Cobalt: Organometallic ChemistryBased in part on the article Cobalt: Organometallic Chemistry by Ulrich Koelle which appeared in theEncyclopedia of Inorganic Chemistry, First Edition.

Abstract

Among the longest known families of organometallic compounds, those of cobalt display great diversity in their structures and oxidation states. The ligands accommodated in organocobalt compounds range from strong σ‐donors such as hydrides and alkyls, to π‐acid ligands such as carbonyls and phosphines. The carbonyl cluster compounds of cobalt are numerous and they play a major role in catalytic reactions of enormous industrial importance such as hydroformylation reactions (conversion of alkenes to aldehydes) and Pauson–Khand cycloaddition (production of cyclic ketones). Cobalt is an electron‐rich late transition metal, a property reflected throughout its organometallic chemistry. Thus, the carbonyl ‘hydride’ HCo(CO) 4 is actually a strong acid and does not have ‘hydridic’ (H − ) properties. Similarly, cobalt does not complex to any group 15 or 16 heterocycles such as pyridine or thiophene. In contrast, boracycles are good ligands for cobalt in a sandwich complex because an electropositive atom such as boron incorporated in a π‐cyclic ring stabilizes the electron‐rich central metal. The cyclopentadienyl ligand is found extensively throughout organocobalt chemistry – not only in cobaltocenes, but also in half‐sandwich complexes of the type (C 5 H 5 )CoL 2 , where L is CO, alkene, alkyne, or phosphine. The (C 5 H 5 )CoL 2 complexes are useful in alkyne dimerization and trimerization. Cobaltocene itself ((C 5 H 5 ) 2 Co) is easily oxidized to the extraordinarily stable 18‐electron cobaltocenium cation, [(C 5 H 5 ) 2 Co] + . Cobalt can be incorporated into carborane cages, which have been found to be efficient extractors of radioactive metal ions such as 90 Sr and 137 Cs in water at low pH. Cobalamins, which consist of a cobalt central atom in a substituted corrin ring, are examples of naturally occurring organometallic compounds, and coenzyme B 12 was the first recognized example of a biological organometallic compound. Methylcobalamins play a role in biology, where they are found in microorganisms and can methylate mercury in the environment.

Balancing Adduct Formation and Ligand Coupling with the Bulky Allyl Complexes [1,3-(SiMe3)2C3H3]2M (M = Fe, Co, Ni)

“…19 Similarly, the addition of iodine to (C 3 H 5 ) 3 Co forms (C 3 H 5 ) 2 CoI and C 3 H 5 I. 31 On the basis of the precedent with A′ 2 Ni, it was thought that similar reactions with A′ 2 M (M = Fe, Co) might also lead to heteroleptic complexes. However, the addition of I 2 to either A′ 2 M complex (eq 7) results in the formation of the coupled allyl dimer 1,3,4,6-tetrakis(trimethylsilyl)-1,5hexadiene and insoluble black precipitates, presumably FeI 2 and CoI 2 , respectively.…”