Synthesis and spectroscopic studies of phenyllead halide and thiocyanate adducts with hexamethylphosphoramide

Hexamethylphosphoramide (HMPA) adducts of the type Ph₃PbX·HMPA (X=Cl, Br, I, and NCS), Ph₂PbX₂·2HMPA (X=Cl, Br, and I), and Ph₂PbX₂·HMPA (X=Br and I), have been prepared and characterized by infrared, Raman, mass, and ³¹P nmr spectroscopy. Molecular weight and infrared solution data show that Ph₃PbX·HMPA adducts dissociate in benzene, the degree of dissociation being NCS«Cl<Br<I. The thiocyanate adducts Ph₃PbNCS·HMPA and Ph₂Pb(NCS)₂·2HMPA have v(CN) and v(CS) frequencies in the solid state, and v(CN) frequencies and absorptivities in benzene solution consistent with N-bonded thiocyanate in the solid state and in benzene solution. Vibrational frequencies are reported in the range 260 to 80 cm⁻¹ and assignments are made for v(Pb-X), v(Pb-O0, and v(Pb-NCS) modes. The 1:1 adducts Ph₃PbX·HMPA are monomeric and trigonal bipyramidal, whereas the 1:2 adducts Ph₂PbX₂·2HMPA are monomeric and cis-octahedral and the Ph₂PbX₂·HMPA appear to be halogen bridged polymers with lead six-coordinate. Coordination of HMPA causes a small upfield change in ³¹P chemnical shift values, and ²J(Pb-P) values vary with X in the order: NCS>I-Br>Cl for Ph₃PbX·HMPA adducts. Corresponding tin and lead adducts are compared with respect to mode of adduct formation.