The concept of a dative metal-metal bond is generally used to designate the donor-acceptor (DA) interaction of an electron-saturated metal center with another electron-deficient--or unsaturated--metal center. This type of DA bonding extended to the field of coordination complexes constitutes a borderline case of weak metal-metal interaction, among which the so-called metallophilic interactions occurring with 4d, 5d, and other late-transition-metal complexes are the most documented and representative examples. From a general standpoint, the peculiar position of the so-called dative metal-metal bond in chemical bonding stems from its presumed covalent character, which contrasts with the situation encountered with metallophilic interactions, which are essentially supported by dispersion and electrostatic forces and somewhat sustained by relativistic effects. In this study, the nature of the metal-metal bond in nonbridged 5d-3d Os-Cr and 5d-5d Os-W adducts, i.e., (Me(3)P)(CO)(4)Os-M(CO)(5) (M = Cr, W) and (CO)(5)Os-Cr(CO)(5), was addressed by resorting to state-of-the-art quantum-chemical methods. Semilocal density functional theory (DFT) approximations like Becke-Perdew or TPSS, the double-hybrid B2PLYP functional, as well as the corresponding dispersion, including TPSS-D and B2PLYP-D functionals and the wave-function-based spin-component-scaled second-order perturbative Moller-Plesset theory (SCS-MP2), were used. Energy decomposition analysis combined with the analysis of pairwise interfragment correlation energies from Pipek-Mezey localized molecular orbitals in combination with SCS-MP2 led to a clear demonstration of the significant role of dispersion (London) forces in the stabilization of the title adducts, wherein the Os-metal DA bond bears a rather low covalent character. These results plead in favor of a systematic recourse to dispersion including DFT approximations when addressing organometallic and coordination complexes.
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