Metal/metallic 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. 

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) ... 

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. 

Similar isomerizations have been noted for a number of complexes. As with metal nitrosyls, IR spectra can be used to indicate the manner of bonding, but there is an overlap region around 2080-2100 cm-1 where i/(C-N) is found for both N- and S-bonded thiocyanates (additionally, S-bonded thiocyanates usually give a much sharper i (C-N) band). 14N NQR has been shown to be a reliable discriminator, but X-ray diffraction is ultimately the most reliable method. 

Principles of structure, bonding and reactivity for metal nitrosyl complexes. J. H. Enemark and R. D. Feltham, Coord. Chem. Rev., 1974,13, 339-406 (126). 

Metal-metal bonding, 1, 137, 169 gravimetry, 1, 525 history7, 1, 21, 23 nomenclature, 1,122, 123 Metal nitrosyls structure, 1, 16 Meta) oxides catalysts... 

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. 

In their extensive studies of metal nitrosyl chemistry, Enemark and Feltham showed that the mode of NO bonding can be altered by the simple addition of another ligand (168, 204, 205). An example of this phenomenon is illustrated by reaction (83b), and has been described by these investigators as stereochemical control of valence (204). 

The propensity of the linearly bonded nitrosyl group to undergo nucleophilic attack is inversely related to the effectiveness of back-donation in metal nitrosyl n-bonding. Below a vN0 value of ca. 1860 cm-1, the nitrosyl is unreactive with nucleophiles. Above that value, and especially above 1900 cm-1, the nitrosyl ligand reacts with a variety of nucleophilic reagents at the N atom (171). This type of attack is the most widely known and best studied in metal nitrosyl chemistry. 

---