Of coordinated nitrite

DeLeo MA, Ford PC. Photoreactions of coordinated nitrite ion. Reversible nitric oxide labilization from the chromium(III) complex [trans-Cr(cyclam)(ONO )2]+. Coord Chem Rev 2000 208 47. 

Other ligands that are the conjugate bases of weak Bronsted acids (e.g., NO2) show similar catalysis by a proton. In the case of coordinated nitrite, the proposed protonated intermediates (32) may actually be isolated in solid salts [here the nitrate [Co(NH3)5(HONO)](N03)3]. In both 31 and 32. A represents the ligand NH3. 

Thus, nitrite complexes are characterized by the highest redox potentials. The nature of the supporting electrolyte is of importance here. For example, potentials measured in aqueous HCl are much higher than those measured in aqueous NaCl. Apparently protonation of coordinated nitrite may be significant. 

The nitrate group, like the nitrite group, is a versatile ligand and numerous modes of coordination have been found in nitrato complexes. ° The uncoordinated N03 ion (isoelectronic with BF3, B03 , C03 , etc.) is planar with N-O near 122 pm this value... 

Figure 2.7 (a) A front view of the nitrite reductase dimer with the five haems in each monomer in white, a bound Ca2+ ion in grey and Lys-133, which coordinates the active site iron of haem 1, in yellow, (b) The haem arrangement. The overall orientation corresponds to (a), with the active site located at haem 1. Reprinted with permission from Einsle et al., 1999. Copyright (1999), Macmillan Magazines Limited. 

Coin-cyclam322-324 and Nin-cyclam322 catalyze the electroreduction of nitrate in aqueous electrolytes with good current efficiencies and turnover numbers, giving mixtures of ammonia, nitrite, and hydroxylamine at a variety of electrode materials. Mechanistic investigations suggested the adsorption of electroreduced Co1- and Ni1 cyclam onto the electrode surface,322 and the formation of an oxo-metal bond via reduction of coordinated nitrate.323... 

The bis-hydroxylamine adduct [Fe (tpp)(NH20H)2] is stable at low temperatures, but decomposes to [Fe(tpp)(NO)] at room temperature. [Fe(porphyrin)(NO)] complexes can undergo one-and two-electron reduction the nature of the one-electron reduction product has been established by visible and resonance Raman spectroscopy. Reduction of [Fe(porphyrin)(NO)] complexes in the presence of phenols provides model systems for nitrite reductase conversion of coordinated nitrosyl to ammonia (assimilatory nitrite reduction), while further relevant information is available from the chemistry of [Fe (porphyrin)(N03)]. Iron porphyrin complexes with up to eight nitro substituents have been prepared and shown to catalyze oxidation of hydrocarbons by hydrogen peroxide and the hydroxylation of alkoxybenzenes. ... 

Table III also shows the values of the equilibrium constants, KVAp for the conversion of iron nitrosyl complexes into the corresponding nitro derivatives. Keq decreases downwards, meaning that the conversions are obtained at a lower pH for the complexes at the top of the table. Thus, NP can be fully converted into the nitro complex only at pHs greater than 10. The NO+ N02 conversion, together with the release of N02 from the coordination sphere, are key features in some enzymatic reactions leading to oxidation of nitrogen hydrides to nitrite (14). The above conversion and release must occur under physiological conditions with the hydroxylaminoreductase enzyme (HAO), in which the substrate is seemingly oxidized through two electron paths involving HNO and NO+ as intermediates. Evidently, the mechanistic requirements are closely related to the structure of the heme sites in HAO (69). No direct evidence of bound nitrite intermediates has been reported, however, and this was also the case for the reductive nitrosylation processes associated with ferri-heme chemistry (Fig. 4) (25).

Potassium Hexanitrocobaltate(III). Add a little acetic acid and an excess of potassium nitrite to a cobalt(II) salt solution. Heat the mixture. What gas evolves What precipitates Write the equation of the reaction. What is the coordination number of cobalt in this compound ... 

The coordination chemistry of nitrite ion has recently been very extensively reviewed.1637 The bonding modes of nitrite ion found in nickel(H) complexes are summarized in Figure 25 and selected nickel(II) complexes containing coordinated nitrite ions are shown in Table 82. 

