SILVER AND GOLD DIMERS - CRYSTAL AND MOLECULAR-STRUCTURES OF AG2(DMPM)2BR2 AND [AU2(DMPM)2](PF6)2 AND RELATION BETWEEN METAL METAL FORCE-CONSTANTS AND METAL-METAL SEPARATIONS

The Ag2(dmpm)2Br2 and [Au2(dmpm)2](PF6)2 compounds (dmpm = bis(dimethylphosphino)methane) have been characterized by X-ray diffraction at 298 K. The molecules are found to contain M2(dmpm)2(2+) core structures with two metal atoms bridged by the dmpm ligands to give eight-membered M2P4C2 rings. The silver compound forms a polymeric chain where the silver atoms of adjacent Ag2(dmpm)2(2+) units are linked by two Br anions (r(AgBr) = 2.7431 (13), 2.9453 (14) angstrom). The intra- and intermolecular Ag...Ag separations are 3.605 (2) and 3.916 (2) angstrom, respectively, while the Au...Au distance in [Au2(dmpm)2](PF6)2 is 3.045 (1) angstrom. In the latter case, some weak Au...F contacts (r(AuF) = 3.44 (1) angstrom) are also noticed. The solid-state low-frequency (40-400 cm-1) vibrational spectra of these compounds and five other related complexes containing Ag2 and Au2 have been analyzed. Intense Raman scatterings associated with the metal-metal stretching frequencies (nu(M2)) along with the metal-metal force contants (F(M2); estimated from the diatomic approximation) have been obtained. The nu(M2) (cm-1) and F(M2) (mdyn angstrom-1) values are as follows, respectively: [Ag2(dmpm)2](PF6)2, 76 and 0.18; [Ag2(dppm)3](PF6)2 (dppm = bis(diphenylphosphino)methane), 76 and 0.18; [Au2(dmpm)2](PF6)2, 68 and 0.27; [Au2(dmpm)3](PF6)2, 69 and 0.28; [Au2(dmpm)2]Cl2, 71 and 0.29; Au2(dmb)(CN)2 (dmb = 1,8-diisocyano-p-menthane), 36 and 0.075. Including literature results, data banks of four and eleven points are accumulated for the silver and gold compounds, respectively, where reparametrized Herschbach-Laurie type relationships (H-L) between r(M2) and F(M2) applied to Ag2 or Au2 systems are designed. For Ag2(dmpm)2Br2 (nu(Ag2) = 48 cm-1), the estimated F(Ag2) for the intramolecular Ag...Ag interactions is 0.03 mdyn angstrom-1. Crystallographic details are as follows. For Ag2(dmpm)2Br2: monoclinic space group P2(1)/n, a = 7.2023 (8) angstrom, b = 10.5734 (5) angstrom, c = 14.1980 (6) angstrom, beta = 82.466 (9)degrees, Z = 2, R = 0.040 for 1382 reflections measured. For [Au2(dmpm)2](PF6)2: monoclinic space group C2/m, a = 10.234 (1) angstrom, b = 13.711 (2) angstrom, c = 9.525 (2) angstrom, beta = 96.74 (1)degrees, Z = 2, R = 0.050 for 1540 reflections where F(o)2 > 3-sigma(F(o)2).