The geometry and energy of the transition state for the exchange of water-coordinating Be2+ in aqueous solution has been investigated by Hartree-Fock (HF) cluster calculations, which include the ion and its near-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 value of -13.6 cm(3)/mol. The discrepancy is attributed to an incomplete description of the second coordination shell, which is needed to accurately 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 from calculations on 5-fold coordinated Be2+ with one Be-OH2 distance constrained to the reaction coordinate. The results suggest a reaction scheme where the exchanging water from the second coordination shell is oriented along a 2-fold axis of the tetramer. Optimization of the complex at the saddle point of the reaction surface resulted in a C-2v transition state with approximate trigonal-bipyramidal geometry. The Be-H2O bonding is dominated by electrostatic interactions with little evidence of dative or covalent bonding between the water lone pairs and the empty 2s and 2p orbitals on Be2+.