Base hydrolysis reactions of [Cr(tmpa)(NCSe)] 2 O 2ϩ , [Cr(tmpa)(N 3 )] 2 O 2ϩ , and follow the pseudo-first-4ϩ 2ϩ [Cr (tmpa) (ϪO)(ϪPhPO )] [ Cr (tmpa) (ϪO)(ϪCO )] 2 2 4 2 2 3
order relationship (excess OH Ϫ ):
For the com-
obsd o b p p 3 plex, Ϫ2 Ϫ1 k (60ЊC) ϭ (1.50 Ϯ 0.03) ϫ 10 s ; ⌬H ‡ ϭ 61 Ϯ 2 kJ/mol, ⌬S ‡ ϭ Ϫ99 Ϯ 7 J/mol K; b 1 Ϫ1 Q (60ЊC) ϭ (3.8 Ϯ 0.3) ϫ 10 M ; ⌬HЊ ϭ 67 Ϯ 2 kJ/mol, ⌬SЊ ϭ 230 Ϯ 7 J/mol K (I ϭ 1.0 M). p An isokinetic relationship among activation parameters for five k (ϭk Q ) OH b p (tmpa)CrOCr(tmpa) complexes shows that all follow essentially the same pathway. Activated complex formation is thought to require nucleophilic attack of coordinated OH Ϫ at the chromium-leaving group bond in the k b step, accompanied by reattachment of a tmpa pyridyl arm displaced by OH Ϫ in the Q p preequilibrium. Abstraction of both thiocyanate ligands was observed upon mixing [Cr(tmpa)(NCS)] 2 O 2ϩ with [Pd(CH 3 CN) 4 ] 2ϩ in CH 3 CN solution. The proposed mechanism requires rapid complexation of both reactant thiocyanate ligands by 8 Ϫ2 Pd(II) (K (25ЊC) ϭ (4.5 Ϯ 0.2) ϫ 10 M ; ⌬HЊ ϭ p prior to rate-limiting Cr9NCS bond-breaking Ϫ32 Ϯ 6 kJ/mol, ⌬SЊ ϭ 59 Ϯ 19 J/mol K) Pd(II)-as-Ϫ3 Ϫ1
(k (25ЊC) ϭ (1.17 Ϯ 0.02) ϫ 10 s ; ⌬H ‡ ϭ 98 Ϯ 2 kJ/mol, ⌬S ‡ ϭ 27 Ϯ 5 J/mol K).
2 sisted NCS Ϫ abstraction is not driven by weakening of the Cr 9 NCS Ϫ bond through ligation of the sulfur atom to palladium, but rather by a favorable ⌬S ‡ resulting from the release of Pd(NCS) ϩ fragments and weak solvation of the activated complex in CH 3 CN solution. ᭧ 1999