Nitrosyl complexes nucleophilic addition

The effect of metal basicity on the mode of reactivity of the metal-carbon bond in carbene complexes toward electrophilic and nucleophilic reagents was emphasized in Section II above. Reactivity studies of alkylidene ligands in d8 and d6 Ru, Os, and Ir complexes reinforce the notion that electrophilic additions to electron-rich compounds and nucleophilic additions to electron-deficient compounds are the expected patterns. Notable exceptions include addition of CO and CNR to the osmium methylene complex 47. These latter reactions can be interpreted in terms of non-innocent participation of the nitrosyl ligand. 

Despite intense study of the chemical reactivity of the inorganic NO donor SNP with a number of electrophiles and nucleophiles (in particular thiols), the mechanism of NO release from this drug also remains incompletely understood. In biological systems, both enzymatic and non-enzymatic pathways appear to be involved [28]. Nitric oxide release is thought to be preceded by a one-electron reduction step followed by release of cyanide, and an inner-sphere charge transfer reaction between the ni-trosonium ion (NO+) and the ferrous iron (Fe2+). Upon addition of SNP to tissues, formation of iron nitrosyl complexes, which are in equilibrium with S-nitrosothiols, has been observed. A membrane-bound enzyme may be involved in the generation of NO from SNP in vascular tissue [35], but the exact nature of this reducing activity is unknown. 

Neutral (cyclohexadienyl)manganese complexes 71, generated by nucleophilic addition to (arene)Mn(CO)3+ cations 65, undergo ligand substitution with nitrosyl hexafluorophosphate to give the corresponding (cyclohexadienyl)Mn(CO)2NO+ cations 72 (Scheme 17)93. Attack by a wide variety of nucleophiles on cations 72... 

In this way, a catalytic reduction of N02- by N2H4 ensues in the appropriate conditions. Interestingly, the addition of N2H4 to other nitrosyl complexes leads to azide complexes, not to N20. The attack of N-binding nucleophiles on bound NO + is at the heart of the mechanisms of N02 reductions in soils by bacteria and reducing enzymes, evolving to gaseous products, N2/N20.4 As anticipated for reaction 7.2, the coordination of N02 is considered the first step toward further reactivity. 

In common with Enemark—Feltham, the new notation makes no attempt to define the formal charges on the nitrosyl ligand and the formal metal oxidation state but focuses attention on the geometry of the nitrosyl, the metal s coordination number and the total electron count. As De La Cruz and Sheppard have recently pointed out [26] in their extensive analysis of the vihratiOTial data for nitrosyl complexes, the great majority of them conform to 18- and 16-electron rules, and therefore, this parameter establishes whether the molecirle has a closed shell. The total electron count has important chemical implications since it indicates whether the compound is likely to undergo electrochemical conversion or nucleophilic addition in order to achieve an 18-electron configuration. 

Nitrosyl complexes of ruthenium react in a similar manner. In this case, supposedly, nucleophilic attack of the coordinated NO molecule on the benzyl halide takes place, or electrophilic addition of PhCH to the coordinated NO group occurs with concomitant addition of X to the metal ... 

The methodology of nucleophilic addition to ri -allyliron complexes has also been applied for natural product synthesis. Thus, the regioselective and stereoselective addition of an organocopper reagent to a neutral Tl -allyl(dicarbonyl)(nitrosyl)iron complex is the key step in the total synthesis of the Cl-Cl 6 fragment of ionomycin (Scheme 4-90). ... 

Additionally, Hofmann s work showed that the reaction of sodium nitroprusside with hydroxylamine leads to formation of Na3lFe(CN)5(H20)] and to generation of nitrous oxide. The kinetics of this reaction have been discussed by several workers. 7,48 Based on the experimental data reported, the process appears to begin with nucleophilic attack on the coordinated nitrosyl ligand by the hydroxylamine N-atom. Then, the decomposition of the resultant intermediate leads to formation of nitrous oxide and the aqua-complex. 

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