Competition between phosphorus lone pairs and ring π-systems in binding to transition metals: Binuclear diphosphacyclobutadiene cobalt carbonyl derivatives

Abstract Metal complexes of the 2,4-di- t- butyl-1,3-diphosphacyclobutadiene ligand have been synthesized by head-to-tail dimerization of t- BuC P: on transition metal sites. In this connection the geometries and energetics of the complete series of binuclear cobalt carbonyl complexes (Me 2 C 2 P 2 ) 2 Co 2 (CO) n ( n  = 5, 4, 3, 2, 1) of the simpler 2,4-dimethyl-1,3-diphosphacyclobutadiene ligand have been investigated by density functional theory. The lowest energy structures of these complexes have bridging η 4 ,η 1 -Me 2 C 2 P 2 ligands donating six electrons to the central Co 2 unit through a tetrahapto ring-metal linkage to one cobalt atom and a phosphorus lone pair to the other cobalt atom. For the tetracarbonyl (Me 2 C 2 P 2 ) 2 Co 2 (CO) 4 doubly CO-bridged structures with terminal η 4 -Me 2 C 2 P 2 ligands similar to known (η 5 C 5 H 5 ) 2 Fe 2 (μ-CO) 2 (CO) 2 and (η 4 -Me 4 C 4 ) 2 Co 2 (μ-CO) 2 (CO) 2 structures lie only ∼5  kcal/mol in energy above the η 4 ,η 1 -Me 2 C 2 P 2 bridged structures. For the unsaturated (Me 2 C 2 P 2 ) 2 Co 2 (CO) 3 and (Me 2 C 2 P 2 ) 2 Co 2 (CO) 2 systems, structures with one or two bridging η 4 ,η 1 -Me 2 C 2 P 2 ligands, respectively, are preferred energetically over isomeric structures with formal Co Co double bonds and Co Co triple bonds, respectively. The lowest energy structure for the monocarbonyl (Me 2 C 2 P 2 ) 2 Co 2 (CO) is a triplet structure in which a (Me 2 C 2 P 2 ) 2 Co sandwich unit functions as a tridentate ligand to a CoCO unit through a phosphorus atom on each ring as well as the central cobalt atom.