The physical chemistry of [M(H2O)4(NO3)2] (M = Mn2+, Co2+, Ni2+, Cu2+, Zn2+) complexes: computational studies of their structure, energetics and the topological properties of the electron density

The complexes of M2+ (M = Mn, Co, Ni, Cu, Zn), trans-[M(H2O)4(NO3)2] in their high-spin ground electronic states have been investigated theoretically for the first time using several correlated DFT levels as well as with the MP2 method in conjunction with two different basis sets, 6-311++G(d, p) and LANL2TZ+/6-311++G(d, p), to examine their equilibrium structures and stabilities. Among the correlated methods, the X3LYP level together with the 6-311++G(d, p) basis set gives the best estimate of geometries in these complexes. The metal–ligand binding energies obtained follow the trend Cu2+ > Ni2+ > Zn2+ > Co2+ > Mn2+ across the series examined, in precise agreement with the Irving-Williams series. The DFT methods largely overestimate the binding energies compared to the MP2 level and the trend follows the order PW91PW91 < PBEPBE < X3LYP < B3LYP < MP2. The use of an ECP basis on the central metal cation and a 6-311++G(d, p) basis set on the main group elements increases the binding energies of the complexes compared to that found using the full-core basis set 6-311++G(d, p) and the energy difference between them can be as large as 20 kcal mol−1. There are significant differences between the structures calculated in the gas phase and those calculated with the PCM model to simulate the effect of solvent. Solvation shortens the M–OH2 bonds and lengthens the M–ONO2 bonds such that the difference between the computed and the crystallographically observed bond lengths tends to decrease; it increases complex stability; and that it leads to the disappearance of two intramolecular H bonds between OH2 and NO3– ligands that are present in the gas-phase structures. While there are differences between the natural populated atomic charges and Bader’s approach of the quantum theory of atoms in molecules (QTAIM), all show charge transfer from the ligands to the metal ion. However, the MP2-level-computed charges were found to be unreliable compared with the DFT-derived charges. The metal–ligand bonding and the intramolecular H bonding in the complex are explored with QTAIM and the insight gained into the electronic structure of these complexes is discussed.