Metal carbonyls, thermal dissociation

Replacement of Other Ligands in Mononuclear Metal Carbonyls Thermal Substitution.—The substitutions of tertiary phosphines L in the complex cw-[Mo(CO)4L2] by CO occur by dissociative mechanisms, the CO entering... 

Two U.S. patents issued to the Barium Steel Corporation in 1957 claim the formation of the heptacarbonyls M(CO)7 (M = Ti, Zr, Hf) as intermediates for the purification of these metals (9,10). In this described refining process, the finely divided metal is treated with CO at 300-400°C and 4-8 atm. The resulting liquid heptacarbonyl compound is then thermally dissociated to the pure metal and CO. The alleged existence of these binary carbonyls seems highly unlikely without supporting evidence. 

Thermal Substitution.—Carbonyl substitutions in Group VI metal carbonyls occur predominantly by dissociative pathways, associative pathways also contributing when the more nucleophilic incoming ligands are considered. The kinetics of the reaction of [W(CO)3(CS)(phen)] with ligands L (phosphines or phosphites) to give [W(CO)3(CS)L(phen)] show first- and second-order terms... 

Dinuclear Carbonyls.—Substitution reactions of [Mn2(CO)io] do not occur by CO dissociation or by associative reaction with incoming ligand, but by thermal or photochemical formation of [Mn(CO)5 ]. Rapid substitution of metal carbonyl radicals has been frequently noted but the mechanisms of these substitutions are... 

Energy-minimized conformations from molecular mechanics are useful to estimate the energetics of metal-ligand bond dissociation. A plausible thermal dissociation reaction for generic zero-valent metal carbonyl complexes that corresponds to the onset of the glass transition is... 

Mo(CO)j(THF) upon UV irradiation. In this case one of the CO ligands is removed and replaced by THE Since THE is easily dissociated, it can be replaced by another ligand in the second reaction step. This reaction is particnlarly useful for carbonyla-tion chemistry since metal carbonyls resist thermal snbstitntion and do not dissociate when heated. 

A film grows as a result of metal carbonyl adsorption on a low-index plane of the metal, followed by thermal dissociation of the carbonyl, diffusion of the resulting adatom into the atomic step of the growing metal crystal, and addition to the crystal (Fig. 1). 

CO substitution in heteronuclear cobalt carbonyl complexes has also been studied. Several phosphines were used in exchange reactions with CO in the complex CpMo(CO)3-Co(CO)4. Both a CO dissociative pathway, leading to substitution, and a radical chain pathway, initiated by associative attack of the phosphine and subsequent disproportionation (see Disproportionation), were found depending on reaction conditions (thermal or photochemical treatment) and the basicity and sterics of the phosphine ligands. As seen in substitution reactions with Co2(CO)g, strongly basic phosphines add to the complex via disproportionation, while less basic phosphines add via substitution. However, the metal centers help determine where the phosphine adds because the rates of substitution on each metal center differ depending on which pathway, dissociative or radical chain, is operative. ... 

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). 

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