Interactions of Metal Ions with Water:  Ab Initio Molecular Orbital Studies of Structure, Vibrational Frequencies, Charge Distributions, Bonding Enthalpies, and Deprotonation Enthalpies. 2. Monohydroxides

• Published in: Inorganic chemistry 2001-08, Vol.40 (17), p.4230-4241
• Main Authors: Trachtman, Mendel, Markham, George D, Glusker, Jenny P, George, Philip, Bock, Charles W
• Format: Article
• Language: English
• Subjects: Metals - chemistry; Molecular Structure; Thermodynamics
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic010008p

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 orbital 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 M n + are from groups 1A, 2A, 3A, and 4A in the second, third, and fourth periods of the Periodic Table and from the first transition series. Geometrical parameters, vibrational frequencies, atomic charge distributions, orbital occupancies, and bonding enthalpies are reported. The M n +−O distances are shorter in the hydroxides than in the corresponding hydrates (published previously as Part 1, Inorg. Chem. 1998, 37, 4421−4431) due to a greater electrostatic 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 = 1 and 2 a significant amount of electron density is consistently transferred from the hydroxide ion to the bound metal ion. Deprotonation enthalpies for the hydrates have been evaluated according to the simple dissociation process, M n +[OH2] → M n +[OH-] + H+, and also via proton transfer to another water molecule, M n +[OH2] + H2O → M n +[OH-] + H3O+. The drastic reduction in these deprotonation enthalpies as H2O molecules are sequentially bonded in the first coordination shell of the metal ion (amounting 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 hydrates. Proton transfer to bases other than water, for example side chain groups of certain amino acids, could more than offset the decrease in deprotonation energy due to the filling of the first coordination shell. Linear relationships have been found between the pK a values for ionization of the Mg2+, Ca2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, and Zn2+ aquo ions, and Δ for the bonding of the first water molecule, for the bonding of the hydroxide ion, and for proton dissociation from the monohydrate. Similar relationships have also been found between the pK a values and the reciprocal of the M−O bond lengths in both the monohydrates and hydroxides. Thus the ionization of metal hydrates in water echoes the properties of the monomeric species M n +[OH2].