We studied the attraction between [Pt(PH3)(3)] and Tl(I) in the [Pt(PH3)(3)]-Tl+ complex using ab initio methodology. We found that the changes around the equilibrium distance Pt-Tl and in the interaction energies are sensitive to the electron correlation potential. This effect was evaluated using several levels of theory, including HF, MPn (n = 2-4), CCSD and CCSD(T). The obtained interaction energies differences at the equilibrium distance R-e (Pt-Tl) range from 134 to 205 kJ/mol at the different levels used. At long-distances, the behaviour of the [Pt(PH3)(3)]-Tl+ interaction may be related mainly to charge-induced dipole and dispersion terms, both involving the individual properties of [Pt(PH3)3] and thallium ion. However, the charge-induced dipole term (R -4) is found as the principal contribution in the stability at the long and short distances. The dispersion interaction is smaller, but not negligible near equilibrium distance. (c) 2005 Elsevier B.V. All rights reserved.