The physical chemistry of coordinated aqua-, ammine-, and mixed-ligand Co2+ complexes: DFT studies on the structure, energetics, and topological properties of the electron density

Abstract

Spin-unrestricted DFT-X3LYP/6-311++G(d,p) calculations have been performed on a series of complexes of the form [Co(H(2)O)(6-n)(NH(3))(n)](2+) (n = 0-6) to examine their equilibrium gas-phase structures, energetics, and electronic properties in their quartet electronic ground states. In all cases Co(2+) in the energy-minimised structures is in a pseudo-octahedral environment. The calculations overestimate the Co-O and Co-N bond lengths by 0.04 and 0.08 A, respectively, compared to the crystallographically observed mean values. There is a very small Jahn-Teller distortion in the structure of [Co(H(2)O)(6)](2+) which is in contrast to the very marked distortions observed in most (but not all) structures of this cation that have been observed experimentally. The successive replacement of ligated H(2)O by NH(3) leads to an increase in complex stability by 6 +/- 1 kcal mol(-1) per additional NH(3) ligand. Calculations using UB3LYP give stabilisation energies of the complexes about 5 kcal mol(-1) smaller and metal-ligand bond lengths about 0.005 A longer than the X3LYP values since the X3LYP level accounts for the London dispersion energy contribution to the overall stabilisation energy whilst it is largely missing at the B3LYP level. From a natural population analysis (NPA) it is shown that the formation of these complexes is accompanied by ligand-to-metal charge transfer the extent of which increases with the number of NH(3) ligands in the coordination sphere of Co(2+). From an examination of the topological properties of the electron charge density using Bader's quantum theory of atoms in molecules it is shown that the electron density rho(c) at the Co-O bond critical points is generally smaller than that at the Co-N bond critical points. Hence Co-O bonds are weaker than Co-N bonds in these complexes and the stability increases as NH(3) replaces H(2)O in the metal's coordination sphere. Several indicators, including the sign and magnitude of the Laplacian of the charge density nabla(2)rho(c), the ratio of the local potential and kinetic energy densities, |V(c)|/G(c), the sign of the total energy density H(c), and the delocalisation index delta(Co,X), X = O, N, are used to show that whilst the metal-ligand bonds are predominantly ionic in nature, they gain covalent character as NH(3) replaces H(2)O, and the Co-N bond is significantly more covalent than the Co-O bond. We have shown that the delocalisation index delta(Co,X), X = O, N, is strongly correlated with the zero-point corrected stabilisation energy E demonstrating that delta can be used as a measure of the bond stability in these complexes.

Antioxidant and antimalarial properties of butein and homobutein based on their ability to chelate iron (II and III) cations: a DFT study in vacuo and in solution

“…In such systems, the value of H is usually negative and close to zero, as found for strong interactions, but with a positive value of ∇ 2 ρ, which is a characteristic of weak interactions [75,76]. Another parameter that is utilised in the characterisation of the bonding is the |V|/G ratio: |V|/G < 1 represents closed-shell interactions; |V|/G > 2 represents shared interactions; 1 < |V|/G < 2 represents interactions of intermediate character [77,78]. The bond critical point data for butein and homobutein complexes with Fe n+ cations are reported in Tables 5 and 6, respectively, for the results in vacuo.…”

The Nature of Bond Critical Points in Dinuclear Copper(I) Complexes

“…More recently, Jenkins and Morrison, [23] Espinosa et al, [24] and subsequently Varadwaj and Marques [25] have suggested that the ratio of the potential-energy density j V c j and the kinetic-energy density G c at the critical point can be used to characterize a bond in a better fashion. When j V c j /G c < 1, interactions in a chemical system are characteristic of closed-shell interactions; those with j V c j /G c > 2 are typically covalent interactions; and when 1 < j V c j /G c < 2, they are of intermediate character.…”

“…When j V c j /G c < 1, interactions in a chemical system are characteristic of closed-shell interactions; those with j V c j /G c > 2 are typically covalent interactions; and when 1 < j V c j /G c < 2, they are of intermediate character. Varadwaj and Marques [25] have made use of this condition to characterize the bond between CoÀN and…”

“…The ratio j V c j /G c is claimed to be a better descriptor of bonding. [23][24][25] Metal-metal bonding in several metal clusters has also been examined by using AIM. [26] To the best of our knowledge, very few studies exist on the BCP in dinuclear or polynuclear copper complexes.…”

DFT/TDDFT study on the electronic structures and optoelectronic properties of a series of iridium(III) complexes based on quinoline derivatives in OLEDs

“…The discrepancy between our calculations and experiments is not only due to the fact that the theoretical results refer to the gas phase with respect to the close-packed crystal lattice in experiment, but it is well known that B3LYP overestimates the structural parameters (particularly bond lengths) in transition metal complexes. [27][28][29] Table 1 shows that in the assumed complexes 3-6, their corresponding ligands are nearly coplanar because the dihedral angles C3-C5-C7-C9 and C4-C6-C8-C10 are close to 0 , indicating that they possess rigid skeletal structures, which may effectively prevent the nonradiative decay. The rigid skeletal structures result in the shorter Ir-N bonds for 3-6 than those of 1 and 2.…”