We study the dependence of the intramolecular M-I-M-I interaction on electron correlation effects in eight-membered rings of type [M-2(PH2CH2PH2)(2)](2+), [M-2(NHCHNH)(2)], [M-2(SCHS)(2)] (M = Au, Ag, Cu), [Au-2(PH2CH2PH2)(2)]-Cl-2, halogenometal(I) [M2X4](2-) (M = Au, Ag, Cu; X = Cl, Pr, I), and [Au2Te4](2-) at the quasirelativistic pseudopotential ab initio MP2 and Hartree-Fock levels. The intramolecular M-I-M-I distances, R, at the MP2 level fall in the same range as the experimental ones. The R values are reduced from HF to MP2 level. All the calculations suggest that correlation effects are essential. The reduction of R depends on the particular M-L combination (L = ligand). In the rings, short Cu-I-Cu-I distances are recovered for the first time. The explicit inclusion of the counterions is unimportant for M-I and essential for MU, as shown by a study of the oxidative laddition of Cl-2 to the model [Au-2(CH2PH2CH2)(2)]. The Au(I) is then oxidized to Au(II), and the Au-Au distance is shortened to 261 pm, corresponding to a sigma bond. This value is in agreement with experiment.