Diverse bonding modes of the pentalene ligand in binuclear cobalt carbonyl complexes

The 2007 synthesis of the hexamethylpentalene complex cis-(η5,η5-C8Me6)Co2(CO)4 by O’Hare and coworkers makes of interest a theoretical study of the unsubstituted binuclear pentalene cobalt carbonyls (C8H6)Co2(CO)n (n = 6, 5, 4, 3). In this connection an unbridged unsubstituted cis-(η5,η5-C8H6)Co2(CO)4 structure analogous to the experimental hexamethyl structure is found to be the lowest energy (C8H6)Co2(CO)4 structure. Furthermore, this (C8H6)Co2(CO)n structure is predicted to be the product from reactions of pentalene derivatives with cobalt carbonyls based on the thermochemistry of CO dissociation energies in the sequence n = 6 → 5 → 4 → 3 → 2. The low-energy structures for the unsaturated (C8H6)Co2(CO)n (n = 3, 2) derivatives also have the pentalene ligand bonded to each cobalt as a pentahapto ligand. They may be derived from either this unbridged cis-(η5,η5-C8H6)Co2(CO)4 structure or a doubly CO-bridged variation thereof by removal of one or two CO groups. The carbonyl-rich (C8H6)Co2(CO)n (n = 6, 5) structures, which may be considered as precursors to cis-(η5,η5-C8H6)Co2(CO)4, illustrate considerable diversity of bonding modes of the pentalene ligand to the central Co2 unit. Structures without direct Co–Co bonds are found exhibiting either cis or trans stereochemistry in which trihapto allylic and/or pentahapto cyclopentadienylic subunits are bonded to Co(CO)3 and Co(CO)2 moieties, respectively. In addition, (C8H6)Co2(CO)n (n = 6, 5) structures exhibiting cis stereochemistry are found in which CC bonds of a butadiene or fulvene subunit of the pentalene ligand replace CO groups in Co2(CO)8, retaining the formal Co–Co single bond.