Nitrosyl ligand, coordination

Coordination about the osmium in 47 is best regarded as distorted trigonal bipyramidal with axial triphenylphosphine ligands. The distortion is toward a square pyramidal geometry with an apical nitrosyl ligand. Coordination about Os in the six-coordinate phenylcarbene complex is octahedral, as expected. 

NO reacts with both ferric and ferrous centers in hemoproteins to form the respective iron(II) and iron(III) nitrosyl adducts, whose structural features are similar to those observed for iron (II) and iron(III) porphyrin nitrosyls. These analogies are also reflected in similar chemical reactivity observed for nitrosylated ferri- and ferroproteins and their respective porphyrin models. For example, NO-adducts of Fe(III) undergo reductive nitrosylation in the presence of an excess of NO, and a similar process is commonly observed for synthetic Fe(III) porphyrins. The first step of this reaction involves nucleophilic attack of OH on the nitrosyl ligand coordinated to the iron center, as presented in reaction (13) (33,60) ... 

Besides ruthenium porphyrins (vide supra), several other ruthenium complexes were used as catalysts for asymmetric epoxidation and showed unique features 114,115 though enantioselectivity is moderate, some reactions are stereospecific and treats-olefins are better substrates for the epoxidation than are m-olcfins (Scheme 20).115 Epoxidation of conjugated olefins with the Ru (salen) (37) as catalyst was also found to proceed stereospecifically, with high enantioselectivity under photo-irradiation, irrespective of the olefmic substitution pattern (Scheme 21).116-118 Complex (37) itself is coordinatively saturated and catalytically inactive, but photo-irradiation promotes the dissociation of the apical nitrosyl ligand and makes the complex catalytically active. The wide scope of this epoxidation has been attributed to the unique structure of (37). Its salen ligand adopts a deeply folded and distorted conformation that allows the approach of an olefin of any substitution pattern to the intermediary oxo-Ru species.118 2,6-Dichloropyridine IV-oxide (DCPO) and tetramethylpyrazine /V. V -dioxide68 (TMPO) are oxidants of choice for this epoxidation. 

The nitrosyls RuH(NO)(PR3)3 are 5-coordinate with trigonal bipyramidal structures and linear Ru-N-O geometries the hydride and nitrosyl ligands occupy the apical positions (for RuH(NO)(PPh3)3, z/(Ru-H) 1970 cm-1, i/(N—O) 1640 cm-1 H NMR, 8 = +6.6 ppm for the hydride resonance). The high-field NMR line is a quartet showing coupling with three equivalent phosphines, which would not be possible in a square pyramidal... 

Nucleophilic reactions with coordinated NO are illustrated by the well known reversible reaction of hydroxide ion with the nitrosyl ligand of... 

Method (i) is a route commonly utilized in monometal nitrosyl complexes. The nitrosyl ligand may function as (formally) a three-electrop donor (NO+) with a linear bonding mode, or as (formally) a one-electron donor (NO ) with a bent (—120°) M-N-0 arrangement. Conversion of the M-NO system to a M-NO system has two effects. First, it increases the metal oxidation state by two second, it generates a vacant coordination site. The dinitrosyl cluster Os3(CO)8(NO)2, which has... 

Treatment of [W(NO)2Cl2] with AgBF4 in MeCN leads to cw-[W(NO)2(MeCN)4](BF4)2. NMR measurements have shown that this complex undergoes stereospecific exchange of coordinated MeCN via a dissociative pathway. The two labile MeCN groups are those trans to the nitrosyl ligands.399... 

Although the ultimate fate of the nitrosyl ligands has not been established, a reasonable pathway for their removal can be postulated based on our IR data and recent literature results. Shriver et al. (42) have found that organoaluminum species will complex with the oxygen of metal-coordinated carbonyl groups. For example, the six-coordinate... 

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. 

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-. 

Reaction of amines with coordinated nitrosyl ligand 133... 

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