Carbon monoxide, substitution

Most of the reported reactions between tetranuclear clusters and alkynes involve mixed-metal cluster species. In these systems hydride and carbon monoxide substitution generally occurs [Eq. (11)] (194-200), although in some cases Me3NO has been used to activate the starting material (201, 202), and in still others cluster breakdown takes place even under mild reaction conditions (203). Rh4(CO)12 (204) and Ir4(CO)12 (205) retain their nuclearity in reactions with alkynes, but in the latter case the metal framework geometry is altered (Fig. 7). The use of [Ir4(CO)11Br] instead of Ir4(CO)12 in reactions with alkenes produces alkene-substituted tetranuclear complexes (189), as shown in Fig. 7. Few other homonuclear clusters have been found to react with alkynes (206-208). In the reaction between the tetranuclear cluster Cp2W2Ir2(CO) 0 and diphenylacetylene two independent processes... 

As discussed above, reduction of metal-metal bonded complexes is common, particularly for carbon monoxide-substituted complexes. The electrochemical potential of these reactions has been widely studied (see Electrochemistry Applications in Inorganic Chemistry). In addition, Meyer has shown that the metal-metal bond strength can be estimated via electrochemical techniques. The results applied to Mn2(CO)io are in agreement with values obtained by other methods and could provide a means of generating a wide range of metal-metal bond strengths. The legs of the electrochemical cycle are shown in equation (99). 

As seen from Table III, iron pentacarbonyl reacts satisfactorily in spite of its inertness towards carbon monoxide substitution under the normal conditions 189). In benzene at 80° C, however, Fe(CO)5 dissociates rapidly (190). The Fe(CO)4 generated displays a nucleophilic reactivity which should promote an A-type mechanism. In spite of the specificities discussed, Maitlis et al. 177) have proposed the following mechanism for the metal carbonyl exchange reactions. 

A method to elude those defects, induced reconstructions, or anion adsorption is to transfer the electrodes under well-controlled conditions including atmosphere. Thus, undesirable effects from oxygen adsorption or impurities as a source of voltammogram modifications can be avoided. These requirements are fulfilled in the iodine-carbon monoxide substitution method which was proposed for the preparation of clean and well-ordered Pt( 111) [66] and applied to Pt(100) clean surface preparation [67]. An interesting alternative to this method would be to find experimental conditions that maintain a carbon monoxide adlayer for surface protection during the transfer, assuming that this adsorption is innocuous for the surface structure itself. If this efficient protection makes no detectable surface-order modifications for Pt(100) electrodes as deduced from the cyclic voltammetric contour, we can conclude that this protection method is convenient for studying the influence of anion adsorption on the surface structure in transfer experiments. 

The lowering of carbonyl stretching frequencies by the introduction of has provided innumerable systems in which the course of carbon monoxide substitution, insertion, and migration could be readily followed by infrared. The advent of Fourier Transform nmr and high field nmr spectrometers has provided straightforward analysis not only of labeling, but of H, and as well as direct detection of many metal nuclei. The use of nmr appears to provide a particularly useful technique for the study of carbonyls in metal systems with I > 2 nuclei, such as cobalt, for which resonances are often broad or unobservable. 

The solvent also plays a significant role in the reaction of [Fe(77-C5Hg)(CO)2]2 with iodine. In chloroform there is evidence for ionic intermediates [Fe(ir-C6H5)(CO)2]2l X there is no evidence for such an intermediate in the poorer solvent n-hexane. The product, however, is the same in both solvents—again as in the above Mn example. Rates of carbon monoxide substitution by phosphites in the closely related monomer... 

Often the free trimetallasubstituted ligands, e.g. [Co3(CO)9Cm3-E)] (E = P,As), [H2Ru3(CO)9( 3-S)], and [Cos( 6-As)(a4-AsPh)2(CO)i7( 3-As)], are rather difficult to isolate in a pure crystdline state because of their instability under the experimental conditions required for their synthesis. In these cases, such clusters of clusters as [Co,( 4-E)3(CO)24] (Fig. 3-24a), [392] [H6Ru,( 4-S)3(CO)24], [396] and [Co,6( M6-As)2(/<4-As)20u4-AsPh)4(CO)32] (Figure 3-24b) [313] can be formed from their intermolecular dimerization or trimerization reactions via carbon monoxide substitution and directly isolated. 

Carbon monoxide and excess steam are normally passed over a cobalt catalyst at about 250-300 C resulting in greater than 99% conversion of CO to COj. This conversion reaction is widely used in oil or solid fuel gasification processes for the production of town gas or substitute natural gas. ... 

Polyimides have been synthesized by Diels-Alder cycloaddition of bismaleimides and substituted biscydopentadienones (81,82). The iatermediate tricychc ketone stmcture spontaneously expeU carbon monoxide to form dihydrophthalimide rings, which are readily oxidized to imides ia the presence of nitrobenzene. 

Lithium 1,2,4-triazolate with [Rh2( j,-Ph2PCH2PPh2)(CO)2( j.-Cl)]PFj. gives the A-framed complex 177 (L=L = CO) (86IC4597). With one equivalent of terf-butyl isocyanide, substitution of one carbon monoxide ligand takes place to yield 177 (L = CO, L = r-BuNC), whereas two equivalents of rerr-butyl isocyanide lead to the product of complete substitution, 177 (L = L = r-BuNC). The starting complex (L = L = CO) oxidatively adds molecular iodine to give the rhodium(II)-rhodium(II) cationic species 178. 

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