Nucleophilic and electrophilic promotion of ligand substitution reactions in oxo-bridged, dinuclear chromium(III) complexes

Base hydrolysis reactions of [Cr(tmpa)(NCSe)](2)O2+, [Cr(tmpa)(N-3)](2)O2+, [Cr-2(tmpa)(2)(mu-O)(mu-PhPO4)](4+) and [Cr-2(tmpa)(2)(mu-O)(mu-CO3)](2) follow the pseudo-first-order relationship (excess OH-): k(obsd) = k(o) + k(b)O(p)[OH-]/(1 + O-p[OH-]). For the CO32- complex, k(b)(60 degrees C) = (1.50 +/- 0.03) x 10(-2) s(-1); Delta H double dagger = 61 +/- 2 kJ/mol, Delta S double dagger = -99 +/- 7 J/mol K; Q(p)(60 degrees C) = (3.8 +/- 0.3) x 10(1) M-1; Delta H degrees = 67 +/- 2 kJ/mol, Delta S degrees = 230 +/- 7 J/mol K (I = 1.0 M). An isokinetic relationship among k(OH)(= k(b)O(p)) activation parameters for five (tmpa)CrOCr(tmpa) complexes shows that ail follow essentially the same pathway. Activated complex formation is thought to require nucleophilic attack of coordinated OH- at the chromium-leaving group bond in the k(b) step, accompanied by reattachment of a tmpa pyridyl arm displaced by OH- in the Q(p) preequilibrium. Abstraction of both thiocyanate ligands was observed upon mixing [Cr(tmpa)(NCS)](2)O2+ with [Pd(CH3CN)(4)](2+) in CH3CN solution. The proposed mechanism requires rapid complexation of both reactant thiocyanate ligands by Pd(II) (K-p(25 degrees C) = (4.5 +/- 0.2) x 10(8) M-2; Delta H degrees = -32 +/- 6 kJ/mol, Delta S degrees = 59 +/- 19 J/mol K) prior to rate-limiting Cr-NCS bond-breaking (k(2)(25 degrees C) = (1.17 +/- 0.02) X 10(-3) s(-1); Delta H double dagger = 98 +/- 2 kJ/mol, Delta S double dagger = 27 +/- 5 J/mol K). Pd(II)-assisted NCS- abstraction is not driven by weakening of the Cr-NCS- bond through ligation of the sulfur atom to palladium, but rather by a favorable Delta S double dagger resulting from the release of Pd(NCS)(+) fragments and weak solvation of the activated complex in CH3CN solution. (C) 1999 John Wiley & Sons, Inc.