Isomerization and chelation studies of (OC)(5)M[eta(1)-PPh2CH2CH2P(p-tolyl)(2)] and (OC)(5)M [eta(1)-P(p-tolyl)(2) CH2CH2PPh2] (M = Cr, Mo, W)

Six dangling phosphine complexes have been synthesized to provide three pai rs of linkage isomers, (OC)(5)M[eta (1)-PPh2CH2CH2P(p-tolyl)(2)] (3Cr, 3Mo, 3W) and (OC)(5)M[eta (1)-P(p-tolyl)(2)CH2CH2PPh2] (4Cr, 4Mo, 4W). Rate con stants for isomerization in toluene of 3Cr, 3Mo, 3W, 4Cr, 4Mo, and 4W at 55 degreesC are 1.23 x 10(-8), 7.76 x 10(-6), 2.3 x 10(-8), 6.52 x 10(-9), 4. 24 x 10(-6), and 9.7 x 10(-9) s(-1), respectively. Rate constants for chela tion of 3Cr, 3Mo, 4Cr, and 4Mo are 1.15 x 10(-8), 3.9 x 10(-7), 1.23 x 10(- 8), and 3.9 x 10(-7) s(-1), respectively. Chelation rates for 3W and 4W wer e too slow to conveniently measure. The position of equilibrium favors dito lylphosphino coordination with equilibrium constants of 1.88, 1.83, and 2.4 for the Cr, Mo, and W isomer pairs, respectively. Isomerization of 3Mo and 4Mo was studied at 25, 40, and 55 degreesC to give activation parameters, DeltaH(double dagger) and DeltaS(double dagger), of 102.4 and 103.6 kJ/mol and -32 and -33 J/(K mol), respectively, DeltaH(double dagger) and DeltaS(d ouble dagger) for chelation of 3Mo and 4Mo were determined to be 113.5 and 115.5 kJ/mol and -22 and -16 J/(K mol), respectively. These results suggest that the transition states for both isomerization and chelation are signif icantly associative and become more so in descending order through group 6.