The nature of metallophilic interactions in closed-shell d 8 -d 8 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]⋯[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.