The d 6 complexes of formula (η 5 -Cp)Ru(CO)(ER 3 )ONO 2 , where ER 3 ) AsPh 3 , PPh 3 , P(panisyl) 3 , PPh 2 (o-anisyl), and P(OPh) 3 , were prepared by reaction of their chloro analogues with AgNO 3 in CH 2 Cl 2 . They underwent moderately slow substitution of the relatively weakly coordinating nitrate by chloride in dry CH 2 Cl 2 in the presence of excess [N(PPh 3 ) 2 + ]Cl -. A kinetics study showed the reaction to be first order in nitrato 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 < PPh 3 < P(p-anisyl) 3 < AsPh 3 , PPh 2 (oanisyl), with k 1 at 25 °C for the first four in the series from 1.4 to 4.4 × 10 -5 s -1 . Activation parameters for conversion of (η 5 -Cp)Ru(CO)(AsPh 3 )ONO 2 to its chloride are ∆H ‡ ) 17(2) kcal/mol and ∆S ‡ ) -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 PPh 2 (o-anisyl) was approximately 2 orders of magnitude more reactive (k 1 ) 4.9 × 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 ratelimiting conversion of the neutral nitrato complex into a coordinatively unsaturated ionpaired 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 AsPh 3 nitrato and chloro complexes and for the PPh 3 chloro complex.