Carbon monoxide bridging

Loss of Carbon Monoxide Accompanied by Carbon Monoxide Bridge Formation... 

Table 3. Formation of Carbon Monoxide Bridged Complexes ...

If the reaction temperature is raised to 430 K and the carbon monoxide pressure to 3 atm, coordination of the metal atom in the rearranged product occurs via the phosphorus site, as in 159 (M = Cr, Mo, W) [84JOM(263)55]. Along with this product (M = W) at 420 K, formation of the dimer of 5-phenyl-3,4-dimethyl-2//-phosphole, 160 (the a complex), is possible as a consequence of [4 - - 2] cycloaddition reactions. Chromium hexacarbonyl in turn forms phospholido-bridged TiyP)-coordinatedcomplex 161. At 420 K in excess 2,3-dimethylbutadiene, a transformation 162 163 takes place (82JA4484). 

Studies of the bonding of carbon monoxide to the metal surfaces produced structures in which the carbon atom is linked to one, two, or three metal atoms. The existence of bonds to two or three atoms (bridged bonds) has been questioned on the basis of theoretical calculations. None of these bondings, however, clarify the mechanism to any extent. 

This technique is the most widely used and the most useful for the characterization of molecular species in solution. Nowadays, it is also one of the most powerful techniques for solids characterizations. Solid state NMR techniques have been used for the characterization of platinum particles and CO coordination to palladium. Bradley extended it to solution C NMR studies on nanoparticles covered with C-enriched carbon monoxide [47]. In the case of ruthenium (a metal giving rise to a very small Knight shift) and for very small particles, the presence of terminal and bridging CO could be ascertained [47]. In the case of platinum and palladium colloids, indirect evidence for CO coordination was obtained by spin saturation transfer experiments [47]. 

Balch et al.560 prepared the cluster [Pd3(/u-dppm)3(//3-I)(//3-PF3)]PF6 [dppm = bis-(diphenylphos-phino)methane] (shown in Figure 115), which contains a PF3 molecule triply bridging a central triangular Pd3 core. However, exceptions are not limited to PF3, an atypical phosphine often considered closer to carbon monoxide. For further examples, see the section on polynuclear Pd1 complexes. 

The diarsine and arsine/phosphine analogues of dppm have been used to prepare bridged diplatinum(I) complexes, in both cases with terminal chloro ligands.116,117 Both complexes react with carbon monoxide to produce carbonyl-bridged species. The mixed thio/phosphine ligand Ph2PCH2SMe (PS) also forms a diplatinum(I) complex by conproportionation of its dichloroplat-inum(II) complex with [Pt(dba)2].118 This dimer reacts with carbon monoxide to produce an unsupported dimer [PtCl(PS)(CO)]2 with the thioether arm of the Ph2PCH2SMe unbound. 

Although we have described terminal and bridging carbon monoxide, it is known to bond to metals in other ways. Some of the other types of linkages are shown as follows ... 

It was recognized during the development of propene hydroformylation that propene provided some stabilization for the catalyst. In the absence of the alkene, but in the presence of carbon monoxide and hydrogen, the catalyst can undergo what has been termed intrinsic deactivation. [3 3] Apparently after oxidative addition of triphenyl-phosphine to rhodium, diphenylphosphido bridged rhodium complexes are formed. 

A dissociative adsorption of methanol forming surface methoxy groups is suggested as the initial step. This is followed by the slow step, the formation of some form of adsorbed formaldehyde species. Evidence.for the bridged species is not available, experiments with °0 labeled methanol are expected to clarify this. Continued surface oxidation leads to a surface formate group and to carbon monoxide. All the byproducts can be obtained by combination of the appropriate surface species. 

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