Almost all of the transition metals form binary complexes in which carbon monoxide acts as the ligand and the metal is in a low oxidation state, most often, zero. The resulting complexes are called metal carbonyls.
Mononuclear metal carbonyls contain only one metal atom and they have comparatively simple structures, obtainable from valence bond considerations, on the basis of Effective Atomic Number (EAN) rules. Stable mononuclear and binuclear carbonyl complexes of first row transition metals include V(CO)6, Cr(CO)6, Mn2(CO)10, Fe(CO)5, Fe2(CO)9, Co2(CO)8 and Ni(CO)4.
Metals having even number of electrons satisfy the 18 electron (EAN) rule by simply adding up required number of CO molecules. Obviously, metals having odd number of electrons cannot conform to it in that manner. For example, Mn(CO)5 and Co(CO)4 are both 17 electron species and consistent with expectation, do not exist as stable molecules. They achieve stability through dimerization, forming dinuclear complexes. However, the corresponding anions of these 17 electron species, Mn(CO)5- and Co(CO4)- are stable speces and conform to the 18 electron rule. Nickel tetracarbonyl is tetrahedral, the pentacarbonyl of iron is trigonal bipyramidal whereas the hexacarbonyl of vanadium and chromium are octahedral. Among the mononuclear carbonyls the only exception to the EAN rule is V(CO)6 which exists as a 17 electron molecule, albeit less stable.
The structures of dinuclear metal carbonyls involve either metal-metal bonds or bridging CO groups, or both. For example, the structure of Fe2(CO)9 diiron nonacarbonyl contains three CO ligands that form bridges between the iron atoms and each iron atom also has three other CO groups attached only to that atom. The structure of Mn2(CO)10, on the other hand, involve a metal-metal bond, so its formula can be written as (CO)5M-M(CO)5. Two isomers are known for Co2(CO)8, one has a metal-metal bond between two cobalt atoms, and the other has two bridging CO ligands and a metal-metal bond. The arrangement of four carbonyls around each Co is squarish, but not exactly planar.