First-principles study of energetic complexes (II): (5-Cyanotetrazolato- N²) pentaammine cobalt (III) perchlorate (CP) and Ni, Fe and Zn analogues

First-principles methods using the TPSS density functional level of theory with the basis set 6-31G^** were applied to study (5-cyanotetrazolato-N²) pentaammine cobalt (III) perchlorate (CP) and Ni, Fe and Zn analogues in the gas phase. The optimized lowest-energy geometry of CP was calculated from reported experimental structural data using the TPSS method. The calculated values are in good agreement with those measured by X-ray diffraction. Ni, Fe and Zn analogues were constructed and calculated on the same basis. NBO results showed that the metal-ligand interactions have covalent character. Donor-acceptor analyses predicted that the delocalization energy E₂ decreases from Co to Zn, so the covalent nature of the complexes increases in the order Co>Fe>Ni>Zn. In addition, HOMO-LUMO composition was investigated to determine the stability of the title compounds.