Attribution of Water-Exchange Mechanisms of Transition-Metal Hexaaqua Ions Using Quantum Chemical Methods

The mechanism for the water-exchange reaction with the transition-metal aqua ions from ScIII through ZnII has been investigated. The exchange mechanisms are analyzed on a model that involves the metal ion with six or seven water molecules. The structures of the reactants/products, transition states, and penta- or heptacoordinated intermediates have been computed with Hartree-Fock or CAS-SCF methods. Each type of mechanism, associative, concerted, or dissociative, proceeds via a characteristic transition state. The calculated activation energies agree with the experimental ΔG‡298 or ΔH‡298 values, and the computed structural changes indicate whether an expansion or compression takes place during the transformation of the reactant into the transition state. These changes are in perfect agreement with the changes deduced from the experimental volumes of activation. The dissociative mechanism is always feasible, but it is the only possible pathway for high-spin d8, d9, and d10 systems. In contrast, the associative mechanism requires that the transition-metal ion does not have more than seven 3d electrons. Thus, ScIII, TiIII, and VIII react via the A, NiII, CuII, and ZnII via the D (or Id) mechanism, whereas all pathways are feasible for the elements in the middle of the periodic table

The Water-Exchange Mechanism of the [UO2(OH2)5]2+ Ion Revisited: The Importance of a Proper Treatment of Electron Correlation

The water‐exchange mechanism of [UO2(OH2)5]2+ has been reinvestigated by using ab initio molecular orbital (MO) methods. The geometries and the vibrational frequencies were computed with CAS‐SCF(12/11)‐SCRF and CAS‐SCF(12/11)‐PCM methods, which take into account static electron correlation (using the complete active space self‐consistent field (CAS‐SCF) technique, based on an active space of 12 electrons in 11 orbitals) and hydration (using the self‐consistent reaction field (SCRF) and polarizable continuum model (PCM) techniques). The total energies were computed with multiconfiguration quasi‐degenerate second‐order perturbation theory, the MCQDPT2(12/11)‐PCM method, which treats static and dynamic electron correlation as well as hydration. The adequacies of other currently used quantum chemical methods, MP2, CCSD(T), B3 LYP, and BLYP, are discussed. For the associative and dissociative pathways, thermodynamic activation parameters (ΔH ≠, ΔS ≠, and ΔG ≠) were computed. For the associative mechanism, the calculated ΔH ≠ and ΔG ≠ values agree with experiment, whereas for the dissociative mechanism, they are higher by ≈20 kJ mol−1. The dissociative mechanism is preferred for substitution reactions of uranyl(VI) complexes with ligands that are stronger electron donors than water. The question of whether a concerted (Ia or Id) or a stepwise (A or D) mechanism operates is discussed on the basis of the computed lifetime of the respective intermediate, and the duration of the vibration with which the intermediate is transformed into the product

Base hydrolysis of acidato pentaamine complexes with inert metal centers: electronic structure of the intermediates, requirements for their formation, and the unique reactivity of the complexes of cobalt(III)

Molecular-orbital calculations have been performed for the conjugate base cis-[Co(NH3)4(NH2)Cl]+ and trans-[M(NH3)4(NH2)Cl]+ (M = CrIII, CoIII, and RhIII), the hexacoordinated intermediates cis- and trans-[Co(NH3)4(NH2)Cl]+, the square-pyramidal and trigonal bipyramidal pentacoordinated intermediates apical-[Co(NH3)4(NH2)]2+, basal-[Co(NH3)4(NH2)]2+, and equatorial-[M(NH3)4(NH2)]2+ (M = Co and Rh), respectively, using modified extended Hückel and SCF MS-X methods. The LUMO of the conjugate bases is an antibonding metal centered d* orbital which is stabilized during the dissociative activation of the MCl bond. For the above conjugate bases, separations of the highest doubly occupied MO (HDOMO) and the LUMO of 1.3, 2.5, 1.8, and 2.5 eV, respectively, have been calculated using the SCF MS-X model. Only in the conjugate base cis-[Co(NH3)4(NH2)Cl]+ with the smallest HDOMO - LUMO gap, the singlet electronic structure of the ground state may be stabilized by changing into a triplet when the CoCl bond is activated. This situation is unique to (acidato)(pentaamine)cobalt(III) complexes with deprotonated amine ligand cis to the leaving group and the reason for the existence of intermediates and their reactivity as well. Base hydrolysis of the analogues CrIII and RhIII complexes - the latter taken to represent the second- and third-row transition-metal amines - is unlikely to proceed via intermediates; a concerted substitution process is expected to take place