Octahapto cyclooctatetraene rings and metal-metal multiple bonds in binuclear niobium carbonyl chemistry

Theoretical studies on (C(8)H(8))(2)Nb(2)(CO)(n) (n = 6, 5, 4, 3, 2, 1) predict structures mainly with octahapto and tetrahapto C(8)H(8) rings. In all cases, the lowest energy singlet spin state structures lie below the corresponding lowest energy triplet spin state structures. Thus the lowest energy (C(8)H(8))(2)Nb(2)(CO)(4) structure has two eta(8)-C(8)H(8) rings and an unbridged NbeNb single bond of length similar to 3.15 angstrom. The lowest energy (C(8)H(8))(2)Nb(2)(CO)(n) structure has two eta(8)-C(8)H(8) rings but a doubly bridged Nb Nb triple bond of length similar to 2.64 angstrom. The lowest energy structure of ((C(8)H(8))(2)Nb(2)(CO)(3) also has a formal Nb Nb triple bond of similar length (2.66 ) but with only one of the rings fully coordinated as an octahapto eta(8)-C(8)H(8) ligand. The other C(8)H(8) ring in this tricarbonyl has " slipped" to form a hexahapto eta(6)-C(8)H(8) ligand. The lowest energy structure of the monocarbonyl ((C(8)H(8))(2)Nb(2)(CO) again has two octahapto eta(8)-C(8)H(8) rings and an extremely short Nb Nb distance of 2.45 angstrom, suggesting a formal quadruple bond. The lowest energy structures for the carbonyl-richer species ((C(8)H(8))(2)Nb(2)(CO)(n) (n = 6, 5) have one eta(8)-C(8)H(8) and one eta(4)-C(8)H(8) ring (n = 5) and two eta(4)-C(8)H(8) rings (n = 6). The qualitatively assigned NbeNb bond orders are consistent with the Wiberg bond indices obtained from the Weinhold natural bond orbital analysis. Comparison of the ((C(8)H(8))(2)Nb(2)(CO)(n) (n = 6, 5, 4, 3, 2, 1) derivatives with the isovalent (C(7)H(7))(2)Mo(2)(CO)(n) is made. (C) 2011 Elsevier B. V. All rights reserved.