Synthesis, crystal structure, electrochemistry and molecular-orbital analysis of the piano-stool dimer [Mo2(η-C5H5)2(CO)4(NC5H4PPh2-2)2]

Abstract

The addition of 2-(diphenylphosphino)pyridine (NC₅H₄PPh₂-2) to an in situ prepared m-xylene solution of [Mo₂(η-C₅H₅)₂(CO)₄](molar ratio 2 : 1), at room temperature, yields the complex [Mo₂(η-C₅H₅)₂(CO)₄(NC₅H₄PPh₂-2)₂]1. Compound 1 is easily converted into the cationic molybdenum(II) mononuclear complex [Mo(η-C₅H₅)(CO)₃(NC₅H₄PPh₂-2)]PF₆2 by reaction with AgPF₆. Reduction of 1 with Na or Na/Hg, in tetrahydrofuran, affords an air-sensitive solution containing Na[Mo(η-C₅H₅)(CO)₂(NC₅H₄PPh₂-2)]3, together with minor products. Electrochemical measurements show that 1 undergoes a reversible one-electron oxidation followed by relatively slow decomposition of the electrogenerated species. The molecular structure of the diethyl ether solvate of 1 was determined by X-ray diffraction methods: monoclinic, space group P2₁/c, with a= 13.218(4), b= 19.485(6), c= 11.019(4)Å, β= 110.10(2)° and Z= 2. The centrosymmetric complex 1 is a typical piano-stool dimer in which two units share the leg coinciding with the Mo–Mo vector. Similarly to other compounds of this type, the M–M separation is quite long [Mo–Mo 3.276(3)Å], ca. 0.5 Å longer than the sum of the metal radii. The evidence for the single metal–metal bond (predicted by counting rules) and its role in providing the system's stability is discussed in terms of qualitative molecular orbital theory. The extended-Hückel method used appears sufficiently reliable as the total electronic energy minimizes for an intermetal separation close to the experimental one. The loss of σ bonding, on elongating Mo–Mo, is counterbalanced by the diminished repulsion between metal lone pairs (filled xy orbitals), thus the intermetallic distance is an evident compromise between attractive and repulsive electronic forces. Steric factors may not be so important. The theoretical implications for the eventual homolytic cleavage of the dimer are also underlined. A rationale is provided for the effects that follow the removal of one electron from the system.