Metal carbonyls arecoordination complexes oftransition metals withcarbon monoxideligands. Metal carbonyls are useful inorganic synthesis and as catalysts or catalyst precursors inhomogeneous catalysis, such ashydroformylation andReppe chemistry. In theMond process,nickel tetracarbonyl is used to produce purenickel. Inorganometallic chemistry, metal carbonyls serve as precursors for the preparation of other organometallic complexes.
Metal carbonyls are toxic by skin contact, inhalation or ingestion, in part because of their ability to carbonylatehemoglobin to givecarboxyhemoglobin, which prevents the binding ofoxygen.[1]
The nomenclature of the metal carbonyls depends on the charge of the complex, the number and type of central atoms, and the number and type of ligands and their binding modes. They occur as neutral complexes, as positively-charged metal carbonylcations or as negatively charged metalcarbonylates. The carbon monoxide ligand may be bound terminally to a single metal atom orbridging to two or more metal atoms. These complexes may behomoleptic, containing only CO ligands, such asnickel tetracarbonyl (Ni(CO)4), but more commonly metal carbonyls are heteroleptic and contain a mixture of ligands.[citation needed]
Mononuclear metal carbonyls contain only one metal atom as the central atom. Exceptvanadium hexacarbonyl, only metals with even atomic number, such aschromium,iron,nickel, and their homologs, build neutral mononuclear complexes. Polynuclear metal carbonyls are formed from metals with odd atomic numbers and contain ametal–metal bond.[2] Complexes with different metals but only one type of ligand are called isoleptic.[2]
Carbon monoxide has distinct binding modes in metal carbonyls. They differ in terms of theirhapticity, denotedη, and their bridging mode. Inη2-CO complexes, both the carbon and oxygen are bonded to the metal. More commonly only carbon is bonded, in which case the hapticity is not mentioned.[3]
The carbonyl ligand engages in a wide range of bonding modes in metal carbonyl dimers and clusters. In the most common bridging mode, denotedμ2 or simplyμ, the CO ligand bridges a pair of metals. This bonding mode is observed in the commonly available metal carbonyls: Co2(CO)8, Fe2(CO)9, Fe3(CO)12, and Co4(CO)12.[1][4] In certain higher nuclearity clusters, CO bridges between three or even four metals. These ligands are denotedμ3-CO andμ4-CO. Less common are bonding modes in which both C and O bond to the metal, such asμ3η2.[citation needed]
Carbon monoxide bonds to transition metals using "synergisticpi* back-bonding". The M–C bonding has three components, giving rise to a partial triple bond. Asigma (σ) bond arises from overlap of the nonbonding (or weakly anti-bonding)sp-hybridized electron pair on carbon with a blend ofd-,s-, andp-orbitals on the metal. A pair ofpi (π) bonds arises from overlap of filled d-orbitals on the metal with a pair of π*-antibonding orbitals projecting from the carbon atom of the CO. The latter kind of binding requires that the metal have d-electrons, and that the metal be in a relatively low oxidation state (0 or +1) which makes the back-donation of electron density favorable. As electrons from the metal fill the π-antibonding orbital of CO, they weaken thecarbon–oxygen bond compared with free carbon monoxide, while the metal–carbon bond is strengthened. Because of the multiple bond character of the M–CO linkage, the distance between the metal and carbon atom is relatively short, often less than 1.8 Å, about 0.2 Å shorter than a metal–alkyl bond. The M-CO and MC-O distance are sensitive to other ligands on the metal. Illustrative of these effects are the following data for Mo-C and C-O distances in Mo(CO)6 and Mo(CO)3(4-methylpyridine)3: 2.06 vs 1.90 and 1.11 vs 1.18 Å.[5]
Infrared spectroscopy is a sensitive probe for the presence of bridging carbonyl ligands. For compounds with doubly bridging CO ligands, denotedμ2-CO or often justμ-CO, the bond stretching frequencyνCO is usually shifted by 100–200 cm−1 to lower energy compared to the signatures of terminal CO, which are in the region 1800 cm−1. Bands for face-capping (μ3) CO ligands appear at even lower energies. In addition to symmetrical bridging modes, CO can be found to bridge asymmetrically or through donation from a metal d orbital to the π* orbital of CO.[6][7][8] The increased π-bonding due to back-donation from multiple metal centers results in further weakening of the C–O bond.[citation needed]
