Nitric oxide, metal carbonyl complexes

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. ... 

Metal nitrosyls are the transition metal complexes of nitric oxide (NO) containing a metal-nitrogen bond. Roussin s red salt, Na2[Fe2(NO)4S2], and Roussin s black salt, Na[Fe4(NO)7S3], were the earliest known metallic nitrosyls. In line with the inclusion of metal carbonyl complexes under the category of organometaUic compounds, the metallic nitrosyl complexes are also recently included in organometaUic compounds. The nitric oxide cation is isoelectronic with carbon monoxide. Hence, there is quite a bit of resemblance in the chemistry of metaUic carbonyls and nitrosyls. However, the contrasts in these chemistries are also noteworthy. 

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

It is possible that nitric oxide may be displaced from the metal centre prior to oxidation of the phosphine. Indeed, such a nitrosyl displacement in the complex CrCl(7t-Cp)(NO)2, leading to derivatives of the type CrCl(jr-Cp)NO(L), has been reported.118 However, such behaviour is rare (compared with CO displacements in metal carbonyls) and it seems likely that the above reduction takes place by an alternative process. 

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. 

Apart from the work on binary metal carbonyls and metal carbonyl hydrides, Flieber and his school also greatly extended the field of carbonyl(ni-trosyl) metal complexes. The first compound of the general composition M(CO)x(NO)- was obtained by Robert L. Mond and Albert Wallis as early as in 1922 [67]. While studying the reactivity of Co2(CO)8 toward various substrates, they observed that slowly at room temperature, but almost instantaneously at 40°C, nitric oxide gas reacts with cobalt tetracarbonyl to form a cherry-red liquid, with the evolution of carbon monoxide . This liquid was... 

The catalytic implications of the alternative coordination modes of nitric oxide in transition metal complexes were first noted by Collman (12). He argued that the linear bent transformation, concomitant with a change in the formal oxidation state of nitrogen from (III) to (I), results in the withdrawal of electron density from the metal center and facilitates the coordination of another ligand into a vacant site. Thus, the mixed carbonyl nitrosyl complex [Co(CO)3(NO)] undergoes thermal CO substitution by an associative mechanism, whereas the iso-electronic, homoleptic carbonyl [Ni(CO)4] reacts by a dissociative pathway (13). 

The discovery that the nitrosyl ligand is capable of binding to transition metals in two isomeric valence forms1 is one of the most dramatic recent developments in organometallic chemistry. Since bent NO donates 2 fewer electrons to the metal than the linear isomer does, linear-bent tautomerism raises the possibility of coordinative unsaturation and catalysis.2 In fact, complexes of nitric oxide are receiving increasing attention as catalysts,3 since they are more reactive than the corresponding carbonyls.4... 

Nitric oxide reacts readily with oxygen to form brown nitrogen dioxide at ordinary temperatures. Several metal ions form complex ions with nitric oxide. It can also replace carbon monoxide in certain metal carbonyls. 

The complex Cr(NO)4 is as yet unknown. Similarly, V(NO)(CO)5 is isoelectronic with Cr(CO)6, Mn(NO)(CO)4 with Fe(CO)5, and Mn2(NO)2(CO)7 with Fe2(CO)g. All nitrosyl carbonyls and their substitution products conform to the inert gas formalism, in which for the purposes of electron accounting the nitric oxide molecule is assumed to donate three electrons and the carbonyl group two, to the metal atom. 

Of particular interest is the oxidation of the complexes Fea CjoH R Og (XXXI R = R = CH3 or H) with aqueous ferric chloride to give a product of composition (RC2R)Fe(CO)6 [190). Oxidation of the dimethyl derivative with cold concentrated nitric acid gives dimethylmaleic anhydride (XXXII). The derivative (HC2H)Fe(CO)6 forms a Diels-Alder adduct (XXXIII) with cyclopentadiene the adduct may be oxidized to succinic acid. All of these data indicate that the derivatives (RC2R)Fe(CO)6 are cyclic derived from maleic acid derivatives. Four of the six carbonyl groups are metal... 

The bluish white, hard, yet ductile, metal is inert to all acids and highly non-abrasive. Used for heavy-duty parts in electrical contacts and spinning jets. Reflectors are prepared from the mirror-smooth surfaces (e.g. head mirrors in medicine). Thin coatings provide a corrosion-resistant protective layer, for example, for jewelry, watches, and spectacle frames. The metal is a constituent of three-way catalysts. Rhodium complexes are used with great success in carbonylations (reactions with CO) and oxidations (nitric acid) in industry. Platinum-rhodium alloys are suitable thermocouples. 

---