Mercury(II) halide adducts of esters of 2-pyridinecarboxylic acid. Crystal structures and structural variations within the series [HgCl2(C5H4NCOOR)], R=Me, Et, Pr-n, Pr-i

The series of complexes [HgX2(C5H4NCOOR)] (X = Cl, Br or I; R = Me, Et, Pr-n or P-i) has been obtained by the addition reaction of esters of 2-pyridinecarboxylic acid to mercury(II) halides irrespective of the ester to metal molar ratio used. The X-ray crystal structures of the four chloro complexes [HgCl2(C2H5NCOOR)], R = Me (1), Et (4), Pr-n (7) and Pr-i (10), have been determined. There is a remarkable structural variety within such a closely related family, probably due to different sizes of the substituents. Complexes 1 and 4 adopt polymeric structures. However, the one-dimensional chain in 1 consists of consecutive symmetrically bridged Hg(mu-Cl)(2)Hg dimeric units linked by secondary Hg ... Cl interactions while in 4 it is formed by monomeric HgCl2L units, L=C5H4NCOOEt, bound by a unique non-planar bis(chloride) asymmetric Hg2Cl2 bridge. In both structures, mercury achieves an effective six-coordination. The structures of 7 and 10 comprise only discrete centrosymmetric dimers and tetramers, respectively. The latter can be described as a central ClLHg(mu-Cl)(2)HgLCl unit, L = C5H4NCOOPri, bound to two additional HgCl2L molecules by means of long-range Hg ... Cl interactions. Mercury shows an effective five-coordination in 7 and five-and six-coordination in 10. The four structures, based on a backbone containing mercury and chlorine atoms exclusively, have been correlated. The geometric parameters of their basic Hg(mu-Cl)(2)Hg structural motifs are also found in other HgCl2(L) adducts and seem to be independent of the coordination number and geometry about mercury as well as of the unidentate or bidentate nature of the L ligand. (C) 1997 Elsevier Science S.A.