The nature of metallophilic interactions in closed-shell d8-d8 metal complexes

We have quantum chemically analyzed the closed-shell d(8)-d(8) metallophilic interaction in dimers of square planar [M(CO)(2)X-2] complexes (M = Ni, Pd, Pt; X = Cl, Br, I) using dispersion-corrected density functional theory at ZORA-BLYP-D3(BJ)/TZ2P level of theory. Our purpose is to reveal the nature of the [X-2(CO)(2)M]& ctdot;[M(CO)(2)X-2] bonding mechanism by analyzing trends upon variations in M and X. Our analyses reveal that the formation of the [M(CO)(2)X-2](2) dimers is favored by an increasingly stabilizing electrostatic interaction when the M increases in size and by more stabilizing dispersion interactions promoted by the larger X. In addition, there is an overlooked covalent component stemming from metal-metal and ligand-ligand donor-acceptor interactions. Thus, at variance with the currently accepted picture, the d(8)-d(8) metallophilicity is attractive, and the formation of [M(CO)(2)X-2](2) dimers is not a purely dispersion-driven phenomenon.