Mercury is present in the structure of several heterometallic clusters, usually bridging three or four transition metal centres.[1] Heterobimetallic compounds with a single M-Hg
bond were prepared in the past, and the simpliest situation is represented by the {HgX} fragment (X = halide or pseudohalide) bonded to a transition metal fragment. Compounds
of this type can be obtained following different synthetic approaches, such as the reaction of HgX2 with suitable precursors or the insertion of Hg in a M-X bond. The most common
transition metals belong to the Groups 6 - 9, with CO, cyclopentadienide (Cp) and related species as ancillary ligands. Selected examples are the complexes [M(HgX)(Cp)(CO)3] (M = Cr, Mo, W), [M(HgX)(CO)5], (M = Mn, Re), [Fe(HgX)(Cp)(CO)2] and [Co(HgX)(CO)4].[2] To the best of our knowledge, the nature of the M-Hg bond was never computationally investigated. In this communication we report the outcomes of DFT calculations on model compounds having general formulae [M(HgX)(Cp)(CO)n] (M = Cr, Mo, W, n = 3; M = Fe, Ru, Os, n = 2; X = Cl, Br, I) and [M(HgX)(CO)n] (M = Mn, Tc, Re, n = 5; M = Co, Rh, Ir, n = 4; X = Cl, Br, I). The nature of the M-Hg bond, the factors affecting the bond strength and the behaviour of {HgX} as Lewis acid or base were studied in detail.
