Computational analysis of M-O covalency in M(OC₆H₅)₄ (M = Ti, Zr, Hf, Ce, Th, U)

A series of compounds M(OC6H5)4 (M = Ti, Zr, Hf, Ce, Th, U) is studied with hybrid density functional theory, to assess M-O bond covalency. The series allows for the comparison of d and f element compounds that are structurally similar. Two well-established analysis methods are employed: Natural Bond Orbital and the Quantum Theory of Atoms in Molecules. A consistent pattern emerges; the U-O bond is the most covalent, followed by Ce-O and Th-O, with those involving the heavier transition metals the least so. The covalency of the Ti-O bond differs relative to Ce-O and Th-O, with the orbital-based method showing greater relative covalency for Ti than the electron density-based methods. The deformation energy of r(M-O) correlates with the d orbital contribution from the metal to the M-O bond, while no such correlation is found for the f orbital component. f orbital involvement in M-O bonding is an important component of covalency, facilitating orbital overlap and allowing for greater expansion of the electrons, thus lowering their kinetic energy.