FURTHER-STUDIES ON THE ROLE OF NEIGHBORING GROUP PARTICIPATION IN CO SUBSTITUTION-REACTIONS OF GROUP-6 METAL CARBOXYLATES

The reactions between W(CO)5O2CR- (R = -C(CH3)3, -CH2CN, and -CF3) and P(OCH3)3 result in cis CO replacement via a dissociative mechanism with concomitant formation of cis-W(CO)4[P(OCH3)3]O2CR-. From the temperature dependence of the rate constants, activation parameters for the reactions where R = -CH2CN and -C(CH3)3 were determined to be DELTA-H* = 25.3 +/- 0.8 kcal.mol-1 and DELTA-S* = 7.7 +/- 2.5 eu and DELTA-H* = 24.2 +/- 1.5 kcal.mol-1 and DELTA-S* = 9.7 +/- 5.1 eu, respectively. These rate data, coupled with those previously reported for R = -CH3 and -H, display a linear free energy relationship when plotted against Taft's polar substituent constant, sigma-*. The reaction constant, rho-*, was found to be -0.66, indicating the CO substitution reaction to be facilitated by electron-releasing substituents. This observation, taken together with the ability of the carboxylate ligand to chelate to the metal center, is interpreted as involvement of the distal oxygen atom of the monodentate carboxylate in cis CO substitution reactions. The X-ray structure of the cyanoacetate derivative, [Et4N][W(CO)5O2CCH2CN], is also reported. The complex crystallizes in the triclinic centrosymmetric space group P1BAR (No. 2) with a = 7.292 (2) angstrom, b = 11.552 (4) angstrom, c = 12.564 (4) angstrom, alpha = 76.03 (3)-degrees, beta = 73.65 (2) angstrom, gamma = 86.48 (2) angstrom, V = 985.6 (5) angstrom-3, and Z = 2. Refinement converged at R = 2.36% and R(w) = 3.23% for those 3380 reflections with I > 2-sigma-(I) and T = 193 K.