Far more numerous are the complexes of nickel(II) which contain a variety of either aliphatic or aromatic amines and coordinated nitrite (Table 82). Most of these complexes are prepared by the reaction of the amine in methanolic solution with a methanolic solution of nickel nitrite prepared metathetically from hydrated nickel nitrate and sodium nitrite.1654-1659... 

The structure determination of Zn(en)(NCS)Cl reveals monomeric tetrahedral zinc coordinated to thiocyanate via nitrogen.179 A structure determination of Cd(en)(N02)2 reveals a polymer containing cadmium in a seven-coordinate pentagonal prismatic environment the en functions as a bridging ligand, as does one of the nitrites.180... 

An especially interesting and important observation is the transformation of coordinated NO to NH3 on the complex [Ru(trpy)(cipy)L]2+ (trpy = 2,2,2"-terpyridine and bipy = 2,2 -bipyridine) through the nitrosyl complex [Ru(trpy)NO]3+. This models the reaction of the enzyme nitrite reductions. 

The dissimilatory nitrite reductase (cytochrome cdlt see below) catalyzes the nitrosation of several nucleophiles by nitrite.1531 This may occur through a heme-nitrosyl in which an NO+ group is present. Coordinated nitrite in simple metal complexes such as [Run(NH3)5(N02)]+... 

The coordinated nitroxide (NO-) is slowly autoxidized to coordinated nitrite. RhNO(OEP) is also formed by action of NO upon the hydride RhH(OEP) (paths p,t) or even RhCl(OEP) (path v) in toluene. The latter... 

The photodissociation of methyl nitrite in the first absorption band, CH30N0(Si) — CH3O + NO(n, j), exemplifies indirect photodissociation (Hennig et al. 1987). Figure 1.11 shows the two-dimensional potential energy surface (PES) of the S electronic state as a function of the two O-N bonds. All other coordinates are frozen at the equilibrium values in the electronic ground state. Although these two modes suffice to illustrate the overall dissociation dynamics, a more realistic picture... 

Let us consider, as a simple prototype, the photodissociation of the nitrites X-NO with X=CHsO, HO, Cl, or F, for example. The dissociation of CH3ONO has been discussed in detail in Sections 7.3 and 7.4. The excited-state PES has a barrier along the X-NO dissociation coordinate R, as sketched in Figure 10.12(a), which traps the excited complex for an appreciable time before it breaks apart. Figure 10.12(b) shows schematically a two-dimensional representation featuring the three essential re-... 

The coordination of the nitrite ion appears to differ with the salt. For example, in AgN03, Ag+ is closer to N than to O, whereas in NaN02, Na+ is closer to O than to N.2 The periodic table existence chart (Figure 5.1) which lists the known nitrites omits bonding information. 

Classical methods for the synthesis of frans-dinitro-bis(ethylenediamine)cobalt(III) nitrate involve conversion of the cis isomer by heat1 or action of sodium nitrite on frans-chloronitrobis(othylenediamine)cobalt(III) nitrate.2 These methods involve isolation of intermediate coordination compounds and consequently are subject to low yields, based upon original cobalt(II) salt. The following procedure for the preparation of the trans compound gives an 84% yield it depends upon oxidation of cobalt(II) to cobalt(III) in the presence of sodium nitrite, ethylene-diamine, and nitric acid. 

The nitrite ion is an excellent coordinating agent, generally forming metal —N02 bonds (although there are a few cases of metal —ONO bonds reported). One of its most interesting reactions takes place with dipositive cobalt in the presence of dilute acid. In this reaction, part of the nitrite coordinates With the cobalt, and part of the nitrite oxidizes the cobalt to the +3 state (NO being formed). Thus one cobalt ion consumes seven nitrite ions to yield the complex Co(N02)e8, the ion often used in qualitative analysis to precipitate potassium from solution. 

Fig. 4 A linear coordination polymer of silver with 4,4 -bipyridyl and bidentate nitrite anions,13 There is a 3.0 A contact between the silver and the nitrogen of the nitrite ion, shown as a dotted line, if this was a full covalent bond, the system would be two-dimensional. Gray spheres denote carbon, white hydrogen, dark blue nitrogen, red oxygen, and green silver.

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