Chromium–Chromium Bonding in Binuclear Azulene Chromium Carbonyl Complexes

Abstract

The experimentally known cis-[(η 5 ,η 5 -C 10 H 8 )Cr 2 (CO) 6 ] structure with a rather long formal Cr―Cr single bond of about 3.3 A is predicted by density functional theory to be the lowest energy C 10 H 8 Cr 2 (CO) 6 structure. However, a trans-[(η 7 ,η 5 -C 10 H 8 )Cr 2 (CO) 6 ] structure lies in energy within ca. 4 kcal/mol of this global minimum. The lowest energy structures for the unsaturated derivatives C 10 H 8 Cr 2 (CO) n (n = 5, 4, 3, 2) are all related structures with cis stereochemistry of the two chromium atoms, a chromium-chromium bond, and a single bridging carbonyl group. Such structures include singlet cis-[(η 5 ,η 5 -C 10 H 8 )Cr 2 (CO) 4 (μ-CO)], singlet cis-[(η 7 ,η 5 -C 10 H 8 )Cr 2 (CO) 3 (μ-CO)], triplet cis-[(η 7 ,η 5 -C 10 H 8 )Cr 2 -(CO) 2 (μ-CO)], and triplet cis-[(η 7 ,η 5 -C 10 H 8 )Cr 2 (CO)(η 2 -μ-CO)], for n = 5, 4, 3, and 2, respectively. The C 10 H 8 Cr 2 (CO) 5 structure is thermodynamically viable with respect to disproportionation into C 10 H 8 Cr 2 (CO) 6 and C 10 H 8 Cr 2 (CO) 4 , in contrast to its cyclopentadienyl analogue [(η 5 -C 5 H 5 ) 2 Cr 2 (CO) 5 ]. However, the C 10 H 8 Cr 2 (CO) 4 structure is thermodynamically unfavorable with respect to disproportionation into C 10 H 8 Cr 2 (CO) 5 and C 10 H 8 Cr 2 (CO) 3, in contrast to its very stable and experimentally known cyclopentadienyl analogue [(η 5 -C 5 H 5 ) 2 Cr 2 (CO) 4 ]. The lowest energy structure cis-[(η 7 ,η 5 -C 10 H 8 )Cr 2 (μ-CO)] for the monocarbonyl C 10 H 8 Cr 2 (CO) is a triplet structure with an unsymmetrical bridging carbonyl group and a Cr=Cr distance of about 2.4 A, suggesting a formal triple bond. The qualitatively assigned Cr―Cr formal bond orders (one through five) are remarkably consistent with the Wiberg bond indices obtained from the Weinhold natural bond order analysis.