Theoretical studies on the multiple metal–metal bonds in the bimetallic molecules and the ultrashort V–Mn bonds in the complexes

The present work analyzed the multiple bimetallic bond of Cr–Cr, V–Mn, V–Tc, Nb–Mn, Cr–Mo, Mo–Mo, and Nb–Tc using the multiconfiguration second-order perturbation theory (CASPT2) in conjunction with treating the relativistic effects by calculating the spin–orbit coupling and using ANO-RCC basis set and the DKH Hamiltonian. The bimetallic bond length, natural bond order (NBO), effective bond order (EBO), formal shortness ratio (FSR) values, and Mulliken charge populations are calculated using the multiconfigurational methods. Our calculations indicate that the V–Mn molecule has smaller bond length value and larger FSR value than other heterobimetallic molecules, and the V–Mn bimetallic core could be considered as a good candidate to investigate the multiple heterobimetallic bonds in complexes. And then the VMn(NHCHNH)2 and VMn(NHCHO)2 molecules were chosen as two models to study the influences of the ligands using the CASSCF and BP86 methods. The steric hindrances and electronic effects of the substituent groups on the ligands were considered by calculating four kinds of complexes with 2-aminopyridine, 2-hydroxypyridine, amidinate, and guanidinate as ligands using the BP86, BPW91, and B3LYP methods. On the basis of our calculations, we would conclude that the ligands with the O or N atom coordinated to the V–Mn core could elongate the V–Mn bond significantly. The steric hindrances from the substituent groups on the ligands could shorten the V–Mn bond to a certain extent and the electronic effects of the substituents could elongate or shorten the V–Mn bond depending on the properties and the positions of the substituents on the ligands. But it may be unfeasible to contract the bimetallic bond length in complex to a certain limit as that in the bimetallic molecule.