Most mononuclear carbonyl complexes are colorless or pale yellow, volatile liquids or solids that are flammable and toxic.[9]Vanadium hexacarbonyl, a uniquely stable 17-electron metal carbonyl, is a blue-black solid.[1] Dimetallic and polymetallic carbonyls tend to be more deeply colored.Triiron dodecacarbonyl (Fe3(CO)12) forms deep green crystals. The crystalline metal carbonyls often aresublimable in vacuum, although this process is often accompanied by degradation. Metal carbonyls are soluble in nonpolar and polar organic solvents such asbenzene,diethyl ether,acetone,glacial acetic acid, andcarbon tetrachloride. Some salts of cationic and anionic metal carbonyls are soluble in water or lower alcohols.[10]
Apart fromX-ray crystallography, important analytical techniques for the characterization of metal carbonyls areinfrared spectroscopy and13C NMR spectroscopy. These two techniques provide structural information on two very different time scales. Infrared-activevibrational modes, such as CO-stretching vibrations, are often fast compared to intramolecular processes, whereas NMR transitions occur at lower frequencies and thus sample structures on a time scale that, it turns out, is comparable to the rate of intramolecular ligand exchange processes. NMR data provide information on "time-averaged structures", whereas IR is an instant "snapshot".[11] Illustrative of the differing time scales, investigation ofdicobalt octacarbonyl (Co2(CO)8) by means of infrared spectroscopy provides 13νCO bands, far more than expected for a single compound. This complexity reflects the presence of isomers with and without bridging CO ligands. The13C NMR spectrum of the same substance exhibits only a single signal at achemical shift of 204 ppm. This simplicity indicates that the isomers quickly (on the NMR timescale) interconvert.[citation needed]
Iron pentacarbonyl exhibits only a single13C NMR signal owing to rapid exchange of the axial and equatorial CO ligands byBerry pseudorotation.[citation needed]
An important technique for characterizing metal carbonyls isinfrared spectroscopy.[13] The C–O vibration, typically denotedνCO, occurs at 2143 cm−1 for carbon monoxide gas. The energies of theνCO band for the metal carbonyls correlates with the strength of the carbon–oxygen bond, and inversely correlated with the strength of theπ-backbonding between the metal and the carbon. The π-basicity of the metal center depends on a lot of factors; in the isoelectronic series (titanium toiron) at the bottom of this section, the hexacarbonyls show decreasing π-backbonding as one increases (makes more positive) the charge on the metal. π-Basic ligands increase π-electron density at the metal, and improved backbonding reduces νCO. TheTolman electronic parameter uses the Ni(CO)3 fragment to order ligands by their π-donating abilities.[14][15]
The number of vibrational modes of a metal carbonyl complex can be determined bygroup theory. Only vibrational modes that transform as theelectric dipole operator will have nonzerodirect products and are observed. The number of observable IR transitions (but not their energies) can thus be predicted.[16][17][18] For example, the CO ligands of octahedral complexes, such asCr(CO)6, transform asa1g,eg, andt1u, but only thet1u mode (antisymmetric stretch of the apical carbonyl ligands) is IR-allowed. Thus, only a singleνCO band is observed in the IR spectra of the octahedral metal hexacarbonyls. Spectra for complexes of lower symmetry are more complex. For example, the IR spectrum ofFe2(CO)9 displays CO bands at 2082, 2019 and 1829 cm−1. The number of IR-observable vibrational modes for some metal carbonyls are shown in the table. Exhaustive tabulations are available.[13] These rules apply to metal carbonyls in solution or the gas phase. Low-polarity solvents are ideal for high resolution. For measurements on solid samples of metal carbonyls, the number of bands can increase owing in part to site symmetry.[19]
| Compound | νCO (cm−1) | 13C NMR shift (ppm) | average M-CO distance (pm) | average C-O distance (pm) | |
|---|---|---|---|---|---|
| CO | 2143 | 181 | |||
