Quantum-chemical analysis of formation reactions of Со²⁺ complexes

Abstract

Based on the analysis of quantum chemical calculations results (GAMESS, density functional theory, B3LYP method) as to coordination compounds of Co²⁺ions with H₂O, NH₃, OH^–, F^–, Cl^–, Br^–, I^–, CN^–, Ac^–, Ak^– generally given by
[Co(H₂O)_6–nL_n]^2+nx, it has been demonstrated that within the selected series of ligands, there is no correlation between the amount of energy of monosubstituted cobalt aqua complexes formation(∆Е) and pK₁,just like between the effective nuclear charge of the central atom (z*_Со) and pK₁. According to the behavior of ∆Е and z*_Со,we identified two groups of ligands. The first group (OH^–, F^–, Ac^–, Ak^–, CN^–, NH₃) demonstrates logical ∆Е decrease caused by the growth of z*_Со. On the contrary, the second group (Cl^–, Br^–, I^–) demonstrates ∆Е increase caused by the growth of z*_Со. This phenomenon is explained by the change in electronegativity and polarizability of donor atoms in groups and periods of the periodic table. It is established that linear correlations given by lgK = A + B·z*_Со can be actualized only for complexes having ligands with similar donor atoms. Referring to the literature on stepwise complex formation of hydroxide, amine and chloride cobalt complexes in combination with z*_Со calculations results, we determined A and B constants of lgK, z*_Со-correlations for the atoms of oxygen (30.2, –17.7); nitrogen (125.4, –69.9) and chlorine
(–6.3, 5.8). The existence of the detected correlation series enables us to lean on lgK,z^*_М–dependence parameters for the fixed donor atom and to determine K_n values for various complexes with complex-based ligands using calculations and z^*_М data. This applies to complexes having central atoms of the same nature as well as simple monodentate ligands. The mentioned approach was used to calculate the stability constants for acrylate cobalt complexes (lgK₁ = 1.2 и lgК₂ = 4.3), which are not covered in literature.