Reactivity of Cyclopentadienyl Molybdenum Compounds towards Formic Acid: Structural Characterization of CpMo(PMe3)(CO)2H, CpMo(PMe3)2(CO)H, [CpMo(μ-O)(μ-O2CH)]2, and [Cp*Mo(μ-O)(μ-O2CH)]2.

The molecular structures of CpMo(PMe3)(CO)2H and CpMo(PMe3)2(CO)H have been determined by X-ray diffraction, thereby revealing four-legged piano-stool structures in which the hydride ligand is trans to CO. However, in view of the different nature of the four basal ligands, the geometries of CpMo(PMe3)(CO)2H and CpMo(PMe3)2(CO)H deviate from that of an idealized four-legged piano stool, such that the two ligands that are orthogonal to the trans H-Mo-CO moiety are displaced towards the hydride ligand. While CpRMo(PMe3)3-x(CO)xH (CpR = Cp, Cp*; x = 1, 2, 3) are catalysts for the release of H2 from formic acid, the carbonyl derivatives, CpRMo(CO)3H, are also observed to form dinuclear formate compounds, namely, [CpRMo(μ-O)(μ-O2CH)]2. The nature of the Mo···Mo interactions in [CpMo(μ-O)(μ-O2CH)]2 and [Cp*Mo(μ-O)(μ-O2CH)]2 have been addressed computationally. In this regard, the two highest occupied molecular orbitals of [CpMo(μ-O)(μ-O2CH)]2 correspond to metal-based δ* (HOMO) and σ (HOMO-1) orbitals. The σ2δ*2 configuration thus corresponds to a formal direct Mo-Mo bond order of zero. The preferential occupation of the δ* orbital rather than the δ orbital is a consequence of the interaction of the latter orbital with p orbitals of the bridging oxo ligands. In essence, lone-pair donation from oxygen increases the electron count so that the molybdenum centers can achieve an 18-electron configuration without the existence of a Mo-Mo bond, whereas a Mo═Mo double bond is required in the absence of lone-pair donation.