| Ti(CO)2− 6 | 1748 | 245 | 204[20] | 116 | |
| V(CO)− 6 | 1859 | (paramagnetic) | 200, 193 (PPN+ salt)[21] | 113[21] | |
| Cr(CO)6 | 2000 | 212 | 191[22] | 114 | |
| Mn(CO)+ 6 | 2100 | ||||
| Fe(CO)2+ 6 | 2204 | 191 | 112 (BF4− salt)[23] | ||
| Fe(CO)5 | 2022, 2000 | 209 | 180[24] | 112 | |
| Ru(CO)5 | 2038, 2002[25] | ||||
| Ni(CO)4 | 181 | 113 |
| Carbonyl | νCO,μ1 (cm−1) | νCO,μ2 (cm−1) | νCO,μ3 (cm−1) |
|---|---|---|---|
| Rh2(CO)8 | 2060, 2084 | 1846, 1862 | |
| Rh4(CO)12 | 2044, 2070, 2074 | 1886 | |
| Rh6(CO)16 | 2045, 2075 | 1819 |
Metal carbonyls are often characterized by13C NMR spectroscopy. To improve the sensitivity of this technique, complexes are oftenenriched with13CO. Typicalchemical shift range for terminally bound ligands is 150 to 220 ppm. Bridging ligands resonate between 230 and 280 ppm.[1] The13C signals shift toward higher fields with an increasing atomic number of the central metal.
NMR spectroscopy can be used for experimental determination of thefluxionality.[26]
Theactivation energy of ligand exchange processes can be determined by the temperature dependence of the line broadening.[27]
Mass spectrometry provides information about the structure and composition of the complexes. Spectra for metal polycarbonyls are often easily interpretable, because the dominant fragmentation process is the loss of carbonyl ligands (m/z = 28).
Electron ionization is the most common technique for characterizing the neutral metal carbonyls. Neutral metal carbonyls can be converted to charged species byderivatization, which enables the use ofelectrospray ionization (ESI), instrumentation for which is often widely available. For example, treatment of a metal carbonyl withalkoxide generates an anionicmetallaformate that is amenable to analysis by ESI-MS:
Some metal carbonyls react withazide to giveisocyanato complexes with release ofnitrogen.[28] By adjusting the cone voltage or temperature, the degree of fragmentation can be controlled. Themolar mass of the parent complex can be determined, as well as information about structural rearrangements involving loss of carbonyl ligands under ESI-MS conditions.[29]
Mass spectrometry combined withinfrared photodissociation spectroscopy can provide vibrational informations for ionic carbonyl complexes in gas phase.[30]
In the investigation of the infrared spectrum of theGalactic Center of theMilky Way, monoxide vibrations of iron carbonyls ininterstellar dust clouds were detected.[32] Iron carbonyl clusters were also observed inJiange H5 chondrites identified by infrared spectroscopy. Four infrared stretching frequencies were found for the terminal and bridging carbon monoxide ligands.[33]
In the oxygen-rich atmosphere of the Earth, metal carbonyls are subject tooxidation to the metal oxides. It is discussed whether in the reducing hydrothermal environments of the prebiotic prehistory such complexes were formed and could have been available as catalysts for the synthesis of criticalbiochemical compounds such aspyruvic acid.[34] Traces of the carbonyls of iron, nickel, and tungsten were found in the gaseous emanations from thesewage sludge of municipaltreatment plants.[35]
Thehydrogenase enzymes contain CO bound to iron. It is thought that the CO stabilizes low oxidation states, which facilitates the binding ofhydrogen. The enzymescarbon monoxide dehydrogenase andacetyl-CoA synthase also are involved in bioprocessing of CO.[36] Carbon monoxide containing complexes are invoked for thetoxicity of CO and signaling.[37]
The synthesis of metal carbonyls is a widely studied subject of organometallic research. Since the work of Mond and then Hieber, many procedures have been developed for the preparation of mononuclear metal carbonyls as well as homo- and heterometallic carbonyl clusters.[38]
Nickel tetracarbonyl andiron pentacarbonyl can be prepared according to the following equations by reaction of finely divided metal withcarbon monoxide:[39]
Nickel tetracarbonyl is formed withcarbon monoxide already at 80 °C and atmospheric pressure, finely divided iron reacts at temperatures between 150 and 200 °C and a carbon monoxide pressure of 50–200 bar.[40] Other metal carbonyls are prepared by less direct methods.[41]
