Carbonyl clusters, nucleophilic attack

However, while ruthenium carbonyl was found to decompose the formate ion in basic media, the rate was slower (<0.1 mmol trimethyl ammonium formate to H2 and C02 per hour) than the rate of the water-gas shift reaction (>0.4 mmol H2/hr) at 5 atm CO and 100 °C. Increasing CO pressure decreased the formate decomposition rate. However, it was observed that increasing the CO pressure from 5 atm CO to 50 atm increased the H2 production rate to 10 mmol/hr. They proposed, in a similar manner to Pettit et al.,34 a mechanism that involved nucleophilic attack by amine (instead of hydroxide). Activation of the metal carbonyl (e.g., Ru3(CO) 2 cluster to Ru(CO)5) was suggested to be favored by dilution, increases in CO pressure, or, in the case of Group VIb metal carbonyl complexes, photolytic promotion. The mechanism is shown below in Scheme 9 ... 

Figure 4 Proposed ACS mechanisms. The major difference between the two mechanisms is the oxidation state of the proximal Ni. In both cases, CO emerges from the channel and binds to the proximal metal of the A-Cluster to form a Ni-CO intermediate. Then, the Ni center performs nucleophilic attack on the methyl group of methylated CFeSP to form a (CH3)(CO) intermediate state of the A-Cluster. Then, carbon-carbon bond formation occurs by condensation of the methyl and carbonyl groups to form an acetyl-metal species. Finally, CoA binds to cleave the acetyl group as acetyl-CoA. See text for more details...

The cobalt carbonyls are prepared by the disproportionation reaction of [Co2(CO)g] in the presence of Lewis bases or by the reduction of cluster cobalt carbonyls with the alkali metals. The iridium compounds are obtained during reduction of [Ir4(CO)i2] with sodium in ether solution. The rhodium carbonyls are usually synthesized by reduction of [Rh2Cl2(CO)4] or [RhClg] " with carbon monoxide in basic medium or by nucleophilic attack of bases on the carbonyl group of carbonyl clusters (see preparation of [M4(CO)i2] and [M6(CO)i6]). 

At this point, it only remains to report on an alternative synthesis for nitride clusters Equation (3.33), which is based on a different approach as the previously described cleavage of NO. A nucleophilic attack by Nj on coordinated CO followed by the elimination of dinitrogen will generate a coordinated isocyanate ligand. The subsequent step to form the nitride is based on the observation that the preformed [Ru4(CO)i3(NCO)]" isocyanate substituted carbonyl cluster slowly converts into the [Ru4(m4-N)(CO)i2] nitride derivative on standing in solution. [268, 363]... 

From a mechanistic perspective, the metal s role in a catal3rtic mechanism such as illustrated by Scheme 1 is several-fold. Coordination to the metal center serves to activate the CO toward nucleophilic attack and stabilize the hydroxy-carbonyl moiety (Scheme 1, step a). For example, the reaction of OH with the coordinated CO of the triangular cluster Ru3(CO)i2 to form a hydroxycarbonyl adduct RU-CO2H has been determined to be 1.2 x 10 s , a value many... 

OssC(CO) (C02R)X] (X = H or I) has been determined, and the metal core shown to exhibit the expected bridged-butterfly geometry. Another preliminary report on nucleophilic attack at a coordinated CO of a carbido carbonyl cluster compound has also appeared. The heterometallic carbido cluster anion [Co6Ni2C2 (C0)i6 ] results from the reaction of [003(00)9(001)] with [Nig (C0)i2 ] and contains two C atoms located in a cavity in the cluster and separated by only... 

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