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 sd^x-hybridized bonds, while the main group elements have sp^x-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 valence d orbitals. The energetically lower lying valence orbital concentrates in bonds toward the more electropositive methyl substituents yielding bond angles C-X-C > Cl-X-Cl when X is a main group element and C-X-C < Cl-X-Cl when X is a transition metal. It is suggested that Bent's rule should be formulated in a more general way: "The energetically lower lying valence orbital concentrates in bonds directed toward electropositive substituents".