Diverse bonding modes and coupling reactions of the boronyl ligand in binuclear cyclopentadienyl cobalt derivatives: Analogies with isoelectronic binuclear cyclopentadienyliron carbonyls

The structures and thermochemistry of the binuclear cyclopentadienylcobalt carbonyl boronyls Cp2Co2(BO)2(CO)n (Cp = η5-C5H5; n = 2, 1, 0) were investigated by density functional theory. Low energy structures were found of the following types: (1) Structures in which the BO groups are all one-electron donors analogous to the isoelectronic Cp2Fe2(CO)n+2 structures; (2) Structures in which the two BO groups have coupled to form bridging B2O2 ligands, which may be either four- or six-electron donors; (3) Structures with three-electron donor bridging BO groups, bonded to one metal through the boron atom and to the other metal through the oxygen atom. The lowest energy Cp2Co2(BO)2(CO)2 structures are singlet trans and cis isomers of doubly bridged structures analogous to the well-known isoelectronic Cp2Fe2(CO)2(μ-CO)2. The lowest energy Cp2Co2(BO)2(CO) structure is a triplet triply bridged structure analogous to the isoelectronic stable triplet state molecule Cp2Fe2(μ-CO)3. Higher energy Cp2Co2(BO)2(CO) structures have trans or cis bridging B2O2 ligands. The lowest energy Cp2Co2(BO)2 structure is a triplet state structure with a six-electron donor bridging trans-η4-μ-B2O2 ligand. The lowest energy singlet Cp2Co2(BO)2 structure has two one-electron donor bridging μ-BO groups and a formal Cotriple bond; length of mdashCo triple bond similar to the predicted lowest energy Cp2Fe2(CO)2 structure.