Metal carbonyl

Metal carbonyls are coordination complexes of transition metals with carbon monoxide ligands. Metal carbonyls are useful in organic synthesis and as catalysts or catalyst precursors in homogeneous catalysis, such as hydroformylation and Reppe chemistry. In the Mond process, nickel tetracarbonyl is used to produce pure nickel. In organometallic 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 carbonylate hemoglobin to give carboxyhemoglobin, which prevents the binding of oxygen.

Nomenclature and terminology

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 carbonyl cations or as negatively charged metal carbonylates. The carbon monoxide ligand may be bound terminally to a single metal atom or bridging to two or more metal atoms. These complexes may be homoleptic, containing only CO ligands, such as nickel tetracarbonyl, but more commonly metal carbonyls are heteroleptic and contain a mixture of ligands.
Mononuclear metal carbonyls contain only one metal atom as the central atom. Except vanadium hexacarbonyl, only metals with even atomic number, such as chromium, iron, nickel, and their homologs, build neutral mononuclear complexes. Polynuclear metal carbonyls are formed from metals with odd atomic numbers and contain a metal–metal bond. Complexes with different metals but only one type of ligand are called isoleptic.
Carbon monoxide has distinct binding modes in metal carbonyls. They differ in terms of their hapticity, denoted η, and their bridging mode. In η²-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.
The carbonyl ligand engages in a wide range of bonding modes in metal carbonyl dimers and clusters. In the most common bridging mode, denoted μ₂ or simply μ, the CO ligand bridges a pair of metals. This bonding mode is observed in the commonly available metal carbonyls: Co₂₈, Fe₂₉, Fe₃₁₂, and Co₄₁₂. In certain higher nuclearity clusters, CO bridges between three or even four metals. These ligands are denoted μ₃-CO and μ₄-CO. Less common are bonding modes in which both C and O bond to the metal, such as μ₃η².

Structure and bonding

Carbon monoxide bonds to transition metals using "synergistic pi* back-bonding". The M–C bonding has three components, giving rise to a partial triple bond. A sigma bond arises from overlap of the nonbonding sp-hybridized electron pair on carbon with a blend of d-, s-, and p-orbitals on the metal. A pair of pi 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 which makes the back-donation of electron density favorable. As electrons from the metal fill the π-antibonding orbital of CO, they weaken the carbon–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 Mo₆ and Mo₃₃: 2.06 vs 1.90 and 1.11 vs 1.18 Å.
Infrared spectroscopy is a sensitive probe for the presence of bridging carbonyl ligands. For compounds with doubly bridging CO ligands, denoted μ₂-CO or often just μ-CO, the bond stretching frequency ν_CO 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 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. The increased π-bonding due to back-donation from multiple metal centers results in further weakening of the C–O bond.

Physical characteristics

Most mononuclear carbonyl complexes are colorless or pale yellow, volatile liquids or solids that are flammable and toxic. Vanadium hexacarbonyl, a uniquely stable 17-electron metal carbonyl, is a blue-black solid. Dimetallic and polymetallic carbonyls tend to be more deeply colored. Triiron dodecacarbonyl forms deep green crystals. The crystalline metal carbonyls often are sublimable in vacuum, although this process is often accompanied by degradation. Metal carbonyls are soluble in nonpolar and polar organic solvents such as benzene, diethyl ether, acetone, glacial acetic acid, and carbon tetrachloride. Some salts of cationic and anionic metal carbonyls are soluble in water or lower alcohols.

Analytical characterization

Apart from X-ray crystallography, important analytical techniques for the characterization of metal carbonyls are infrared spectroscopy and ¹³C NMR spectroscopy. These two techniques provide structural information on two very different time scales. Infrared-active vibrational 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". Illustrative of the differing time scales, investigation of dicobalt octacarbonyl by means of infrared spectroscopy provides 13 ν_CO bands, far more than expected for a single compound. This complexity reflects the presence of isomers with and without bridging CO ligands. The ¹³C NMR spectrum of the same substance exhibits only a single signal at a chemical shift of 204 ppm. This simplicity indicates that the isomers quickly interconvert.
Iron pentacarbonyl exhibits only a single ¹³C NMR signal owing to rapid exchange of the axial and equatorial CO ligands by Berry pseudorotation.

Infrared spectra

An important technique for characterizing metal carbonyls is infrared spectroscopy. The C–O vibration, typically denoted ν_CO, occurs at 2143 cm⁻¹ for carbon monoxide gas. The energies of the ν_CO band for the metal carbonyls correlates with the strength of the carbon–oxygen bond, and inversely correlated with the strength of the π-backbonding between the metal and the carbon. The π-basicity of the metal center depends on a lot of factors; in the isoelectronic series at the bottom of this section, the hexacarbonyls show decreasing π-backbonding as one increases the charge on the metal. π-Basic ligands increase π-electron density at the metal, and improved backbonding reduces ν_CO. The Tolman electronic parameter uses the Ni₃ fragment to order ligands by their π-donating abilities.
The number of vibrational modes of a metal carbonyl complex can be determined by group theory. Only vibrational modes that transform as the electric dipole operator will have nonzero direct products and are observed. The number of observable IR transitions can thus be predicted. For example, the CO ligands of octahedral complexes, such as Cr₆, transform as a_1g, e_g, and t_1u, but only the t_1u mode is IR-allowed. Thus, only a single ν_CO 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 of Fe₂₉ 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. 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.

Compound	ν_CO	¹³C NMR shift	average M-CO distance	average C-O distance	-
CO	2143	181			-
	1748	245	204	116	-
	1859		200, 193	113
Cr₆	2000	212	191	114	-
	2100				-
	2204		191	112	-
Fe₅	2022, 2000	209	180	112	-
Ru₅	2038, 2002				-
Ni₄			181	113	-

Carbonyl	ν_CO, μ₁	ν_CO, μ₂	ν_CO, μ₃
Rh₂₈	2060, 2084	1846, 1862
Rh₄₁₂	2044, 2070, 2074	1886
Rh₆₁₆	2045, 2075		1819