The cobalt carbonyl complexes (CF 3 NC) 2 Co 2 (CO) n (n=7, 6, 5, 4) and (CF 3 NC)Co(CO) n (n=4, 3, 2) of the strongly π-accepting trifluoromethyl isocyanide ligand have been investigated by density functional theory. The lowest energy (CF 3 NC) 2 Co 2 (CO) 6 structures are doubly bridged structures with the global minimum having two bridging CF 3 NC ligands. The lowest energy unbridged (CF 3 NC) 2 Co 2 (CO) 6 structure lies ∼19kcal/mol in energy above this global minimum. This differs from unsubstituted Co 2 (CO) 8 for which the doubly bridged and unbridged isomers have similar energies so both can be observed experimentally. The lowest energy structures of the formally unsaturated (CF 3 NC) 2 Co 2 (CO) n (n=5, 4) derivatives are also doubly bridged structures. Bridging CF 3 NC groups are energetically preferred over bridging CO groups. Four-electron donor bridging η 2 -μ-CF 3 NC groups are found in higher energy structures. Coupling of CF 3 NC groups to form a coordinated bis(trifluoromethyldiimine) ligand, CF 3 NCCNCF 3 is observed in carbonyl-rich (CF 3 NC) 2 Co 2 (CO) 7 structures. However, such (CF 3 NC) 2 Co 2 (CO) 7 structures do not appear to be viable since CO dissociation from the lowest energy such structure to give (CF 3 NC) 2 Co 2 (CO) 6 is predicted to be essentially thermoneutral.