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

Theoretical studies on (C8H8)2Nb2(CO)n (n = 6, 5, 4, 3, 2, 1) predict structures mainly with octahapto and tetrahapto C8H8 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 (C8H8)2Nb2(CO)4 structure has two η8-C8H8 rings and an unbridged Nb–Nb single bond of length ∼3.15 Å. The lowest energy (C8H8)2Nb2(CO)2 structure has two η8-C8H8 rings but a doubly bridged Nbtriple bond; length of mdashNb triple bond of length ∼2.64 Å. The lowest energy structure of (C8H8)2Nb2(CO)3 also has a formal Nbtriple bond; length of mdashNb triple bond of similar length (2.66 Å) but with only one of the rings fully coordinated as an octahapto η8-C8H8 ligand. The other C8H8 ring in this tricarbonyl has “slipped” to form a hexahapto η6-C8H8 ligand. The lowest energy structure of the monocarbonyl (C8H8)2Nb2(CO) again has two octahapto η8-C8H8 rings and an extremely short Nb6-point quadruple bond; length half of m-dashNb distance of 2.45 Å, suggesting a formal quadruple bond. The lowest energy structures for the carbonyl-richer species (C8H8)2Nb2(CO)n (n = 6, 5) have one η8-C8H8 and one η4-C8H8 ring (n = 5) and two η4-C8H8 rings (n = 6). The qualitatively assigned Nb–Nb bond orders are consistent with the Wiberg bond indices obtained from the Weinhold natural bond orbital analysis. Comparison of the (C8H8)2Nb2(CO)n (n = 6, 5, 4, 3, 2, 1) derivatives with the isovalent (C7H7)2Mo2(CO)n is made.