Thermodynamics for the clustering of ammonia onto Pb sup + in the gas phase

Equilibrium constants together with the enthalpy, entropy, and Gibbs free energy changes were determined for the gas-phase stepwise addition reactions of up to four ammonia molecules onto the Pb{sup +} ion. Standard enthalpy changes of -28.3, -19.2, -13.0, and -10.7 kcal/mol and entropy changes of -23.3, -27.3, -22.0, and -24.8 cal/K {center dot} mol were measured for the first, second, third, and fourth steps of the clustering, respectively. These results, along with earlier ones for the clustering of ammonia onto Li{sup +}, Na{sup +}, K{sup +}, and Rb{sup +}, provide evidence concerning the structure and types of bonding in metal ion-ammonia clusters. Since the binding in Pb{sup +}NH{sub 3} and Pb{sup +}(NH{sub 3}){sub 2} is much stronger than that expected by electrostatic calculation, partial covalent interaction of Pb{sup +} with NH{sub 3} is likely in these two clusters. Theoretical calculations were also carried out with the Sakur-Tetrode equation to determine the contributions of the translational, rotational, and vibrational components to the entropy change for each of the above clustering reactions.