Carbon monoxide bridged complexes

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). 

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. 

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. 

Table III shows the bonding modes that have been established for carbon monoxide in these polynuclear aggregates. In addition to the terminally bonded group 0 ) observed for the mononuclear complexes, a variety of bridging carbonyl species have also been identified, involving two-electron and four-electron donation from the carbonyl group to the cluster unit.

In conclusion, the presented dinuclear iron structure is the first example of a bio-mimetic iron compound, which can be regarded as a first generation model for the class of [Fe]-only hydrogenases. The complex incorporates both relevant carbon monoxide ligands, as well as three bridging thiolato ligands, which could be possibly present in the active site of these enzymes. 

The cis dicarbonyl intermediate 23 could not be isolated, although it was longer lived when cp = cp [57] than when cp = cp. The cis trans isomerization of the dicarbonyl complex is catalyzed by CO, presumably via an unstable CO-bridged intermediate, which would decay to the tram isomer and carbon monoxide (Sch. 15) [56]. 

My last comment concerns the reaction of palladium olefin complexes with carbon monoxide discovered by Tsuji. I agree that this is most likely to proceed by an insertion rather than an ionic mechanism. Chloride attack on coordinated olefin is rare however. Chloride ion is an inhibitor, for example in the palladous chloride catalyzed hydration of ethylene (0). I, therefore, wondered whether carbon monoxide was affecting the ease with which chloride attacks olefin. One can postulate that carbon monoxide participates in this insertion either as a gas phase reactant or by first forming a carbonyl olefin complex. Such complexes of the noble metals were unknown, but examining the reaction between carbon monoxide and the halogen bridged olefin complexes of platinum revealed that they are formed very readily... 

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