Cobalt nitrosyl coordination

Enemark, J. H., Feltham, R. D., Riker-Nappier, J., and Bizot, K. F. (1975). Stereochemical control of valence. IV. Comparison of the structures and chemical reactivities of five-and six-coordinate diarsine complexes of the cobalt nitrosyls CoNO) group. Inorg. Chem. 14, 624-632. 

General accounts of the reactivity of coordinated nitrosyls are available.106 121 124 A metal ion or metal complex is unable to generate the reactivity of ionic NO+ towards nucleophiles (i.e. OH-, RS-, RHN-) or of NO- towards electrophiles (i.e. H+, PhCH2X), but it can present NO in a less reactive form under conditions inappropriate to the free ion. Thus linear coordination with vNO > 1886 cm 1 may promote nucleophilic reactions at N, and bent coordination with the higher electron density on N (vNO < 1700cm-1) may promote electrophilic attack by H+, 02, NO and PhCHjBr.125 No cobalt nitrosyl is known to undergo nucleophilic attack on the nitrosyl group. Normally linear coordination is associated with four or five coordination and the nucleophile adds to the unsaturated metal instead. However a number of electrophilic reactions are known (Table 13). A recent discussion of these is recommended.124... 

PtG. 20. The proposed mechanism for the reaction of 5-coordinate cobalt nitrosyl complexes with oxygen. 

Well-characterized complexes of the MNO 8 class are scarce (Table 7.2).62 Five-coordinate species with bound NO and square pyramidal geometries are dominant,10 18 probably related to the great trans-effect of NO-. Most common are the cobalt nitrosyl porphyrins.18 By reacting with monodentate ligands (NCS-, Cl-, Br-, etc.), six-coordinate complexes may be obtained. Reaction 7.28 describes the preparation of the first compound of this type studied by X-ray methods.13s... 

One important characteristic of cobalt porphyrins is their ability to bind or react with small molecules, such as NO [27, 67, 70, 91, 93, 100], CO [36, 114, 115], O2 [314-320], or CO2 [321], and several studies have focused on the chemical and/or electrochemical reactivity of (P)Co toward these small molecules. The interaction of cobalt porphyrins with NO and the electrochemical properties of the resulting cobalt-nitrosyl porphyrins have been investigated by several research groups [7]. (TPP)Co(NO) exhibits two oxidations and three reductions at a microelectrode in CH2CI2 [90]. The NO group remains coordinated after electrooxidation and the initial electron abstraction from (TPP)Co(NO) was proposed to involve the porphyrin jr-ring system. Other electrode reactions were accompanied by a dissociation of NO from the compound and the site of electron transfer could not be determined. 

The hexamine cobalt (II) complex is used as a coordinative catalyst, which can coordinate NO to form a nitrosyl ammine cobalt complex, and O2 to form a u -peroxo binuclear bridge complex with an oxidability equal to hydrogen peroxide, thus catalyze oxidation of NO by O2 in ammoniac aqueous solution. Experimental results under typical coal combusted flue gas treatment conditions on a laboratory packed absorber- regenerator setup show a NO removal of more than 85% can be maitained constant. 

Oxygen-transfer reactions have been shown to occur from cobalt(III)-nitro complexes to alkenes coordinated to palladium.472 Thus ethylene and propene have been oxidized stoichiometrically in quantitative yields to acetaldehyde and acetone respectively, with the concomitant reduction of the nitro- to the nitrosyl-cobalt analog. A catalytic transformation with turnover numbers of 4-12 can be achieved at 70 °C in diglyme. The mechanism shown in Scheme 11 has been suggested. 

Identification of an efficient metal chelate for optimum absorption of NO requires knowledge of the thermodynamics and kinetics of the coordination of NO to various metal chelates. Knowledge is also needed of the kinetics and mechanisms of the reaction between nitrosyl metal chelates and absorbed SO2 in solution to calculate the regeneration rate of metal chelates and to control the products of reaction by adjusting the scrubber operating conditions. Not much of this information is available in the literature, although several ferrous and cobalt chelates have been used as additives for testing in bench-scale wet flue gas simultaneous desulfurization and denitrification scrubbers. 

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