Some metal carbonyls are prepared by thereduction ofmetal halides in the presence of high pressure of carbon monoxide. A variety of reducing agents are employed, includingcopper,aluminum,hydrogen, as well as metal alkyls such astriethylaluminium. Illustrative is the formation of chromium hexacarbonyl from anhydrouschromium(III) chloride inbenzene with aluminum as a reducing agent, andaluminum chloride as the catalyst:[39]
The use of metal alkyls, such astriethylaluminium anddiethylzinc, as the reducing agent leads to the oxidative coupling of the alkyl radical to form thedimeralkane:
Tungsten,molybdenum,manganese, andrhodium salts may be reduced withlithium aluminium hydride.Vanadium hexacarbonyl is prepared withsodium as a reducing agent inchelating solvents such asdiglyme.[9]
In the aqueous phase, nickel or cobalt salts can be reduced, for example bysodium dithionite. In the presence of carbon monoxide, cobalt salts are quantitatively converted to the tetracarbonylcobalt(−1) anion:[9]
Some metal carbonyls are prepared using CO directly as thereducing agent. In this way, Hieber and Fuchs first prepareddirhenium decacarbonyl from the oxide:[42]
If metal oxides are usedcarbon dioxide is formed as a reaction product. In the reduction of metal chlorides with carbon monoxidephosgene is formed, as in the preparation ofosmium carbonyl chloride from the chloride salts.[38] Carbon monoxide is also suitable for the reduction ofsulfides, wherecarbonyl sulfide is the byproduct.
Photolysis orthermolysis of mononuclear carbonyls generates di- and polymetallic carbonyls such asdiiron nonacarbonyl (Fe2(CO)9).[43][44] On further heating, the products decompose eventually into the metal and carbon monoxide.[citation needed]
The thermal decomposition of triosmium dodecacarbonyl (Os3(CO)12) provides higher-nuclear osmium carbonyl clusters such as Os4(CO)13, Os6(CO)18 up to Os8(CO)23.[9]
Mixed ligand carbonyls ofruthenium,osmium,rhodium, andiridium are often generated by abstraction of CO from solvents such asdimethylformamide (DMF) and2-methoxyethanol. Typical is the synthesis ofIrCl(CO)(PPh3)2 from the reaction ofiridium(III) chloride andtriphenylphosphine in boiling DMF solution.[45]
Salt metathesis reaction of salts such as KCo(CO)4 with [Ru(CO)3Cl2]2 leads selectively to mixed-metal carbonyls such as RuCo2(CO)11.[46]
The synthesis of ionic carbonyl complexes is possible by oxidation or reduction of the neutral complexes. Anionic metal carbonylates can be obtained for example by reduction of dinuclear complexes with sodium. A familiar example is the sodium salt of iron tetracarbonylate (Na2Fe(CO)4,Collman's reagent), which is used in organic synthesis.[47]
The cationic hexacarbonyl salts ofmanganese,technetium andrhenium can be prepared from the carbonyl halides under carbon monoxide pressure by reaction with aLewis acid.[citation needed]
The use of strong acids succeeded in preparing gold carbonyl cations such as [Au(CO)2]+, which is used as a catalyst for the carbonylation ofalkenes.[48] The cationic platinum carbonyl complex [Pt(CO)4]2+ can be prepared by working in so-calledsuperacids such asantimony pentafluoride.[49] Although CO is considered generally as a ligand for low-valent metal ions, the tetravalent iron complex [Cp*2Fe]2+ (16-valence electron complex) quantitatively binds CO to give the diamagnetic Fe(IV)-carbonyl [Cp*2FeCO]2+ (18-valence electron complex).[50]
Metal carbonyls are important precursors for the synthesis of other organometallic complexes. Common reactions are thesubstitution of carbon monoxide by other ligands, the oxidation or reduction reactions of the metal center, and reactions at the carbon monoxide ligand.[1]
The substitution of CO ligands can be induced thermally orphotochemically by donor ligands. The range of ligands is large, and includesphosphines,cyanide (CN−), nitrogen donors, and even ethers, especially chelating ones.Alkenes, especiallydienes, are effective ligands that afford synthetically useful derivatives. Substitution of 18-electron complexes generally follows adissociative mechanism, involving 16-electron intermediates.[51]
Substitution proceeds via adissociative mechanism:
Thedissociation energy is 105 kJ/mol (25 kcal/mol) fornickel tetracarbonyl and 155 kJ/mol (37 kcal/mol) forchromium hexacarbonyl.[1]
Substitution in 17-electron complexes, which are rare, proceeds viaassociative mechanisms with a 19-electron intermediates.
