Formation of Metal Nitrosyl Complexes

Pell and Armor found entirely different products in alkaline solution. Above pH 8.3, the sole ruthenium product of the reaction of Ru(NH3)g+ with NO was the dinitrogen complex Ru(NH3)5(N2)2+. Under these conditions the rate law proved to be first-order in [Ru(NH3)g+], [NO] and [OH-]. A likely mechanism is the reversible reaction of Ru(NH3)3+ with OH- to give the intermediate Ru(NH3)5(NH2)2+, followed by electrophilic NO attack at the amide ligand and release of water. However, the kinetic evidence does not exclude other sequences. 

Several Ru(III) salen complexes of the type Ruin(salen)(X)(NO) (X=C1-, ONO-, H20 salen = N,AP-bis(salicylidene)-ethylenediamine dianion) have been examined as possible photochemical NO precursors (19). Photo-excitation of the Rum(salen)(NO)(X) complex labilizes NO to form the respective solvento species Ruin(salen)(X)(Sol). The kinetics of the subsequent back reactions to reform the nitrosyl complexes (e.g. Eq. (8)) were studied as a function of the nature of the solvent (Sol) and reaction conditions. The reaction rates are dramatically dependent on the identity of Sol, with values of kNO (298 K, X = C1-) varying from 5 x 10-4 M-1 s-1 in acetonitrile to 4 x 107 M-1 s-1 in toluene, a much weaker electron donor. In this case, Rum Sol bond breaking clearly 

Ligand substitution reactions of NO leading to metal-nitrosyl bond formation were first quantitatively studied for metalloporphyrins, (M(Por)), and heme proteins a few decades ago (20), and have been the subject of a recent review (20d). Despite the large volume of work, systematic mechanistic studies have been limited. As with the Rum(salen) complexes discussed above, photoexcitation of met allop or phyr in nitrosyls results in labilization of NO. In such studies, laser flash photolysis is used to labilize NO from a M(Por)(NO) precursor, and subsequent relaxation of the non-steady state system back to equilibrium (Eq. (9)) is monitored spectroscopically. 

In the presence of excess NO, the observed transient spectra are expected to decay exponentially with an observed rate constant (feobs) equal to  

The kinetics of reactions of NO with ferri- and ferro-heme proteins and models under ambient conditions have been studied by time-resolved spectroscopic techniques. Representative results are summarized in Table I (22-28). Equilibrium constants determined for the formation of nitrosyl complexes of met-myoglobin (metMb), ferri-cytochrome-c (Cyt111) and catalase (Cat) are in reasonable agreement when measured both by flash photolysis techniques (K= konlkQff) and by spectroscopic titration in aqueous media (22). Table I summarizes the several orders of magnitude range of kon and kQs values obtained for ferri- and ferro-heme proteins. Many k0f[ values were too small to determine by flash photolysis methods and were determined by other means. The small values of kQ result in very large equilibrium constants K for the 

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