Metallic Carbonyls

Metal carbonyls are transition-metal complexes of carbon monoxide containing a direct metal-carbon bond, in which the CO molecule acts as a neutral ligand; the first homoleptic carbonyl, nickel tetracarbonyl [Ni(CO)₄], was reported by Mond in 1890, and metal carbonyls are now widely studied for their industrial, catalytic, and CO-releasing importance. They are classified two ways. By the number of metal atoms: mononuclear carbonyls contain just one metal atom and have comparatively simple structures -- [Ni(CO)₄] (nickel tetracarbonyl) is tetrahedral, [Fe(CO)₅] (iron pentacarbonyl) is trigonal bipyramidal, and [Cr(CO)₆] (chromium hexacarbonyl) is octahedral; polynuclear carbonyls contain two or more metal atoms and may be homonuclear (e.g. Co₂(CO)₈, Mn₂(CO)₁₀, Fe₃(CO)₁₂) or heteronuclear (e.g. MnCo(CO)₉, MnRe(CO)₁₀). By structure: a carbonyl ligand bonded to only one metal atom is a terminal carbonyl, while one bonded simultaneously to two metal atoms is a bridging carbonyl. Non-bridged metal carbonyls contain no bridging CO at all -- some, like [Ni(CO)₄], [Fe(CO)₅] and [Cr(CO)₆], have only terminal carbonyls, while others, like Mn₂(CO)₁₀ (more precisely written (CO)₅Mn-Mn(CO)₅, and similarly Tc₂(CO)₁₀ and Re₂(CO)₁₀), combine terminal carbonyls with a genuine metal-metal bond. Bridged carbonyls contain one or more bridging CO ligands alongside terminal carbonyls and at least one metal-metal bond -- Fe₂(CO)₉ (di-iron nonacarbonyl) has three bridging CO ligands and six terminal CO groups, while Co₂(CO)₈ (dicobalt octacarbonyl) exists as two distinct isomers, one with a direct Co-Co bond and no bridges, the other with two bridging CO ligands. The metal-CO bond itself has two components, together called synergic bonding: first, a sigma (M ← CO) bond forms by the carbonyl's carbon atom donating an electron pair into a vacant d orbital on the metal, which makes the metal electron-rich; to relieve that extra electron density, a filled metal d orbital then donates back in …