Formal chromium–chromium triple bonds and bent rings in the binuclear cycloheptatrienylchromium carbonyls (C7H7)2Cr2(CO)n (n = 6, 5, 4, 3, 2, 1, 0): A density functional theory study

Binuclear cycloheptatrienylchromium carbonyls of the type (C7H7)2Cr2(CO)n (n = 6, 5, 4, 3, 2, 1, 0) have been investigated by density functional theory. Energetically competitive structures with fully bonded heptahapto η7-C7H7 rings are not found for (C7H7)2Cr2(CO)n structures having two or more carbonyl groups. This result stands in contrast to the related (CnHn)2M2(CO)n (M = Mn, n = 6; M = Fe, n = 5; M = Co, n = 4) systems. Most of the predicted (C7H7)2Cr2(CO)n structures have bent trihapto or pentahapto C7H7 rings and Crtriple bond; length of mdashCr distances in the range 2.4–2.5 Å suggesting formal triple bonds. In some cases rearrangement of the heptagonal C7H7 ring to a tridentate cyclopropyldivinyl or tridentate bis(carbene)alkyl ligand is observed. In addition structures with CO insertion into the C7H7–Cr bond are predicted for (C7H7)2Cr2(CO)n (n = 6, 4, 2). The global minima found for the (C7H7)2Cr2(CO)n derivatives for n = 6, 5, and 4 are (η5-C7H7)(OC)2Crtriple bond; length of mdashCr(CO)4(η1-C7H7), (η3-C7H7)(OC)2Crtriple bond; length of mdashCr(CO)3(η2,1- C7H7), and (η5-C7H7)2Cr2(CO)4, respectively. The global minima for (C7H7)2Cr2(CO)n (n = 3, 2) have rearranged C7H7 groups. Singlet and triplet structures with heptahapto η7-C7H7 rings are found for the dimetallocenes (η7-C7H7)2Cr2(CO) and (η7-C7H7)2Cr2, with the singlet structures being of much lower energies in both cases.