Crystal structures and spectroscopic studies of the mononuclear complex [AgBr(PPh3)2] and binuclear [Ag2X2(PPh3)4]·2CHCl3(X = Cl or Br)

Abstract

The structure of the silver(I) complexes [AgBr(PPh₃)₂] and [Ag₂X₂(PPh₃)₄]·2CHCl₃(X = Cl or Br) have been determined by single-crystal X-ray diffraction. The complex [AgBr(PPh₃)₂] crystallizes in the monoclinic space group C2/c and contains discrete monomeric [AgBr(PPh₃)₂] units with essentially trigonal-planar AgBrP₂ co-ordination, and a crystallographic two-fold axis of symmetry coincident with the Ag–Br bond. The geometric parameters for the silver atom environment are: Ag–Br 2.568(1), Ag–P 2.458(2)Å, P–M–P 124.14(5), P–M–Br 117.93(3)°. The complexes [Ag₂X₂(PPh₃)₄]·2CHCl₃(X = Cl or Br) are isomorphous, monoclinic, space group C2/c, and contain [Ag₂X₂(PPh₃)₄] dimers. Each of the two silver atoms in the structure is four-co-ordinated by forming bonds with the P atoms of the two phosphine ligands and the two doubly bridging halide atoms. The Ag and X atoms lie in a plane, and each of the molecules in the unit cell has a C₂ axis which passes through the two X atoms. A chloroform molecule is hydrogen bonded to each X atom. The far-IR spectra of these complexes show bands which are assigned to ν(AgX) modes, and the spectra of these and the unsolvated dimer [Ag₂Cl₂(PPh₃)₄] are analysed to yield information about the Ag–X bonding. The Raman spectrum of [AgBr(PPh₃)₂] shows a band which is assigned to a ν(AgP) mode, an assignement which is confirmed by the observation of similar bands in the Raman spectra of the isostructural gold(I) complexes [AuX(PPh₃)₂](X = Cl, Br or I). The solid-state cross-polarization magic-angle spinning (CP MAS)³¹P NMR spectra of the silver complexes show multiplets die to ¹J(AgP) coupling. The spectra of the dimers show separate chemical shifts for the crystallographically inequivalent phosphorus atoms, and ²J(PP) coupling between these atoms. The splitting patterns are interpreted in terms of the silver co-ordination environment.