A study of complexes Mg(NH3)(n)(+center dot) and Ag(NH3)(n)(+), where n=1-8: Competition between direct coordination and solvation through hydrogen bonding

Density functional calculations at B3LYP/6-31+G(d) and B3LYP/DZVP are reported for Mg(NH3)(n)(+.), where n = 1-6 and for some solvated ions Mg(NH3)(n)(+.)...NH3 (n = 1-3, 6). After correction for basis set superposition errors, the enthalpies for sequential addition of NH3 to Mg+. resulting from direct coordination to the metal are 38.1, 26.6, 21.2, 13.7, 12.1, and 11.3 kcal mol(-1). The free energies for these same addition reactions are all negative, although for complexes with n greater than or equal to 4 the values are very small. Attempts at optimising structures with higher coordination numbers all resulted in the formation of solvated octahedral complexes. Enthalpies for solvation through hydrogen bonding to one of the ligated NH3 molecules are all less than 16 kcal mol(-1) and decrease rapidly as the number of ligated NH3 molecules increases. Molecular orbital calculations at B3LYP/DZVP have been used to optimise structures for ions Ag(NH3)(n)(+), where n = 1-6. The five-coordinate and six-coordinate structures have very small binding enthalpies (4.3 and 2.6 kcal mol(-1)) and the free energies for formation of these ions are positive. The binding energies for the addition of the first and second NH3 molecules added to Ag+ are 40.1 and 36.1 kcal mol(-1), while those for the third and fourth additions are much smaller (15.1 and 11.0 kcal mol(-1)). Adducts up to n = 3 have been detected in electrospray experiments. The first three adducts of Ag+ With NH3 have been formed in the selected ion flow tube apparatus and multicollision induced dissociation experiments show Ag(NH3)(3)(+) to have a lower binding enthalpy than both Ag(NH3)(2)(+) and Ag(NH3)(+). (C) 2000 Elsevier Science B.V.