel, although technetium's radioactivity poses problems that limit this application to self-contained systems.[121] While (for example)CrO2−
4 can also inhibit corrosion, it requires a concentration ten times as high. In one experiment, a specimen of carbon steel was kept in an aqueous solution of pertechnetate for 20 years and was still uncorroded.[120] The mechanism by which pertechnetate prevents corrosion is not well understood, but seems to involve the reversible formation of a thin surface layer (passivation). One theory holds that the pertechnetate reacts with the steel surface to form a layer of technetiumdioxide which prevents further corrosion; the same effect explains how iron powder can be used to remove pertechnetate from water. The effect disappears rapidly if the concentration of pertechnetate falls below the minimum concentration or if too high a concentration of other ions is added.[122]
As noted, the radioactive nature of technetium (3 MBq/L at the concentrations required) makes this corrosion protection impractical in almost all situations. Nevertheless, corrosion protection by pertechnetate ions was proposed (but never adopted) for use inboiling water reactors.[122]
The catalytic activity of Re(bpy)(CO)3Cl for carbon dioxide reduction was first studied by Lehn et al.[123] and Meyer et al.[124] in 1984 and 1985, respectively. Re(R-bpy)(CO)3X complexes exclusively produce CO from CO2 reduction withFaradaic efficiencies of close to 100% even in solutions with high concentrations of water orBrønsted acids.[108]
The catalytic mechanism of Re(R-bpy)(CO)3X involves reduction of the complex twice and loss of the X ligand to generate a five-coordinate active species which binds CO2. These complexes will reduce CO2 both with and without an additional acid present; however, the presence of an acid increases catalytic activity.[108] The high selectivity of these complexes to CO2 reduction over the competinghydrogen evolution reaction has been shown bydensity functional theory studies to be related to the faster kinetics of CO2 binding compared to H+ binding.[110]
Bohrium is a synthetic element and is too radioactive to be used in anything.
Manganese compounds are less toxic than those of other widespread metals, such asnickel andcopper.[125] However, exposure to manganese dusts and fumes should not exceed the ceiling value of 5 mg/m3 even for short periods because of its toxicity level.[126] Manganese poisoning has been linked to impaired motor skills and cognitive disorders.[127]
Technetium has low chemical toxicity. For example, no significant change in blood formula, body and organ weights, and food consumption could be detected for rats which ingested up to 15 μg of technetium-99 per gram of food for several weeks.[128] In the body, technetium quickly gets converted to the stableTcO−
4 ion, which is highly water-soluble and quickly excreted. The radiological toxicity of technetium (per unit of mass) is a function of compound, type of radiation for the isotope in question, and the isotope's half-life.[129] However, it is radioactive, so all isotopes must be handled carefully. The primary hazard when working with technetium is inhalation of dust; suchradioactive contamination in the lungs can pose a significant cancer risk. For most work, careful handling in afume hood is sufficient, and aglove box is not needed.[130]
Very little is known about the toxicity of rhenium and its compounds because they are used in very small amounts. Soluble salts, such as the rhenium halides or perrhenates, could be hazardous due to elements other than rhenium or due to rhenium itself.[131] Only a few compounds of rhenium have been tested for their acute toxicity; two examples are potassium perrhenate and rhenium trichloride, which were injected as a solution into rats. The perrhenate had anLD50 value of 2800 mg/kg after seven days (this is very low toxicity, similar to that of table salt) and the rhenium trichloride showed LD50 of 280 mg/kg.[132]
Of the group 7 elements, only manganese has a role in the human body. It is an essential trace nutrient, with the body containing approximately 10milligrams at any given time. It is present as acoenzyme in biological processes that include macronutrient metabolism, bone formation, andfree radical defense systems. It is a critical component in dozens of proteins and enzymes.[133] The manganese in the human body is mainly concentrated in the bones, and the soft tissue remainder is concentrated in the liver and kidneys.[134] In the human brain, the manganese is bound to manganesemetalloproteins, most notablyglutamine synthetase inastrocytes.[135] Technetium, rhenium, and bohrium have no known biological roles. Technetium is, however, used in radioimaging.
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