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.