Ligand nitrosyl

The low-valent ferrate [Fe(CO)3(NO)] 76 or Hieber anion was discovered some 50 years ago by Hieber and Beutner [43, 44] in order to extend the Hieber base reaction [45,46], in which iron pentacarbonyl 78 reacts with alkaline bases to form the [Fe(CO)4] anion [47, 48]. Compared to its homoleptic analogue, the Hieber anion is more stable because the electron-withdrawing character of the nitrosyl ligand stabilizes the negative charge at the iron atom. 

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. 

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. 

Such a mechanism would have to involve the nitrosyl ligand acting in a non-innocent manner, changing from a three-electron donor to a one-electron donor in the intermediate complex. Such participation of the nitrosyl ligand has precedent in related systems (108). 

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... 

This reaction allows the preparation of a new type of complexes with nitrosyl ligands . 

Nitrosyl ligands have relatively poor fx-donor capabilities, but are excellent rr-acceptors. There are synthetic routes to nitrosyl complexes using NO gas, [NO][Bp4] or [NOJIPFg], hydroxyamine hydrochloride, HNO3, NaN02, or A -nitrosoamides. 

The mechanism of equation (3) is claimed to involve concerted binding of one amine molecule to the nitrosyl ligand with base-catalyzed proton removal." Kinetic parameters for decomposition of... 

The nitrosyl ligand in 7 is also activated toward attack by molecular oxygen. 

Butler, A. R., Glidewell, C., Hyde, A. R., and Walton, J. C. (1985). Formation of paramagnetic mononuclear iron nitrosyl complexes from diamagnetic di- and tetranuclear iron-sulphur nitrosyls Characterization by EPR spectroscopy and study of thiolate and nitrosyl ligand exchange reactions. Polyhedron 4, 797-809. 

---