Kinetics of substitution of weakly coordinating nitrate by chloride in (η5-Cp)Ru(CO)(ER3)ONO2 (ER3 = AsPh3, PPh3, P(p-anisyl)3, PPh2(o-anisyl), P(OPh)3) in dichloromethane

The d(6) complexes of formula (eta (5)-Cp)Ru(CO)(ER3)ONO2, where ER3 = AsPh3, PPh3, P(p-anisyl)(3), PPh2(o-anisyl), and P(OPh)(3), were prepared by reaction of their;chloro analogues with AgNO3 in CH2Cl2. They underwent moderately slow substitution of the relatively weakly coordinating nitrate by chloride in dry CH2Cl2 in the presence of excess [N(PPh3)(2)(+)]Cl-. A kinetics study showed the reaction to be first order in nitrate complex and independent of chloride salt concentration under pseudo-first-order conditions. First-order rate constants for nitrate metathesis follow the trend P(OPh)3 < PPh3 < P(p-anisyl)(3) < AsPh3 much less than PPh2(o-anisyl), with k(1) at 25 degreesC for the first four in the series from 1.4 to 4.4 x 10(-5) s(-1). Activation parameters for conversion of (eta (5)-Cp)Ru(CO)(AsPh3)ONO2 to its chloride are DeltaH(double dagger) = 17(2) kcal/mol and DeltaS(double dagger) = -21(5) eu. Even use of a slightly moist solvent increased the rate of nitrate metathesis by 20-30% without changing the form of the rate law. The complex containing PPh2(o-anisyl) was approximately 2 orders of magnitude more reactive (k(1) = 4.9 x 10(-3) s(-1)). A likely explanation is stabilization of a coordinatively unsaturated intermediate by weak coordination of the potentially chelating o-OMe group. A mechanism entailing rate-limiting conversion of the neutral nitrate complex into a coordinatively unsaturated ion-paired species is consistent with this set of data. Semiempirical calculations (PM3(tm)), which model the structures of complexes in these systems quite well, supported such behavior. X-ray crystal structures were determined for the AsPh3 nitrate and chloro complexes and for the PPh3 chloro complex.