Interactions of metal ions with water: Ab initio molecular orbital studiesof structure, vibrational frequencies, charge distributions, bonding enthalpies, and deprotonation enthalpies. 2. Monohydroxides

The formation and properties of a wide range of metal ion monohydroxides, M n+[OH-], where n = 1 and 2, have been studied by ab initio molecular orbita l calculations at the MP2(FULL)/6-311++G**//MP2(FULL)/6-311++G** and CCSD(T )(FULL)/6-311++G**//MP2(FULL)/6-311++G** computational levels. The ions Mn are from groups 1A, 2A, 3A, and 4A in the second, third, and fourth period s of the Periodic Table and from the first transition series. Geometrical p arameters, vibrational frequencies, atomic charge distributions, orbital oc cupancies, and bonding enthalpies are reported. The Mn+-O distances are sho rter in the hydroxides than in the corresponding hydrates (published previo usly as Part 1, Inorg. Chem. 1998, 37, 4421-4431) due to a greater electros tatic interaction in the hydroxides. The natural bond orbitals for most of the first-row transition metal ion hydroxides do not contain a formal metal -oxygen bonding orbital; nevertheless the atomic charge distributions show that for both n = I and 2 a significant amount of electron density is consi stently transferred from the hydroxide ion to the bound metal ion. Deproton ation enthalpies for the hydrates have been evaluated according to the simp le dissociation process, Mn+[OH2] --> Mn+[OH-] + H+, and also via proton tr ansfer to another water molecule, Mn+[OH2] + H2O --> Mn+[OH-] + H3O+. The d rastic reduction in these deprotonation enthalpies as H2O molecules are seq uentially bonded in the first coordination shell of the metal ion (amountin g to 71, 64, 85, and 91 kcal/mol for the bonding of six water molecules to Mg2+, Ca2+, Mn2+, and Zn2+, respectively) is found to be due to the greater decrease in the bonding enthalpies for the hydroxides relative to the hydr ates. Proton transfer to bases other than water, for example side chain gro ups of certain amino acids, could more than offset the decrease in deproton ation energy due to the filling of the first coordination shell. Linear rel ationships have been found between the pK(a) values for ionization of the M g2+, Ca2+, Mn2+, Fe2+, CO2+, Ni2+, Cu2+, and Zn2+ aquo ions, and DeltaH deg rees (298) for the bonding of the first water molecule, for the bonding of the hydroxide ion, and for proton dissociation from the monohydrate. Simila r relationships have also been found between the pKa values and the recipro cal of the M-O bond lengths in both the monohydrates and hydroxides. Thus t he ionization of metal hydrates in water echoes the properties of the monom eric species Mn+[OH2].