Bent's Rule and the Structure of Transition Metal Compounds

The equilibrium geometries of Me2XCl2 for X = C, Si, Ge, Sn, Pb, Ti, Zr, and Hf are calculated at the HF and MP2 levels of theory using valence basis sets of DZ+P quality. The calculated geometries are in good agreement with experimental gas-phase values. The Cl−X−Cl angle is always smaller than the C−X−C angle when X is a main group element C−Pb. This is in agreement with Bent's rule. The opposite relationship is predicted for the transition metal compounds. The calculated Cl−X−Cl angle is significantly larger than the C−X−C angle for X = Ti, Zr, and Hf. The different order of the Cl−X−Cl and C−X−C angles between the main group and the transition metal compounds is explained by the energy levels of the valence orbitals of the central atom X. The transition metals have mainly sdx-hybridized bonds, while the main group elements have spx-hybridized bonds. The valence s orbital of the main group elements is always below the p valence orbitals, but the valence s orbital of the transition metals is above the v...