Kinetic study on the substitution of dimethylacetamide for the terminal aqua ligands in the trinuclear chromium(III) complexes [Cr3(μ3-O)(μ-RCO2)6(H2O)3]+ (R = H, CH3, CH3CH2, CH2Cl, CHCl2, CH3OCH2, (CH3)3C, CH2ClCH2, (CH3CH2)2CH).: Elucidation of the mechanism from the activation volumes and the substituent effects of bridging carboxylate ligands

The substitution of dimethylacetamide (dma) for the terminal aqua ligands in the carboxylate-bridged trinuclear chromium(III) complex [Cr-3(mu(3)-O)(mu-RCO2)(6)(H2O)(3)](+) (R = H, CH3, CH3CH2, CH2Cl, CHCl2, CH3OCH2, (CH3)(3)C, CH2ClCH2, (CH3CH2)(2)CH) in dma was kinetically studied by UV-visible absorption at 25-50 degrees C and 0.1-232 MPa. The time course is uniphasic over all three steps of the ligand replacement. The substitution rate k varied from 2.4(1) x 10(-5) (R = CHCl2) to 9.49(2) x 10(-3) (R = C(CH3)(3)) s(-1) depending on the substituent R at 40 degrees C. Large positive activation parameters Delta H double dagger (98-123 kJ mol(-1)), Delta S double dagger (29-81 J K-1 mol(-1)), and Delta V double dagger (12.4-21.3 cm(3) mol(-1)) for all the complexes suggested a dissociative activation mode (D or I-d mechanism). It is similar to those for terminal ligand substitution of acetate-bridged trinuclear complexes of ruthenium(III) and rhodium(III) with a mu(3)-O ligand and molybdenum with two mu(3)-O ligands. Examination of the substituent effect disclosed a linear relationship between k and Taft's electronic parameters, as well as pK(a) (RCOOH), indicating that the sigma-donor ability of the bridging carboxylate affects the strength of the Cr-OH2 bond in the cis position. The crystals of [Cr-3(mu(3)-O)(mu-RCo2)(6)(H2O)(3)][B(C6H5)(4)]. nH(2)O (R = H (1b) n = 6; R = CH3 (2b), n = 2) were found to be triclinic with space group <P(1)over bar>, a = 9.2080(8) Angstrom, b = 14.724(2) Angstrom, and c = 15.308(2) Angstrom, alpha = 79.369(6)degrees, beta = 86.513(8)degrees gamma = 79.823(8)degrees, Z = 2, and V = 2006.5(4) Angstrom(3) and with space group <P(1)over bar>, a = 8.848(6) Angstrom, b = 15.057(7) Angstrom, c = 17.375(8) Angstrom, alpha = 107.82(3)degrees, beta = 104.57(4)degrees, gamma = 92.27(4)degrees, Z = 2,and V = 2116(2) Angstrom(3) respectively. The relatively longer Cr-OH2 distances (average 2.03(1) and 2.06(2) Angstrom for 1b and 2b, respectively) than those of the mononuclear chromium(III) aqua complexes, due to a trans effect of the central oxide ion and the additional cis effect of the bridging carboxylate, play a role in accelerating the dissociative substitution for the terminal ligands.