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 <i>trans</i> 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 <i>trans</i> H–Mo–CO moiety are displaced towards the hydride ligand. While Cp^RMo­(PMe₃)_3–<i>x</i>(CO)_<i>x</i>H (Cp^R = Cp, Cp*; <i>x</i> = 1, 2, 3) are catalysts for the release of H₂ from formic acid, the carbonyl derivatives, Cp^RMo­(CO)₃H, are also observed to form dinuclear formate compounds, namely, [Cp^RMo­(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 <i>formal</i> 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