A qualitative and semiquantitative study of the weakly bonding nature of M ... M contacts between square-planar d(8)-ML(4) molecules is presented. The results of ab initio calculations on the model dimers cis-[PiCl(2)(CO)(2)](2) (in two different conformations), [PtCl2{HNCH(OH)}2], and [RhCl(CO)(3)](2) are reported. These are predicted to be stable toward dissociation into two monomers, with bonding energies in excess of 3 kcal/mol and M ... M distances well in the range of the experimental values for dimers and chains. The effect of adding axial groups to the ML(4) dimers on the strength of the M ... M interaction has been explored by means of ab initio calculations on several adducts of [PtCl2(CO)(2)] as well as by extended Huckel calculations on the model adducts [Cl4YRh ... RhZCl(4)](n-) (Y, Z = Cl-, CO, H+, or AuCl). A qualitative ordering for the M ... M bond strengths is proposed, which is consistent with the available experimental data.