Influence of terpyridine as π-acceptor ligand on the kinetics and mechanism of the reaction of NO with ruthenium(III) complexes

The kinetics of the unusually fast reaction of cis- and trans-[Ru(terpy)(NH3)(2)Cl](2+) (with respect to NH3; terpy = 2,2':6',2-terpyridine) with NO was studied in acidic aqueous solution. The multistep reaction pathway observed for both isomers includes a rapid and reversible formation of an intermediate Ru-III-NO complex in the first reaction step, for which the rate and activation parameters are in good agreement with an associative substitution behavior of the Ru-III center (cis isomer, k(1) 618 +/- 2 M-1 s(-1), DeltaH(double dagger) = 38+/- 3 kJ mol(-1), DeltaS(double dagger) = -63+/- 8 JK(-1) mol(-1), AV(double dagger) = -17.5 +/- 0.8 cm(3) mol(-1); K-1 0.097 +/- 0.001 s(-1), DeltaH(double dagger) = 27 +/- 8 kJ mol(-1), DeltaH(double dagger) = -173 +/- 28 J K-1 mol(-1), DeltaV(double dagger) = - 17.6 +/- 0.5 cm(3) mol(-1); trans isomer, k(1) = 1637 +/- 11 M-1 s(-1), DeltaH(double dagger) = 34 +/- 3 kJ mol(-)1, DeltaS(double dagger) = -69 +/- 11 J K-1 mol(-1), DeltaV(double dagger) -20 +/- 2 cm(3) mol(-1); k(-1) = 0.47 +/- 0.08 s(-1), DeltaH(double dagger) = 39 +/- 5 kJ mol(-1), DeltaS(double dagger) = -121 +/- 18 J K-1 mol(-1), DeltaV(double dagger) -18.5 +/- 0.4 cm(3) mol(-1) at 25degreesC The subsequent electron transfer step to form Ru-II-NO+ occurs spontaneously for the trans isomer, followed by a slow nitrosyl to nitrite conversion, whereas for the cis isomer the reduction of the Ru-III center is induced by the coordination of an additional NO molecule (cis isomer, k(2) = 51.3 +/- 0.3 M-1 s(-1), DeltaH(double dagger) = 46 +/- 2 kJ mol(-1), DeltaS(double dagger) = -69 +/- 5 J K-1 mol(-1), DeltaV(double dagger) = -22.6 +/- 0.2 cm(3) mol(-1) at 45 degreesC). The final reaction step involves a slow aquation process for both isomers, which is interpreted in terms of a dissociative substitution mechanism (cis isomer, DeltaV(double dagger) = +23.5 +/- 1.2 cm(3) mol(-1); trans isomer, DeltaV(double dagger) = +20.9 +/- 0.4 cm(3) mol(-1) at 55 degreesC that produces two different reaction products, viz. [Ru(terpy)(NH3)(H2O)NO](3+) (product of the cis isomer) and trans-[Ru(terpy)(NH3)(2)(H2O)](2+). The pi-acceptor properties of the tridentate N-donor chelate (terpy) predominantly control the overall reaction pattern.