C= CH—CH2—CH2—CH2; X= Br) react with the hydride donors Li+[HB (iec-Bu) 3]-or Na+[H2Al (OCH2CH2OCH3) 2]-to produce (C5Me5) Rh (PMe3)(R) H. Thecomplexes with R= alkyl or R= vinyl are unstable, undergoing rapid reductive elimination at 25 C, but can be observedby* H NMR below-20 C.(C5Me5) Rh (PMe3)(CH3) H undergoes first-order reductive elimination with k=(6.38±0.10) X 10~ 5 s~* at-17 C. In contrast,(C5Me5) Rh (PMe3)(C6H5) H undergoes a more complicated first-order process in C6D6, producing C6H6 and (C5Me5) Rh (PMe3)(C6D5) D with the overall activation parameters AH*= 30.5±0.8 kcal/mol and AS*= 14.9±2.5 eu. The alkyl and aryl hydride complexes can also be generated byphotochemical extrusion of H2 from (C5Me5) Rh (PMe3) H2 in the presence of alkane or arene solvent. In a competition experiment, a 5.4: 1 selectivity for benzene over cyclopentane was exhibited at-35 C. Irradiation in toluene solvent at-45 C produced products in which activationof all possible CH bonds of toluene was observed: 57% meta, 36% para, 7% ortho, and< 1% benzyl. Thermodynamically controlled competitionbetween activation of benzene and toluene, m-xylene, o-xylene, or p-xylene showed preferences for benzene of 2.7, 12.1, 7.6, and 58.6. The aryl complexes (C5Me5) Rh (PMe3)(aryl) H were found to be in rapid equilibrium with their p2-arene derivatives at temperatures above-15 C. Mechanistic studies revealed a [l, 2]-shift pathway around the ring withAH*= 16.3±0.2 kcal/mol and AS*=-6.3±0.8 eu for the derivative with R= 2, 5-C6H3Me2. Generation of the coordinatively unsaturated species (C5Me5) Rh (PMe3) in the presence of p-[C6H4 (i-Bu) 2] permitted the direct observation of an p2-arene complex at-15 C. The kinetics of CH bond activation in this system are interpreted in terms of two distinct rate-determining reactions, arene coordination vs. alkane oxidative addition. The rhodium-phenyl bond is found to be about 13 kcal/mol stronger than the rhodium-methyl bond.

One of the most beneficial features of transition-metal com-plexes is their ability to promote chemical reactions in organic molecules. Oxidative addition is a fundamental process that produces a species capable of reacting in numerous ways characteristic of the molecule that has been activated. 1 For example, aldehydes and acid chlorides can be decarbonylated following oxidative addition of the CH or C-Cl bonds. 2 The oxidative