Structure and equilibria in triorganolead halide adduct formation

Abstract

The Lewis acidities of triorganolead halides have been studied by 31P and 207Pb NMR spectroscopy and by calorimetry. The formation of 1:1 adducts with a variety of mono- and bidentate bases was demonstrated by the linearity of the plots of chemical shift versus (shift/conc.) 1 2 . The 207Pb chemical shifts of the bidentate adducts indicated only 5-coordinate lead and hence the absence of chelation. Towards triphenyllead chloride basicities varied in the order triethylphosphine oxide > tributylphosphine oxide ⩾ triphenylphosphine oxide > DMSO > tributylphosphine ⩾ pyridine. Triphenyllead chloride was found to be a stronger acid (larger K) than both triphenyllead bromide and triethyllead chloride. The effects of solvent on the equilibrium constants paralleled the trend reported previously for organotin halides. The equilibrium constant for adduct formation with triphenylphosphine oxide (TPPO) is slightly larger for triphenyllead chloride relative to triphenyltin chloride, confirming the expected increase in acidities down Group 14. X-Ray diffraction studies of triphenylphosphine oxide adducts of triphenyllead bromide and triphenyltin chloride showed both structures to be trigonal bipyramidal with the phenyl groups on the equatorial positions. Additionally, the structures are crystallographically isomorphous.