Force constant calculations for the in-plane vibrations of anions [MX3CO]- (where MX is PdCl, PdBr, PtCI, PtBr or Ptl)

Force constants for planar complexes [MX 3 CO]- (MX = PdCl, PdBr, PtC1, PtBr and PtI) have been calculated using a modified valence force field, with constrained off-diagonal force constants based on results of [MX 4 ] 2− systems. The resulting stretching force constants are shown to have little dependence on choice of skeletal deformation assignments. The stretching force constants to palladium are lower than to platinum. The difference is more marked for the MC bonds than for MX bonds demonstrating the much weaker π-donor ability of 4d orbitals of Pd(ll) than 5d of Pt(II). The relationship between MX force constants in [MX 4 ] 2− [M 2 X 6 ] 2− and [MX 3 CO]− is discussed.