Molecular orbital calculations on transition metal complexes. Part XXIV. Bis-(π-pyridine)chromium and its cation

Abstract

INDO SCF MO calculations have been carried out for the formally d 6 system bis-(π-pyridine)chromium, Cr(C 5H 5N) 2, and the corresponding cation. For the neutral molecule the results indicate that the introduction of the nitrogen heteroatom should lead to an ionisation potential about 1.5 eV greater than for the parent bis-(π-benzene) chromium, but as for Cr(C 6H 6) 2 the d-electron ionisation energies follow the ordering a 1(d z 2 ) < e 2(d x 2−y 2 , d xy), and the dominant contributions to the bonding again arise from interactions between the metal e 2(d x 2−y 2 , d xy) and e 1(d xz, d yz) levels and the π-orbitals of the ligand rings. Results for the cation, [Cr(C 5H 5N) 2] +, yield estimates for the one-electron d-orbital splitting parameters, ΔE 1 and ΔE 2, which are quite similar to those found for [Cr(C 6H 6) 2] +, and g ⊥ and A H values comparable to those of the bis-benzene system are expected for the 2A 1(e 2 4a 1) ground state, although the 14N hyperfine interaction should be negligible. Calculations are also made of all the proton hyperfine coupling constants for the experimentally accessible bis-(π-2,6-dimethylpyridine)chromium cation. The calculated splittings of the mainly 3d e 1 and e 2 levels, due to the heteroatom, are in all cases small (<3,000 cm −1), and in contrast to the bis-(π-borabenzene) metal, M(C 5H 5BR) 2, species, the bis-(π-pyridine) derivatives behave in a very similar manner to the homocyclic bis-(π-benzene) systems.