The molecular structures of CpMo(PMe3)(CO)(2)H 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)(2)H 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 (CpMo)-Mo-R(PMe3)(3-x)(CO)(x)H (Cp-R = Cp, Cp*; x = 1, 2, 3) are catalysts for the release of H-2 from formic acid, the carbonyl derivatives, (CpMo)-Mo-R(CO)(3)H, are also observed to form dinudear formate compounds, namely, [(CpMo)-Mo-R(mu-O)(mu-O2CH)](2). The nature of the Mo center dot center dot center dot Mo interactions in [CpMo(mu-O)(mu-O2CH)](2) and [Cp*Mo(mu-O)(mu-O2CH)](2) have been addressed computationally. In this regard, the two highest occupied molecular orbitals of [CpMo(mu-O)(mu-O2CH)](2) correspond to metal-based delta* (HOMO) and sigma (HOMO-1) orbitals. The sigma(2)delta*(2) configuration thus corresponds to a formal direct Mo Mo bond order of zero. The preferential occupation of the delta* orbital rather than the 5 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.