A computational study of the electronic structure, bonding, and spectral properties of tripodal tetraamine Co(III) carbonate complexes

Density functional calculations have been carried out on the experimentally characterized Co(iii) [Co(N₄)(O₂CO)]⁺ carbonate complexes containing a tripodal tetraamine ligand (N₄ = tpa, Metpa, Me₂tpa, Me₃tpa, pmea, pmap, tepa) and also the model [Co(NH₃)₄(O₂CO)]⁺ system. Calculations on the model species, performed using both gas-phase and solvent-corrected procedures, have revealed that the inclusion of a condensed-phase environment is necessary to obtain generally satisfactory results for the structural and bonding properties in these systems. Using the solvent-corrected approach, the observed trends in structural parameters for the metal-ligand bonds, ⁵⁹Co chemical shifts, and changes in visible absorption wavelengths have been satisfactorily reproduced for the [Co(N ₄)(O₂CO)]⁺ complexes. A time-dependent density functional analysis of the electronic excitations indicates that the overall composition and character of the relevant (d-d) transitions remain similar throughout the series, indicating that the changes in the Co-N interactions, associated with the structural variations occurring as the N-donor ligand identity and size change, appear most likely responsible for the particular spectroscopic features displayed by these species. These observations are further supported by molecular orbital and energy decomposition analyses. The results from the present calculations confirm recent findings that the inclusion of a treatment for solvent effects plays a critical role in the computational modelling of coordination complexes involving mixed (anionic and neutral) ligands.