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

Abstract

Trinuclear cyclopentadienylniobium carbonyls are of interest since Cp₃Nb₃(CO)₇ (Cp = η⁵-C₅H₅) is a stable molecule having a unique face-bridging carbonyl group formally donating six electrons to the Nb₃ triangle through one Nb–C σ-bond and two orthogonal π-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 Å 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 ∼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 ∼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 ν(CO) frequencies of one of these stereoisomers, including the unusually low ν(CO) frequency of ∼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 NbNb distances in the Nb₃ triangle are shortened to ∼2.8 Å, suggesting a formal bond order of at least two.