An ab-initio study is presented for the whole series of mercury dihalides and for representative organomercury compounds including the moiety MeHg(+) (Me = CH3) : MeHgX (X = Cl, Br, I), Me(2)Hg, [MeHg(PH3)](+) and [MeHg(PH3)(3)](+). The last compound is used to model the [MeHg(np(3))](+) (np(3) = N(CH(2)CH(2)Ph(2))(3)) complex. A basis set of moderate size has been selected, which enables the accurate reproduction of experimental observables like geometries vibrational frequencies, and thermochemical parameters, while still allowing the study of large molecules. The resistance of the HgC bond to acidic attack has been found to decrease in the order MeHgCl > MeHgBr > MeHgI on the one hand and [MeHg(PH3)](+) > [MeHg(PH3)(3)](+) on the other hand. In agreement with the experimental observations on [MeHg(np(3))](+) the easiest HgC bond cleavage occurs in the [MeHg(PH3)(3)](+) compound.