pound~~~~~~~~,~~ ,~~~'~ and, in particular, to the type of Hg orbitals involved in the HgX bonding. The prominent role of s-dZz hybridization has been noticed already in the work of Orgel in 1958,20 whereas the involvement of pz orbitals in the bonding has never been fully understood. The study of HgzX2"~'~ compounds and of the direct interaction between mercury atoms has also received attention. On the other hand, only a few theoretical studies have been devoted to compounds containing the HgC bond?e,10,17 This is in contrast with the role, noxious as well as useful, of the MeHg+ moiety occumng in nature as well as resulting from human activity.' Motivated by the importance of the HgC bond, we have initiated a series of theoretical studies,8 of which the present publication provides the basis. We present here an ab-initio study on a series of HgMe+ containing molecules. In a first stage, we have selected a suitable basis set through calculation of geometries, vibrational frequencies, and thermochemical parameters of a series of simple compounds for which good experimental data are available. Thereafter, special attention has been focused on the relative stability of the HgC bonds of the molecules in terms of electronic structure and of general trends derived from energetic considerations.