INVESTIGATION OF THE LIGAND-EXCHANGE REACTION FOR THE AQUEOUS BE2+ ION

The geometry and energy of the transition state for the exchange of wa ter-coordinating Be2+ in aqueous solution has been investigated by Har tree-Fock (HF) cluster calculations, which include the ion and its nea r-neighbors. The calculated activation parameters, Delta V and Delta H, are compared to the spectroscopic values. The HF activation volume Delta V was found to be similar to 0 compared to the experimental va lue of -13.6 cm(3)/mol. The discrepancy is attributed to an incomplete description of the second coordination shell, which is needed to accu rately define the volume of the transition-state complex. Including a continuum approximation for the solvent did not improve the agreement. The HF activation enthalpy for the exchange reaction, 0.65 eV, is in good agreement with the experimentally determined value of 0.61 eV. A qualitative description of the reaction mechanism has been developed f rom calculations on 5-fold coordinated Be2+ with one Be-OH2 distance c onstrained to the reaction coordinate. The results suggest a reaction scheme where the exchanging water from the second coordination shell i s oriented along a 2-fold axis of the tetramer. Optimization of the co mplex at the saddle point of the reaction surface resulted in a C-2v t ransition state with approximate trigonal-bipyramidal geometry. The Be -H2O bonding is dominated by electrostatic interactions with little ev idence of dative or covalent bonding between the water lone pairs and the empty 2s and 2p orbitals on Be2+.