Bonding metal nitrosyls

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... 

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 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. 

An alternative approach to describing the bonding in MY2 complexes is to consider the triatomic MY2 unit as a covalent inorganic functional group. Enemark and Feltham have shown that the diverse properties of metal nitrosyl complexes can be conveniently interpreted by describing the complexes as derivatives of the [MNO] group, where n is the total number of electrons in the rf-orbitals of the metal and the 7i -orbitals of the nitrosyl ligand. 

The adsorption of nitric oxide is of further interest from the point of view of metal-nitrosyl interactions in inorganic chemistry. The bonding of nitrosyl ligands to transition metal centers in metal compounds has indicated that the NO ligand is amphoteric, i.e., it can be formally considered as N0+ or NO-(linear or bent) when bonding to a single metal center (16,17). 

The results are consistent with the predictions of the ligand field theory and with the observed photochemical reactions. Bond bending distortions are observed in metal-nitrosyl compounds. The photochemical reactions of CotCCO NO and the photohydrogenation catalyzed by RhCPPhjJ NO provide indirect support for the bending. 

Figure 14. Frontier molecular orbital diagrams of (a) metal-oxo and (b) metal-nitrosyl tt-bonding interactions. [Adapted from (16).]...

Let us turn now to the nature of the bond that is normally found in M—CO groups. In most cases, it is adequate for practical, everyday purposes to regard a ligand simply as an electron pair donor and to think of the bond to the central atom simply as L—>M. However, there are important classes of compounds for which this simple concept is seriously inadequate. The most prominent examples are the metal carbonyls, metal nitrosyls, and compounds of low-valent metals containing phosphines or isonitriles as ligands. 

A structural study of the two SO2 complexes established that the Rh-S distance for L = Cl is ca. 0.1 A shorter than for L = CO, the former being the shortest established distance among the > -pyramidal complexes. The metal atom in [Rh(ttp)(C0)(S02)] lies much closer to the plane of the four basal donor atoms than in RhCl(ttp)(S02), [RhCl(ttp)(NO)], or even Rha(ttp) This feature has been attributed to the excellent r-acceptor carbonyl, which prefers the metal in the basal plane so as to maximize 2r (CO)-

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similar effect on M-NO distances might be present also in metal nitrosyls, but the generally low precision of reported M-NO distances makes this type of analysis difficult ... 

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