The rate of substitution in 18-electron complexes is sometimes catalysed by catalytic amounts of oxidants, viaelectron transfer.[52]
Metal carbonyls react withreducing agents such as metallicsodium orsodium amalgam to give carbonylmetalate (or carbonylate) anions. Polynuclear or electron-imprecise clusters add the reductant to give mononuclear anions:[53]
Conversely, electron-precise mononuclear compounds lose CO and may form clusters:[53]
Mercury can insert into the metal–metal bonds of some polynuclear metal carbonyls:
The CO ligand is often susceptible to attack bynucleophiles. For example,trimethylamine oxide andpotassium bis(trimethylsilyl)amide convert CO ligands toCO2 andCN−, respectively. In the "Hieber base reaction",hydroxide ion attacks the CO ligand to give ametallacarboxylic acid, followed by the release of carbon dioxide and the formation of metal hydrides or carbonylmetalates. A well-studied example of thisnucleophilic addition is the conversion of iron pentacarbonyl tohydridoiron tetracarbonyl anion:
Hydride reagents also attack CO ligands, especially in cationic metal complexes, to give theformyl derivative:
Organolithium reagents add with metal carbonyls to acylmetal carbonyl anions.O-Alkylation of these anions, such as withMeerwein salts, affordsFischer carbenes.
Despite being in low formaloxidation states, metal carbonyls are relatively unreactive toward manyelectrophiles. For example, they resist attack by alkylating agents, mild acids, and mildoxidizing agents. Most metal carbonyls do undergohalogenation.Iron pentacarbonyl, for example, forms ferrous carbonyl halides:
Metal–metal bonds are cleaved by halogens. Depending on the electron-counting scheme used, this can be regarded as an oxidation of the metal atoms:
Most metal carbonyl complexes contain a mixture of ligands. Examples include the historically importantIrCl(CO)(P(C6H5)3)2 and theantiknock agent(CH3C5H4)Mn(CO)3. The parent compounds for many of these mixed ligand complexes are the binary carbonyls, those species of the formula [Mx(CO)n]z, many of which are commercially available. The formulae of many metal carbonyls can be inferred from the18-electron rule.
Large anionic clusters ofnickel,palladium, andplatinum are also well known. Many metal carbonyl anions can be protonated to givemetal carbonyl hydrides.
Nonclassical describes those carbonyl complexes where νCO is higher than that for free carbon monoxide. In nonclassical CO complexes, the C-O distance is shorter than free CO (113.7 pm). The structure of [Fe(CO)6]2+, with dC-O = 112.9 pm, illustrates this effect. These complexes are usually cationic, sometimes dicationic.[60]
Metal carbonyls are used in several industrial processes. Perhaps the earliest application was the extraction and purification of nickel vianickel tetracarbonyl by theMond process (see alsocarbonyl metallurgy).[citation needed]
By a similar processcarbonyl iron, a highly pure metal powder, is prepared by thermal decomposition of iron pentacarbonyl. Carbonyl iron is used inter alia for the preparation ofinductors,pigments, asdietary supplements,[61] in the production ofradar-absorbing materials in thestealth technology,[62] and inthermal spraying.[citation needed]
Metal carbonyls are used in a number of industrially importantcarbonylation reactions. In theoxo process, analkene, hydrogen gas, and carbon monoxide react together with a catalyst (such asdicobalt octacarbonyl) to givealdehydes. Illustrative is the production ofbutyraldehyde frompropylene:
Butyraldehyde is converted on an industrial scale to2-ethylhexanol, a precursor toPVCplasticizers, byaldol condensation, followed by hydrogenation of the resulting hydroxyaldehyde. The "oxo aldehydes" resulting from hydroformylation are used for large-scale synthesis of fatty alcohols, which are precursors todetergents. The hydroformylation is a reaction with highatom economy, especially if the reaction proceeds with highregioselectivity.[citation needed]
Another important reaction catalyzed by metal carbonyls is thehydrocarboxylation. The example below is for the synthesis of acrylic acid and acrylic acid esters:
