Metals nitric oxide complexes

J. Lewis, Transition Metal—Nitric Oxide Complexes , Sci. Prog. (Oxford), 1959, 47, 506. 

In this manner, the field of monohalide-metal nitric oxide complexes was developed, as well as the so-called nitroprussiates extensively studied by Nast and Proschel (104). Since these complexes do not contain CO groups as ligands a discussion of the noteworthy results falls outside the limits of this survey, although the field is closely related to the metal carbonyl nitrosyls. 

Metal carbonyls form several complexes but those of major interest in MOCVD are the carbonyl halides and the carbonyl-nitric-oxide complexes. 

To separate osmium from ruthenium, the aqueous solution is acidified with nitric acid. While nitric acid oxidizes osmate ion to volatile osmium tetroxide, Os04, it converts ruthenium to a nitric oxide complex. Osmium tetroxide is removed from the solution by distillation in air and collected in an aqueous solution of caustic soda containing ethanol. Osmium tetroxide solution is heated with ammonium chloride, upon which osmium precipitates out as a complex chloride, 0s02(NH3)4Cl2. The precipitate is filtered, washed and decomposed by ignition with hydrogen to yield osmium metal. 

These results have been discussed by Seel in Structure and Valence Theory of Inorganic Nitric Oxide Complexes (102). In such complexes, nitric oxide is covalently bonded to the metal atom as the positive ion NO+, it being assumed that an electron is transferred to the metal. This allows the isoelectronic groups NO+, CO, CN to be considered together and permits understanding of such known series as [Fen(CN)5NO]2, [Fen(CN)5CO]3-, and [Fen(CN)5CN]4-. 

The nitric oxide molecule shows many similarities to carbon monoxide in its ability to form complexes with transition metals. Nitric oxide has an extra electron, occupying a n antibonding orbital, which is relatively easily lost. In the case of terminally bound NO, simple MO theory predicts that whilst M—NO+ will be linear, M—NO- may be a bent bond. The potential thus... 

The function of cytochromes in dissimilatory nitrite reduction does not appear to be one of electron transport. Payne et al. (1971) showed that epr studies indicated no metal involvement but there was a formation of a heme-nitric oxide complex during nitrite reductase action. 

A second proteinaceous fraction, nitric oxide reductase, containing a bound c-type cytochrome converts nitric oxide to nitrous oxide (Matsubara and Iwasaki, 1971 Payne et al., 1971 Cox and Payne, 1973). The enzyme is soluble in Pseudomonas perfectomarinus (Cox and Payne, 1973), but is particulate in Alcaligenes faecalis and Pseudomonas denitrificans (Matsubara and Iwasaki, 1971 Miyata et al., 1969). Epr studies indicate no metal involvement but the formation of a different heme nitric oxide complex during release of nitrous oxide (Payne et al., 1971). The physiological electron donors are not known. FADHa, reduced phenazine methosulphate, and reduced viologen dyes have been found to be effective (Thauer et al., 1977). 

Transition metal compounds containing nitric oxide as a coordinated ligand are normally called nitrosyl complexes. However, the term nitrosyl , is only sometimes restricted specifically to complexes which can be regarded as containing a three-electron metal-nitric oxide bond, and the term seems to be used generally for all nitric oxide compounds. Although there are many transition metal-nitrosyl complexes, relatively few also contain metal-carbon linkages and therefore fall within the subject of this chapter. 

In a back titration, a slight excess of the metal salt solution must sometimes be added to yield the color of the metal-indicator complex. Where metal ions are easily hydrolyzed, the complexing agent is best added at a suitable, low pH and only when the metal is fully complexed is the pH adjusted upward to the value required for the back titration. In back titrations, solutions of the following metal ions are commonly employed Cu(II), Mg, Mn(II), Pb(II), Th(IV), and Zn. These solutions are usually prepared in the approximate strength desired from their nitrate salts (or the solution of the metal or its oxide or carbonate in nitric acid), and a minimum amount of acid is added to repress hydrolysis of the metal ion. The solutions are then standardized against an EDTA solution (or other chelon solution) of known strength. 

Unlike nitric oxide, NO, the monomeric radical sulfur nitride, NS, is only known as a short-lived intermediate in the gas phase. Nevertheless the properties of this important diatomic molecule have been thoroughly investigated by a variety of spectroscopic and other physical techniques (Section 5.2.1). The NS molecule is stabilized by coordination to a transition metal and a large number of complexes, primarily with metals from Groups 6, 7, 8 and 9, are known. Several detailed reviews of the topic have been published. ... 

Compounds of several different types might be formed by introduction of nitric oxide into complex ions. If the metal atom provides one of the electrons of the electron-pair bond, NO should assume the 02-like 82 structure. If both bond electrons come originally from NO (which then... 

Carbonyl Nitric Oxides. Another group of metal-carbonyl complexes, worthy of investigation as CVD precursors, consists of the carbonyl nitric oxides. In these complexes, one (or more) CO group is replaced by NO. An example is cobalt nitrosyl tricarbonyl, CoNO(CO)3, which is a preferred precursor for the CVD of cobalt. It is a liquid with a boiling point of 78.6°C which decomposes at 66°C. It is prepared by passing NO through an aqueous solution of cobalt nitrate and potassium cyanide and potassium hydroxide. ... 

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