Nitrosyls bonding

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. 

Based on the principle of microscopic reversibility one may conclude that the intermediate(s) in the off step will be the same as those generated during the k(m pathway, thus iron nitrosyl bond breakage (k 2)... 

In (95), the initial formation of the bent nitrosyl species [Co(NO)(en)2Cl]+ (220) is followed by electrophilic attack of free NO and then by reaction with a second NO molecule leading to products. Since the nitro complex produced in this reaction has the same stereochemistry as the reactant Co111—NO- species, it has been proposed that the Co—N(nitrosyl) bond remains intact throughout the reaction sequence, thus requiring free NO to attack via its oxygen end (193). This proposal requires further proof. 

A versatile reagent is N-methyl-N-nitrosotoluene-p-sulphonamide (MNTS), which effects the replacement of a metal hydride bond and a labile ligand by a metal nitrosyl bond. For example (36) ... 

A number of transition metal nuclei can be studied by Mossbauer techniques (e.g., Fe, Ni, Ru, W, Os, Ir, and Pt). Of these, only Ir, Ru, and Fe have been used to study nitrosyl bonding. The most detailed studies have been on the well-known iron complexes [Fe(CN)5(NO)] - (87. 89) and [Fe(NO)(dtc)z] (88-90) (dtc is N,N-dialkyldithiocarbamato). In the latter, high-spin/low-spin equilibria can be followed by Fe Mossbauer spectroscopy, and the Mossbauer parameters agree well with data from electron spin resonance (ESR) spectroscopy in determining the ground states of these complexes. 

The first accurately documented example of a distinctly bent M-N-0 linkage in a metal nitrosyl complex was presented in 1968. Ibers showed that the cation [IrCl(CO)NO(PPh3)2] (Figure 2) possessed a square pyramidal structure with a metal-nitrosyl bond angle of 124°. Subsequently other species exhibiting similar structural properties have been reported. It has now become clear that there are essentially two distinct orientations in which nitric oxide may be complexed. In one case the ligand is coordinated linearly and in the other it is bent (angle approximately 120°) for structural data available on linear and bent nitrosyls see Tables 1 and 2 respectively. 

Enemark and Feltham s notation deliberately does not specify the oxidation state of the complex or its coordination number. However, recent studies based on independent spectroscopic and structural data may provide compelling evidence for a specific metal oxidation state, which has implications for the metal-nitrosyl bonding. Some authors therefore see a need to communicate this information. 

Dahl and coworkers [116] reported molybdenum-promoted nitrosyl bond activation leading to the formation of nitrido and/or imido hgand-containing metal clusters. A solution of Mo2(r/ -C5Me5)2(CO)4 and Co(CO)3(NO) in tetrahydro-furan, upon photolysis under a slow stream of N2, gives ( / -C5Me5)3M03C02-(CO)8(/t3-NH)(/t4-N). 

---