REACTIONS OF RHENIUM CYCLOPENTADIENYL AND INDENYL DICHLOROMETHANE COMPLEXES OF THE FORMULA [(ETA(5)-CXHY)RE(NO)(PPH3)(CLCH2CL)]+BF4- WITH KETONES - SUBSTITUTION RATES AND MECHANISM

Rates of reaction of the coordinatively saturated cyclopentadienyl and indenyl dichloromethane complexes [(eta5-CxHy)Re(NO)(PPh3)(ClCH2Cl)]+BF4- (x/y = 1, 5/5; 2, 9/7) and cyclohexanone to give [(eta5-CxHy)Re(NO)(PPh3)(eta1-O=C(CH2)4CH2)]+BF4- (x/y = 3, 5/5; 4,9/7) are measured by P-31{H-1} NMR under pseudo-first-order conditions (CH2Cl2, -70 to -30-degrees-C, 10-60-fold excesses of cyclohexanone). These substitutions are first-order in both 1/2 and cyclohexanone, even at cyclohexanone concentrations of 1.0-3.4 M. Eyring plots of the second-order rate constants give DELTAH(double dagger) of 15 +/- 1 kcal/mol and DELTAH(double dagger) of -5 +/- 4-5 eu. There is only a modest kinetic indenyl ligand effect (factor of 6 at -60 to -50-degrees-C). Thus, C(x)H(y) ring slippage is viewed as unlikely during substitution. Faster reactions of 1 with tropone (DELTAH(double dagger) 12 +/- 2 kcal/mol, DELTAS(double dagger) -17 +/- 11 eu) and methyl ethyl sulfide give similar rate data. These results and competition experiments with ethyl chloride exclude pathways involving initial dichloromethane dissociation. An associative mechanism that features a square pyramidal intermediate with a bent nitrosyl ligand is considered. Testable aspects of this model and stereochemical implications are discussed.