Computational Investigations of Metal-Metal Bonding in Molecular Thorium Compounds and Ce and Group IV Analogues.

Abstract

We report quantum chemical investigations of metal-metal bonding in molecular thorium clusters and compare them with cerium and group IV transition metal analogues. We explore periodic trends in metal-metal bonding and the roles of electron delocalization, orbital diffuseness, and oxidation state. As cluster size increases in the series [{Th(η⁸-COT)Cl₂}_nK₂] (COT = C₈H₈, n = 2-5, 2-5) and [Th₄Cl₄(η⁸-COT)₄]²⁺ (T), n-center-2-electron bonding weakens. Quantum theory of atoms in molecules (QTAIM) analysis finds Th-Th bond paths only in 2 and 3, while T exhibits a non-nuclear attractor, indicating charge concentration in the [Th₄Cl₄]¹⁰⁺ core. In the Ce analogues, Ce-Ce bonding is observed only in oxidation states below +3. Calculations on [Ce₃Cl₆]^z (z = 1-3) and Cp-stabilized analogues show Ce-Ce bond shortening with increasing population of the 3-center MOs. The QTAIM confirms Ce-Ce bond paths in [Ce₃Cl₆]⁺, [Ce₃Cl₆(η⁵-Cp)₃]^-, and [Ce₃Cl₆(η⁵-Cp)₃K₂]. Group IV analogues reveal variations in metal-metal bonding on progressing from the contracted 3d AOs of Ti to the more diffuse 6d AOs of Rf. In [M₃Cl₆(η⁸-COT)₃K₂] (3-M), only 3-Rf exhibits QTAIM bond paths similar to the Th analogue, suggesting that only the 6d orbitals are sufficiently diffuse as to support such interactions.