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)2Cl]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 RuIII−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 RuIII center (cis isomer, k1 = 618 ± 2 M-1 s-1, ΔH⧧ = 38 ± 3 kJ mol-1, ΔS⧧ = −63 ± 8 J K-1 mol-1, ΔV⧧
= −17.5 ± 0.8 cm3 mol-1; k-1 = 0.097 ± 0.001 s-1, ΔH⧧ = 27 ± 8 kJ mol-1, ΔS⧧ = −173 ± 28 J K-1 mol-1,
ΔV⧧ = −17.6 ± 0.5 cm3 mol-1; trans isomer, k1 = 1637 ± 11 M-1 s-1, ΔH⧧ = 34 ± 3 kJ mol-1, ΔS⧧ = −69 ±
11 J K-1 mol-1, ΔV⧧ = −20 ± 2 cm3 mol-1; k-1 = 0.47 ± 0.08 s-1, ΔH⧧ = 39 ± 5 kJ mol-1, ΔS⧧ = −121 ±
18 J K-1 mol-1, ΔV⧧ = −18.5 ± 0.4 cm3 mol-1 at 25 °C). The subsequent electron transfer step to form RuII−NO+
occurs spontaneously for the trans isomer, followed by a slow nitrosyl to nitrite conversion, whereas for the cis
isomer the reduction of the RuIII center is induced by the coordination of an additional NO molecule (cis isomer,
k2 = 51.3 ± 0.3 M-1 s-1, ΔH⧧ = 46 ± 2 kJ mol-1, ΔS⧧ = −69 ± 5 J K-1 mol-1, ΔV⧧ = −22.6 ± 0.2 cm3 mol-1
at 45 °C). The final reaction step involves a slow aquation process for both isomers, which is interpreted in terms
of a dissociative substitution mechanism (cis isomer, ΔV⧧ = +23.5 ± 1.2 cm3 mol-1; trans isomer, ΔV⧧ = +20.9
± 0.4 cm3 mol-1 at 55 °C) 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 π-acceptor properties of the tridentate N-donor chelate
(terpy) predominantly control the overall reaction pattern.