Heptahapticity in binuclear (cycloheptatrienyl)molybdenum carbonyl derivatives: The interplay between ring hapticity/planarity and metal-metal multiple bonding

Abstract

Theoretical studies on the bi-nuclear (cycloheptatrienyl)molybde-num carbonyl complexes [(C7H7)2Mo 2(CO)n] (n = 6, 5, 4, 3, 2, 1, 0) indicate structures with fully bonded heptahapto ζ7-C7H7 rings and four or fewer carbonyl groups to be energetically competitive. This is in striking contrast to the corresponding chromium derivatives. The lowest energy of such a structure for [(ζ7-C7H7) 2Mo2(CO)4] is a singlet unbridged structure with a formal Mo-Mo single bond with a length of approximately 3.2 Å. A higher-energy pentahapto structure [(ζ5-C7H 7)2Mo2(CO)4] is also predicted with a formal Mo = Mo triple bond with a length of approximately 2.56 A. Low-energy structures are predicted for [(C7H7)2Mo 2(CO)3] with two heptahapto ζ-C7H 7 rings, either one or two bridging carbonyl groups, and formal Mo=Mo double bonds with a length of approximately 2.8 Å. However, the global minimum for [(C7H7)2Mo2(CO) 3] is a [(ζ-C7H7)(ζ-C 7H7)Mo2(CO)3] structure with a formal Mo = Mo triple bond with a length of approximately 2.53 Å. The lowest-energy structures for [(C7H7)2Mo 2(CO)2] and [(C7H7) 2Mo2(CO)] have heptahapto ζ5-C 7H7 rings and predicted metal-metal bond lengths of approximately 2.54 and 2.31 A, respectively, consistent with the formal triple and quadruple bonds, respectively, needed to give both metal atoms the favored 18-electron configuration. The lowest-energy structures for the carbonyl-richer systems [(C7H7)2Mo2(CO)n] (n = 6, 5) contain one trihapto ζ-C7H7 ring and one pen-tahapto ζ5-C7H7 ring. © 2010 Wiley-VCH Verlag GmbH © Co. KGaA, Weinheim.