Structural studies of phosphine complexes of mercury (II) halides.

Previous investigations of the solid-state structures of (R[3]P)nHgX2 (where X = Cl, Br, or I and n = 1 or 2) complexes have been extended by the determination of the structures of the following: (i) [(Pr[3]P)HgCl[2]][2] (vi) [(NCCH[2]CH[2])[3]PHgCl[2][n]; (ii) L(Pr[3]P)HgBr[2]][2] (vii) [(Ph[3]P)HgI[2]][2]; (iii) [(a-Pr[3]P)HgI[2]n (viii) [(NCCH[2]CH[2])[3]P][2]HgCl[2]; (iv) [(B-Pr[3]P)HgI[2]][2] (ix) L(NCCH[2]CH[2])[3]P][2]HgBr[2]Me[2]C0; (v) I[2]Cd(u-I)[2]Hg(PPr[3])[2] (x) [([2]-thienyl)[3]P][2]HgCl[2]As with previously solved 1:1 mercury(II) halide tertiary phosphine complexes (i) to (vii) show a wide variety of structures. These vary from symmetrical halogen-bridged dimers (e.g. [(Pr[3]P)HgCl[2]][2]) to a polymeric chain structure for [(a-Pr[3]P)HgI[2]]n. Both [(B-Pr[3]P)AgI[2]][2] and I[2]Cd(u-I)[2]Hg(PPr[3])[2] show the unusual feature for halogen-bridged dimers, of having both phosphine ligands attached to the same metal atom. In the case of I[2]Cd(u-I)[2]Hg(PPr[3])[2] both phosphine groups are attached to the mercury atom, snowing mercury to have a stronger affinity than cadmium for phosphorus.The 2:1 complexes all are distorted monomeric tetrahedral structures, [(NCCH[2]CH[2])[3]P][2]HgCl[2] consisting of two independent monomeric units.However, [(NCCH[2]CH[2]CH[2])[3]P][2]HgBr[2]Me[2]CO which contains a molecule of acetone has a trigonal bipyramidal arrangement due to a weak interaction between the mercury atom and the oxygen atom from the acetone molecule.The solid-state structures of these complexes have been rationalised in the terms of:(a) the donor strengths of the R[3]P ligands(b) the donor strengths of the halidesFor 2:1 complexes strong o-phosphine donors give a greater distortion of the structure from the tetrahedral, having large P-Hg-P bond angles and short Hg-P bond lengths. Similarly, for the limited data available, the weaker a-bonding halides (e.g. Cl) give the predicted trends of short Hg-P bond lengths accompanied by larger P-Hg-P bond angles.The situation for the 1:1 compounds is more complex with some o-phosphine donors forming more extended structures. A few of these 1:1 complexes are known to exist in more than one form, for example, Bu[3]PHgCl[2]and Pr[3]PHgI[2]- In both of these cases one form is a halogen-bridged dimer, the other form being a more extended structure; in the former it is tetrameric and in the latter a halogen-bridged chain structure. Theexpected trend of short Hg-P bond lengths and larger P-Hg-X angles forstrong a-phosphine donors is still apparent in these structures as is the shortening of the Hg-P. bonds and increase in the associated X[t]-Hg-P angles for weak a-donor halides.