A qualitative molecular orbital study of d(8) ... d(8) contacts, using as models the dimers of [RhCl(CO)(3)] and cis-[PtCl2(CO)(2)], is presented, focusing on the effect of the pyramidalization of the metal atom on the strength of the metal-metal interaction. The existence of a pyramidality effect in dimers of d(8)-ML(4) complexes is supported also by the results of ab initio MP2 calculations on the dimer of cis-[PtCl2(CO)(2)]. A structural database analysis shows that a correlation between the pyramidality and the M ... M distance exists for several families of dimers, oligomers, and polymers of square planar complexes of NI(II), Pd(II), Pt(II), and Au(III). The importance of the pyramidality effect in those compounds is discussed in comparison with similar effects in binuclear complexes with metal-metal bonds of different multiplicities.