STRUCTURE, BONDING, AND REACTIVITY OF MOLYBDENUM ETA(3)-CYCLOHEXENONECOMPLEXES IN COMPARISON WITH THEIR CYCLOPENTENONE ANALOGS - ETA(3)-ALLYL ETA(4)-DIENE CONVERSION/

The neutral eta(3)-cyclohexenone complexes [Mo(eta(3)-C6H7O)(CO)(2)(Me CN)(2)Br] 1, [Mo(eta(3)-C6H7O)(CO)(2){HB(pz)(3)}] 2, [Mo(eta(3)-C6H7O) (CO)(2)(bipy)Br] 3 (bipy = 2,2'-bipyridine) and [Mo(eta(3)-C6H7O)(CO)( 2)(dppm)Br] 4 (dppm = Ph2PCH2PPh2) have been synthesized. The structur e of 2 has been determined by X-ray crystallography. All these complex es resist hydride abstraction using Ph3C+PF6-, in sharp contrast to th e eta(3)-cyclopentenone analogues where eta(3)-allyl/eta(4)-diene conv ersion is a facile process. A rationale for this different behaviour i s provided by extended-Huckel calculations combined with a Walsh analy sis of hydrogen abstraction. Thus, while in the eta(3)-C5H5O --> eta(4 )-C5H4O conversion a Mobius system is formed upon release of hydride v ia electrophilic attack, this is not possible in the hypothetical eta( 3)-C6H7O --> eta(4)-C6H6O process. Therefore, eta(3)-C6H7O is a C-H ac id. Also, the occurrence of different conformations, exo for allyl and endo for diene complexes, is rationalized.