Binuclear methylphosphinidine complexes of cyclopentadienylruthenium carbonyls: effects of the higher ligand field strength of ruthenium derivatives relative to iron derivatives†

The structures and energetics of the binuclear methylphosphinidene complexes of cyclopentadienylruthenium carbonyls of the type [MePRu₂(CO)_nCp₂] (n = 4, 3, 2, 1) have been investigated for comparison with their previously studied iron analogues. For the tetracarbonyls and tricarbonyls [MePM₂(CO)_nCp₂] (n = 4, 3) substituting ruthenium for iron has relatively little effect on the energetically preferred structures. Thus such structures have two-electron donor bridging MeP groups with no metal–metal bond for the tetracarbonyls and a metal–metal single bond for the tricarbonyls. This leads to favored 18-electron configurations for both ruthenium atoms in all cases. However, the higher ligand field strengths of ruthenium complexes relative to analogous iron complexes have major effects on the energetically preferred structures for the dicarbonyls and monocarbonyls [MePM₂(CO)_nCp₂] (M = Fe, Ru; n = 2, 1). Thus the 11 lowest energy structures for the dicarbonyl [MePFe₂(CO)₂Cp₂] are triplet or quintet spin state structures whereas the 6 lowest energy structures for the ruthenium analogue [MePRu₂(CO)₂Cp₂] are all singlet structures. These low-energy singlet [MePRu₂(CO)₂Cp₂] structures include species in which both ruthenium atoms attain the favored 18-electron configurations in different ways: either by a Ru–Ru single bond and an agostic C–H–Ru interaction from the methyl group, a Ru–Ru single bond and a four-electron donor bridging MeP ligand with PRu double bonds, or a formal RuRu double bond with a two-electron donor bridging MeP ligand. The 8 lowest energy structures for the diiron monocarbonyl [MePFe₂(CO)Cp₂] are all triplet or quintet spin structures whereas the lowest energy structure for the diruthenium monocarbonyl [MePRu₂(CO)Cp₂] by more than 20 kcal mol⁻¹ is a singlet structure with a formal RuRu double bond and bridging CO and four-electron donor MeP groups. Thermochemical information predicts such monocarbonyl derivatives to be the dominant binuclear decarbonylation products of the tricarbonyls [RPRu₂(CO)₃Cp₂] by thermal or photochemical methods.