Nitrosyl complexes isomerization

Most rhenium nitrosyl complexes have been prepared using NO, NOX or HN03 as the nitrosylat-ing agent. Recently, the reductive nitrosylation of [Re04] with NH2OH-HCl in the presence of NCS or Nf in an alkaline medium has been accomplished.556 The complexes formed in this way include [Re(NO)X3(H20)] (X = NCS or N3) and [Re(N0)X2(L)(H20)j (L = bipy or phen), containing the [ReNO]2+ moiety and existing in several isomeric forms. 

Ruthenium forms more nitrosyl complexes than any other element. The dinitrosyl complex [Ru(NO)2(PPh3)2] was first prepared from [RuCl2(CO)2(PPh3)2] and Na[N02] in DMF [Ru(ONO)2(CO)2(PPh3)2] is formed initially but this isomerizes to [Ru(N02)2(CO)2(PPh3)2] which decomposes to the [Ru(NO)2(PPh3)2] complex,1406 probably via [Ru(ONO)(CO)(NO)(PPh3)2].1407... 

The ability of the nitrosyl ligand to behave as an electron pair reservoir has also been considered to play an important part in certain catalytically active systems. The vacant site provided by isomerization of the ligand could enable an unsaturated organic molecule to enter the transition metal s coordination sphere, thus forming an active intermediate. Examples of catalysis by nitrosyl complexes include the hydrogenation of alkenes by Rh(NO)L3 species and the dimerization of dienes in the presence of Fe(CO)2(NO)2 or Fe(n-C3Hs)(CO)2NO. Certain molybdenum dinitrosyl complexes, such as MoCljfNOljfPPhjlj, have also been found to provide very efficient alkene dismutation catalysts. ... 

A 2.134-g. (0.0202-mol) quantity of palladium metal is dissolved in aqua regia (3 ml. of 15 N HNOj + 12 ml. of 12 N HCl) which has been allowed to stand for about 30 minutes. If powdered palladium is used, no heat is required to effect solution for other forms of palladium metal, some heating is usually required. The solution is evaporated on a steam bath to a brown-black syrup, which is then covered with 5 ml. of 12 N hydrochloric acid and evaporated to moist salts. This treatment with hydrochloric acid is repeated twice more to remove nitric acid and to destroy nitrosyl complexes. The resulting tetrachloro-palladic(II) acid is dissolved, with stirring and heating, in a minimum volume of water (approximately 100 ml.), and 4.0 g. (0.068 mol) of sodium chloride is added. The dark brown solution is concentrated to one-half its volume on a hot plate and is then evaporated to dryness on a steam bath. This evaporation removes excess hydrochloric acid, which if allowed to remain leads to contamination of the tetra-amminepalladium(II) tetrachloropalladate(II) with the isomeric hydrochloric acid also prevents loss of product owing to... 

Schaniel et al. studied the photolysis products of some [(NH3)4Pt(NO)X] (X = C1, S04 , NO3 complexes) (F ure 21) (132,133). They reported the first photogeneration of nitrosyl linkage isomers in such Pt nitrosyl complexes, from irradiation of a powdered sample of the [(NH3)4Pt(NO)Cl] complex with h t in the spectral range 550—800 nm. IR spectral data revealed a surprising shift in the 1/440 band from 1673 to 1793 cm that they attributed to a nitrosyl-to-isonitrosyl linkage isomerization to a geometry between / -ON and // -ON (132). 

Figure 21 Nitrosyl-isonitrosyl isomerization in some [(NH3)4Pt(NO)X]" complexes.

Figure 26 Proposed mechanism of nitrite reduction to NO by cdj. (A B) reductive activation of resting (inactive enzyme) and tyrosine displacement (not shown) (B C) nitrite binding (C D) protonation of nitrite complex (D E) cleavage of N-0 bond and elimination of H2O (E F) intramolecular iron oxidation (valence isomerization) (F G H C and F G I C) redox reactions involving heme c and heme cl nitrosyl complex followed by rapid dissociation of NO ( ) enzyme gets trapped in a "dead end" species in the absence of reducing substrate or nitrite. Adapted with permission from Ref (21). Copyright 2014 American Chemical Society.

A five-coordinated complex [CoCl2(NO)(PCH3Ph2)2], shown in Figure 6, is known to exist in two isomeric forms. The trigonal bipyramidal isomer exhibits N—O stretching frequency at 1750 cm while the square pyramidal isomer absorbs at 1650 cm . The bond angle and nitrosyl-stretching frequencies of some mononuclear nitrosyl complexes of the first transition series is shown in Table 1. 

The (77 -allyl)Fe(GO)2NO complex was also shown to undergo rotameric transformations in several RT photochemical studies.These transformations include allyl rotation, GO loss, and nitrosyl linkage isomerism. 

Two reviews by F. Basolo make interesting reading. Both are retrospective, and center mainly on work carried out by the author and his co-workers. One deals with conventional ligand substitutions.the other with substitution reactions of carbonyl and nitrosyl complexes/ and both contain material on four- and five-coordinate compounds. A review by R. Romeo on dissociative reaction pathways in platinum(II) chemistry takes the form of a useful compilation of the various reaction types which might lead to three-coordinate 14-electron species. These include -eliminations and various insertion reactions as well as ligand replacements and isomerizations. ... 

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. 

Scheme 5-14 may be called a two-dimensional system of reactions, in contrast to Scheme 5-1 which consists of a one-dimensional sequence of two acid-base equilibria. In Scheme 5-14 the (Z/E) configurational isomerism is added to the acid-base reactions as a second dimension . The real situation, however, is yet more complex, as the TV-nitrosoamines may be involved as constitutional isomers of the diazohydroxide. In order not to make Scheme 5-14 too complex the nitrosoamines are not included, but are shown instead in Scheme 5-15. The latter also includes the addition reactions of the (Z)- and ( )-diazoates (5.4 and 5.5) to the diazonium ion to form the (Z,Z)-, (Z,E)- and (2 2i)-diazoanhydrides (5.6, 5.7 and 5.8) as well as proto-de-nitrosation reactions (steps 10, 11 and 12). This pathway corresponds to the reverse reaction of diazotization, as amine and nitrosating reagent (nitrosyl ion) are formed in this reaction sequence.

In recent years, the photoinduced linkage isomerization reactions of bound nitrosyl appearing in different complexes have been... 

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