Also the cyclization of acetylene to cyclooctatetraene uses metal carbonyl catalysts:[63]
In theMonsanto andCativa processes,acetic acid is produced from methanol, carbon monoxide, and water usinghydrogen iodide as well as rhodium and iridium carbonyl catalysts, respectively. Related carbonylation reactions affordacetic anhydride.[64]
Carbon monoxide-releasing molecules are metal carbonyl complexes that are being developed as potential drugs to release CO. At low concentrations, CO functions as a vasodilatory and an anti-inflammatory agent. CO-RMs have been conceived as a pharmacological strategic approach to carry and deliver controlled amounts of CO to tissues and organs.[65]
Many ligands are known to form homoleptic and mixed ligandcomplexes that are analogous to the metal carbonyls.[citation needed]
Metal nitrosyls, compounds featuringNO ligands, are numerous. In contrast to metal carbonyls, however, homoleptic metal nitrosyls are rare. NO is a stronger π-acceptor than CO. Well known nitrosyl carbonyls includeCoNO(CO)3 and Fe(NO)2(CO)2, which are analogues of Ni(CO)4.[66]
Complexes containingCS are known but uncommon.[67] The rarity of such complexes is partly attributable to the fact that the obvious source material,carbon monosulfide, is unstable. Thus, the synthesis of thiocarbonyl complexes requires indirect routes, such as the reaction ofdisodium tetracarbonylferrate withthiophosgene:
Complexes ofCSe andCTe have been characterized.[68]
Isocyanides also form extensive families of complexes that are related to the metal carbonyls. Typical isocyanide ligands aremethyl isocyanide andt-butyl isocyanide (Me3CNC). A special case isCF3NC, an unstable molecule that forms stable complexes whose behavior closely parallels that of the metal carbonyls.[69]
The toxicity of metal carbonyls is due to toxicity ofcarbon monoxide, the metal, and because of thevolatility andinstability of the complexes, any inherent toxicity of the metal is generally made much more severe due to ease of exposure. Exposure occurs by inhalation, or for liquid metal carbonyls by ingestion or due to the good fat solubility by skin resorption. Most clinical experience were gained from toxicological poisoning withnickel tetracarbonyl andiron pentacarbonyl due to their use in industry. Nickel tetracarbonyl is considered as one of the strongest inhalation poisons.[70]
Inhalation ofnickel tetracarbonyl causes acutenon-specific symptoms similar to acarbon monoxide poisoning, such asnausea,cough,headache,fever, anddizziness. After some time, severe pulmonary symptoms such as cough,tachycardia, andcyanosis, or problems in thegastrointestinal tract occur. In addition to pathological alterations of the lung, such as by metalation of the alveoli, damages are observed in the brain, liver, kidneys, adrenal glands, and spleen. A metal carbonyl poisoning often necessitates a lengthy recovery.[71]
Chronic exposure by inhalation of low concentrations of nickel tetracarbonyl can cause neurological symptoms such as insomnia, headaches, dizziness and memory loss.[71] Nickel tetracarbonyl is considered carcinogenic, but it can take 20 to 30 years from the start of exposure to the clinical manifestation of cancer.[72]
Initial experiments on the reaction of carbon monoxide with metals were carried out byJustus von Liebig in 1834. By passing carbon monoxide over moltenpotassium he prepared a substance having the empirical formula KCO, which he calledKohlenoxidkalium.[73] As demonstrated later, the compound was not a carbonyl, but the potassium salt ofbenzenehexol (K6C6O6) and the potassium salt ofacetylenediol (K2C2O2).[38]
The synthesis of the first true heteroleptic metal carbonyl complex was performed by Paul Schützenberger in 1868 by passingchlorine and carbon monoxide overplatinum black, where dicarbonyldichloroplatinum (Pt(CO)2Cl2) was formed.[74]
