From two-electron via four-electron to six-electron donor carbonyl groups in trinuclear derivatives of the oxophilic metal niobium.

Trinuclear cyclopentadienylniobium carbonyls are of interest since Cp3Nb3(CO)7 (Cp=eta5-C5H5) is a stable molecule having a unique face-bridging carbonyl group formally donating six electrons to the Nb3 triangle through one Nb-C sigma-bond and two orthogonal pi-bonds. In this connection, the molecules Cp3Nb3(CO)n (n=9, 8, 7, 6) have been examined by density functional theory. The saturated derivative Cp3Nb3(CO)9 is predicted to have a structure with all terminal carbonyl groups and a singly bonded Nb3 triangle with 3.361 A edges. The singly bonded Nb3 triangle is retained in the lowest energy structure for Cp3Nb3(CO)8, which has a four-electron donor edge-bridging carbonyl group in addition to seven terminal carbonyl groups. An alternative structure for Cp3Nb3(CO)8 at approximately 8 kcal mol(-1) above this global minimum has an unusual four-electron donor carbonyl group bridging all three niobium atoms. The two lowest energy structures found for Cp3Nb3(CO)7 are a pair of stereoisomers within approximately 1 kcal mol(-1) of energy, having a six-electron donor carbonyl group bridging all three niobium atoms, in addition to six terminal carbonyl groups. The structure and nu(CO) frequencies of one of these stereoisomers, including the unusually low nu(CO) frequency of approximately 1330 cm(-1) for the six electron donor carbonyl group, are in close agreement with experiment. In contrast to Cp3Nb3(CO)8 and Cp3Nb3(CO)7, the lowest energy structure for the even more unsaturated Cp3Nb3(CO)6 has only two-electron donor carbonyl groups, three of which are edge-bridging and three of which are terminal. However, in this Cp3Nb3(CO)6 structure, two of the Nb[double bond, length as m-dash]Nb distances in the Nb3 triangle are shortened to approximately 2.8 A, suggesting a formal bond order of at least two.