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

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.