Ludwig Mond, one of the founders ofImperial Chemical Industries, investigated in the 1890s with Carl Langer and Friedrich Quincke various processes for the recovery of chlorine which was lost in theSolvay process bynickel metals, oxides, and salts.[38] As part of their experiments the group treated nickel with carbon monoxide. They found that the resulting gas colored the gas flame of aburner in a greenish-yellowish color; when heated in a glass tube it formed a nickel mirror. The gas could be condensed to a colorless, water-clear liquid with a boiling point of 43 °C. Thus, Mond and his coworker had discovered the first pure, homoleptic metal carbonyl,nickel tetracarbonyl (Ni(CO)4).[75] The unusual high volatility of the metal compound nickel tetracarbonyl ledKelvin to the statement that Mond had "given wings to the heavy metals".[76]
The following year, Mond andMarcellin Berthelot independently discoverediron pentacarbonyl, which is produced by a similar procedure as nickel tetracarbonyl. Mond recognized the economic potential of this class of compounds, which he commercially used in theMond process and financed more research on related compounds. Heinrich Hirtz and his colleague M. Dalton Cowap synthesized metal carbonyls ofcobalt,molybdenum,ruthenium, anddiiron nonacarbonyl.[77][78] In 1906James Dewar and H. O. Jones were able to determine the structure of diiron nonacarbonyl, which is produced from iron pentacarbonyl by the action of sunlight.[79] After Mond, who died in 1909, the chemistry of metal carbonyls fell for several years in oblivion.BASF started in 1924 the industrial production of iron pentacarbonyl by a process which was developed byAlwin Mittasch. The iron pentacarbonyl was used for the production of high-purity iron, so-calledcarbonyl iron, and iron oxidepigment.[40] Not until 1927 did A. Job and A. Cassal succeed in the preparation ofchromium hexacarbonyl andtungsten hexacarbonyl, the first synthesis of other homoleptic metal carbonyls.[citation needed]
Walter Hieber played in the years following 1928 a decisive role in the development of metal carbonyl chemistry. He systematically investigated and discovered, among other things, theHieber base reaction, the first known route to metal carbonyl hydrides and synthetic pathways leading to metal carbonyls such asdirhenium decacarbonyl.[80] Hieber, who was since 1934 the Director of the Institute of Inorganic Chemistry at theTechnical University Munich published in four decades 249 papers on metal carbonyl chemistry.[38]
Also in the 1930sWalter Reppe, an industrial chemist and later board member of BASF, discovered a number of homogeneous catalytic processes, such as thehydrocarboxylation, in which olefins oralkynes react with carbon monoxide and water to form products such as unsaturatedacids and their derivatives.[38] In these reactions, for example, nickel tetracarbonyl or cobalt carbonyls act as catalysts.[81] Reppe also discovered thecyclotrimerization and tetramerization ofacetylene and its derivatives tobenzene and benzene derivatives with metal carbonyls as catalysts. BASF built in the 1960s a production facility foracrylic acid by the Reppe process, which was only superseded in 1996 by more modern methods based on the catalyticpropylene oxidation.[citation needed]
For the rational design of new complexes the concept of the isolobal analogy has been found useful. Roald Hoffmann was awarded the Nobel Prize in chemistry for the development of the concept. This describes metal carbonyl fragments of M(CO)n as parts of octahedral building blocks in analogy to the tetrahedral CH3–, CH2– or CH– fragments in organic chemistry. In example dimanganese decacarbonyl is formed in terms of the isolobal analogy of twod7 Mn(CO)5 fragments, that areisolobal to the methyl radicalCH•
3. In analogy to howmethyl radicals combine to formethane, these can combine todimanganese decacarbonyl. The presence of isolobal analog fragments does not mean that the desired structures can be synthesized. In his Nobel Prize lecture Hoffmann emphasized that the isolobal analogy is a useful but simple model, and in some cases does not lead to success.[82]
The economic benefits of metal-catalysedcarbonylations, such asReppe chemistry andhydroformylation, led to growth of the area. Metal carbonyl compounds were discovered in the active sites of three naturally occurring enzymes.[83]
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