Nitrosyl ligands, bent

The first well-characterized example of a bent nitrosyl ligand was that found in a derivative of Vaska s complex ... 

The product is square pyramidal with a bent nitrosyl ligand (Z. Ir—N—O - 124 ) at ihe apical position (Fig. 15.15a).44 Other complexes with a bent M—NO have been found, including the remarkable example [Rubent nitrosyl groups45 (Fig. 15.15b). 

The structural studies of nitrosyls have shown to date that the bent nitrosyl ligand invariably occurs at the apical position of a square based pyramid or a distorted octahedron in which the metal ion configuration assuming NO- coordination is d6. Despite numerous electronic structural descriptions (168, 169,198-200), it is not totally clear why fully bent nitrosyls with M—N—O bond angles of 120° have not been found in other geometries such as the square plane and the trigonal bipyramid. 

In this reaction the nitrosyl ligand bends, undergoing a 2 e reduction, rather than form a 20 e complex. This reaction can also serve to activate the nitrosyl ligand. Whereas the linear nitrosyl may be unreactive if vNO is sufficiently low, the bent nitrosyl is reactive to electrophiles. 

Electrophilic attack by H+ on bent nitrosyl ligands leads to reduction (190, 205, 216). Complexes containing HNO, NHOH, and NH2OH have all been prepared in this manner, raising the possibility, as yet unrealized, of a catalytic reduction of NO to hydroxylamine. The simplest and best characterized of these reactions is (91) reported by Enemark et al. (205). 

A parallel situation appears to obtain for the mixed allyl nitrosyl complex Ru(NO)(C3H5)L2 prepared by Schoonover and Eisenberg (231). This complex which is coordinatively saturated (NO+ and rf -allyl), forms a CO adduct which is assigned a bent nitrosyl structure (231). Further reaction under CO leads to the formation of Ru(CO)3L2 with the possible elimination of acrolein oxime. The coupling of the allyl and nitrosyl ligands can be viewed in this case as nucleophilic attack of NO- on an f/3-allyl species. Unlike in reaction (110), both of the moieties to be coupled lie within the same coordination sphere. The significance of these results is that it lends viability to the notion embodied in (109) in which a migratory insertion of nitrosyl occurs as NO-. 

In this catalysis a surface allyl species is formed which combines with NO to form 3-nitrosopropene. Tautomerism and dehydration then lead to the acrylonitrile product. The coupling of the allyl and nitrosyl ligands in Ru(NO)(CO)(C3H5)L2 (231), via (112) represents a key step in the proposed reaction sequence and suggests the necessity of a bent nitrosyl, at least in a discrete complex case. 

This structure should be contrasted with an alternative formulation of this species as a complex containing one linear and one bent nitrosyl which is suggested in (110) for the Ru analog. The dinitrogen dioxide ligand differs... 

Several mixed carbonyl-nitrosyls having the 18-electron count have been prepared from [Co(NO)(CO)3] or [Co(NO)2Br]2 (Table 11). All are four coordinate and probably tetrahedral, although the low vNO for K3[Co(NO)(CN)3] (1485 cm-1) may imply square-planar coordination with a bent nitrosyl. Rates of substitution in poorly coordinating solvents (THF, xylene equations 11 and 12) follow the rate expression kobs = k, + k,[L], with kt and k2 being attributed to dissociative paths involving the sovlent and ligand respectively.94 For L—L chelates formation of the monoden-tate species is rate determining.95... 

Both mono and dinitrosyl cations may be prepared. Salts of the tetrahedral dinitrosyl cation can be prepared either from rhodium nitrosyls (equation 56) or by addition of nitric oxide to ionic rhodium(I) complexes (equation 57). Both nitrosyl groups in complex (47) are bent. The action of dppe upon the i)A(triphenylphosphine) complex produces N2O. Penta- and hexacoordinate mononitrosyl cations are more common and are usually prepared by the action of NOPFe or NOBF4 on rhodium(I) complexes in organonitrile solvents (equation 58). The principal reactions of the complexes are substitutions of the nitrile ligands (equations 59-61). A noteworthy feature of (48), [Rh(NO)(MeCN)3(PPh3)2]+, is that the MeCN ligand trans to the bent nitrosyl group is said to be bent. 

The NMR chemical shifts and coupling constants for compounds (13)-(16) are tabulated in Table IX all the spectra were obtained from samples with 99% enrichment. The bent nitrosyl ligands resonate at high field whereas linear groups are found at medium field. It is instructive to compare the NMR data for [RuCl(CO)(NO)(PPh3)2] (15)... 

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