Ligand Field Stabilization and Activation Energies Revisited: Molecular Modeling of the Thermodynamic and Kinetic Properties of Divalent, First-Row Aqua Complexes

Abstract

Ligand Field Molecular Mechanics (LFMM) parameters have been developed for the hexaaqua complexes of the divalent first row metals V (2+) through to Zn (2+). The LFMM parameters provide an explicit description of the d electron stabilization energies and are tuned to reproduce both the experimental structures and the relative energetics of the hexaaqua complexes in aqueous solution plus Johnson and Nelson's data ( Inorg. Chem., 1995, 34, 5666-5671) for hypothetical complexes from which the ligand field stabilization energy has been removed. The LFMM calculations automatically reveal the typical, strong Jahn-Teller distortions of the d (4) Cr(II) and d (9) Cu(II) species plus the structurally smaller, but still energetically significant, distortions of the d (6) Fe(II) and d (7) Co(II) complexes. The latter are largely mediated via M-O pi interactions which are explicitly treated in the LFMM method via the Angular Overlap Model e pi perpendicular parameter which treats the pi interactions perpendicular to the ligand plane. Without further parameter modification, we then predict approximate reaction barriers for both associative and dissociative ligand exchange based on simplified model pathways. The correlation between computed barrier heights and experimental exchange rate constants is generally excellent with the slowest reactions (involving V and Ni) having the largest barriers while the fastest reactions (Cu and Cr) have near zero barriers. In agreement with experiment, the LFMM model further confirms that k(V) < k(Ni) and k(Co) < k(Fe) which contrasts with the classical crystal field activation energy analysis of Basolo and Pearson which predicts k(V) = k(Ni) and the reverse order of reactivity for Fe and Co species. LFMM further predicts a mechanistic changeover from an associatively activated process at Mn to dissociatively activated exchange for Fe, Co, and Ni which is consistent with experimental activation volumes and previously published high level quantum chemical calculations.