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)(&mgr;(3)-O)(&mgr;-RCO(2))(6)(H(2)O)(3)](+) (R = H, CH(3), CH(3)CH(2), CH(2)Cl, CHCl(2), CH(3)OCH(2), (CH(3))(3)C, CH(2)ClCH(2), (CH(3)CH(2))(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 = CHCl(2)) to 9.49(2) x 10(-)(3) (R = C(CH(3))(3)) s(-)(1) depending on the substituent R at 40 degrees C. Large positive activation parameters DeltaH() (98-123 kJ mol(-)(1)), DeltaS() (29-81 J K(-)(1) mol(-)(1)), and DeltaV() (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 &mgr;(3)-O ligand and molybdenum with two &mgr;(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-OH(2) bond in the cis position. The crystals of [Cr(3)(&mgr;(3)-O)(&mgr;-RCO(2))(6)(H(2)O)(3)][B(C(6)H(5))(4)].nH(2)O (R = H (1b), n = 6; R = CH(3) (2b), n = 2) were found to be triclinic with space group P&onemacr;, a = 9.2080(8) Å, b = 14.724(2) Å, and c = 15.308(2) Å, alpha = 79.369(6) degrees, beta = 86.513(8) degrees, gamma = 79.823(8) degrees, Z = 2, and V = 2006.5(4) Å(3) and with space group P&onemacr;, a = 8.848(6) Å, b = 15.057(7) Å, c = 17.375(8) Å, alpha = 107.82(3) degrees, beta = 104.57(4) degrees, gamma = 92.27(4) degrees, Z = 2, and V = 2116(2) Å(3), respectively. The relatively longer Cr-OH(2) distances (average 2.03(1) and 2.06(2) Å 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.