Structure of the Transition States and Intermediates Formed in the Water-Exchange of Metal Hexaaqua Ions of the First Transition Series

Abstract

The structures of the transition states and intermediates formed in the water-exchange of hexaaqua complexes of the first row transition elements have been computed with ab initio methods at the Hartree−Fock or CAS-SCF level. As an approximation, water molecules in the second coordination sphere except one, bulk water, and anions have been neglected. For each of the three types of activation, namely associative, concerted, and dissociative mechanism, a representative transition metal complex has been studied, viz. Ti(OH2)6 3+, V(OH2)6 2+, and Ni(OH2)6 2+. Each type of mechanism proceeds via a characteristic transition state. For the A and D mechanisms, respectively, hepta- or pentacoordinated intermediates are formed, and their lifetimes were estimated based on the energy difference between that of the transition state and the corresponding intermediate. The computed activation energies are in agreement with the experimental values and are independent of the mechanism or the charge on the metal center. The bond length changes occurring during the activation agree with the corresponding experimental ΔV ⧧ values. In a recent article, Åkesson et al. (J. Am. Chem. Soc. 1994, 116, 8705) proposed an interpretation of the experimental ΔV ⧧ values that differs from that commonly applied (Merbach, A. E. Pure Appl. Chem. 1987, 59, 161). In particular, they claimed a dissociative activation for the water-exchange of the hexaaqua ions of VII and MnII in spite of their negative volumes of activation. The present computational results on VII are in perfect agreement with the Ia mechanism attributed on the basis of its ΔV ⧧ value. It should be noted that in principle, the D mechanism is possible for all the hexaaqua ions of the first transition series, but in many cases, the associative or concerted pathway is preferred. For a given complex, all the possible mechanisms must be analyzed, before the most favorable pathway can be determined. The presently studied case of VII, where equal energies of activation have been computed for the Ia and D mechanism, illustrates this point. The attribution of the mechanism was only possible by comparison with the experimental volume of activation. Computed energies of activation alone may not suffice to identify the mechanism; a safe attribution can only be made if the structural changes agree with the volume of activation.

Ligand Exchange Processes on Solvated Zinc Cations – DFT Analysis of Hydrogen Cyanide Exchange on [Zn(HCN)6]2+

“…We attribute the difference in stability between the two transition states to bond weakening of the five equatorial HCN 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 Table 1). Rotzinger [14,27] suggested as a quantum-chemical descriptor for the activation volume that the sign of the change of the sum of all M-ligand bond lengths (∆∑) during the activation process should agree with the sign of ∆V ‡ . In accordance with the expected trend, the calculated change in the Zn-N bond lengths during the exchange reaction (∆∑) were found to be positive for the limiting dissociative mechanism and negative for the associative interchange mechanism (see Table 1).…”

Reply to the Comment on “The Water‐Exchange Mechanism of the [UO2(OH2)5]2+ Ion Revisited: The Importance of a Proper Treatment of Electron Correlation” [F. P. Rotzinger, Chem. Eur. J. 2007, 13, 800]

“…[18,19] Rather, with two H 2 Os in the second sphere, many isomers exist, but there are no criteria for the elucidation of the "right" isomer. Thus, VWG should have studied all of these isomers instead of only one.…”

[CpRu((R)-Binop-F)(H2O)][SbF6], a New Fluxional Chiral Lewis Acid Catalyst: Synthesis, Dynamic NMR, Asymmetric Catalysis, and Theoretical Studies

“…36 However, quantum mechanics and molecular mechanics (QM/MM) approaches seem to be well suited for performing explicit solvation simulations. 37, 38 For the calculation of solvent exchange or racemization processes, the solvent is frequently neglected [39][40][41][42] or treated implicitly using a continuum model. 43,44 In this study, solvent effects were taken into account statically by means of polarized continuum model (PCM) calculations 45 using a self-consistent field technique as implemented in Gaussian03.…”