Formal chromium–chromium triple bonds and bent rings in the binuclear cycloheptatrienylchromium carbonyls (C 7H 7) 2Cr 2(CO) n ( n = 6, 5, 4, 3, 2, 1, 0): A density functional theory study

Abstract

Binuclear cycloheptatrienylchromium carbonyls of the type (C 7H 7) 2Cr 2(CO) n ( n = 6, 5, 4, 3, 2, 1, 0) have been investigated by density functional theory. Energetically competitive structures with fully bonded heptahapto η 7-C 7H 7 rings are not found for (C 7H 7) 2Cr 2(CO) n structures having two or more carbonyl groups. This result stands in contrast to the related (C n H n ) 2M 2(CO) n (M = Mn, n = 6; M = Fe, n = 5; M = Co, n = 4) systems. Most of the predicted (C 7H 7) 2Cr 2(CO) n structures have bent trihapto or pentahapto C 7H 7 rings and Cr Cr distances in the range 2.4–2.5 Å suggesting formal triple bonds. In some cases rearrangement of the heptagonal C 7H 7 ring to a tridentate cyclopropyldivinyl or tridentate bis(carbene)alkyl ligand is observed. In addition structures with CO insertion into the C 7H 7–Cr bond are predicted for (C 7H 7) 2Cr 2(CO) n ( n = 6, 4, 2). The global minima found for the (C 7H 7) 2Cr 2(CO) n derivatives for n = 6, 5, and 4 are (η 5-C 7H 7)(OC) 2Cr Cr(CO) 4(η 1-C 7H 7), (η 3-C 7H 7)(OC) 2Cr Cr(CO) 3(η 2,1- C 7H 7), and (η 5-C 7H 7) 2Cr 2(CO) 4, respectively. The global minima for (C 7H 7) 2Cr 2(CO) n ( n = 3, 2) have rearranged C 7H 7 groups. Singlet and triplet structures with heptahapto η 7-C 7H 7 rings are found for the dimetallocenes (η 7-C 7H 7) 2Cr 2(CO) and (η 7-C 7H 7) 2Cr 2, with the singlet structures being of much lower energies in both cases.