Ligand Labilization

Because their rates of substitution tue convenient for study, most work has been done with platinum complexes, and for lhe.se it is found that ligands can be airanged in a fairly consistent order indicating their relative abilities to labilize ligands irans to themselves ... 

Polymerization of alkynes by Ni" complexes produces a variety of products which depend on conditions and especially on the particular nickel complex used. If, for instance, O-donor ligands such as acetylacetone or salicaldehyde are employed in a solvent such as tetrahydrofuran or dioxan, 4 coordination sites are available and cyclotetramerization occurs to give mainly cyclo-octatetraene (cot). If a less-labile ligand such as PPhj is incorporated, the coordination sites required for tetramerization are not available and cyclic trimerization to benzene predominates (Fig. A). These syntheses are amenable to extensive variation and adaptation. Substituted ring systems can be obtained from the appropriately substituted alkynes while linear polymers can also be produced. 

As already remarked, in compounds wer-IrX3 (PMe2Ph)3, the most labile ligand is the group X (X = H, Me, halogen) trans to a phosphine. The Ag+-assisted removal of chloride in (I) yields [IrCl2(H20)(PMe2Ph)3]+ (XII) where the H20 is very weakly bound (Ir—O 2.189 A, compare Ir(H20) + 2.041 A) and readily replaced. 

Additional results indicate that the formation of (XV) is the result of the fact that S-coordinate Ir(III) complexes prefer a square-pyramidal geometry with the vacant octahedral site trans to the most strongly trani-labilizing ligand (COEt) [R. J. Mawby, private communication (March 1972)]. 

RATES OF ACID HYDROLYSIS OF SOME Co(IlI) COMPLEXES AT 250 °C The labile ligand is italicized. 

It was found by Trost that the low reactivity could be circumvented by the employment of labile ligands, such as the propionitrile in the Mo(CO)3(EtCN)3 precatalyst [57]. Instead of directly transferring this procedure to microwave heating applications, a useful and easily handled microwave procedure was developed for rapid and selective molybdenum-catalyzed allylic alkylations under noninert conditions (Eq. 11.39) [12]. The former, more sensitive, two-step reaction was fine-tuned into a robust one-step procedure employing the inexpensive and stable precatalyst Mo(CO)6, used in low concentrations. The alkylations were conducted in air and resulted in complete conversions, high yields, and an impressive enantiomeric excess (98%) in only 5-6 min. Despite the daunting temperatures, up to 250°C with THF... 

Diphosphine mono-chalcogenides are able to bind to metal centres from either one of or both the phosphorus(III) and chalcogen donor centres. This has led to their study as hemi-labile ligands in catalyst systems, whereby one of the donor centres binds strongly to the metal centre and the other one is weakly bound and hence can be readily dissociated or reattached to reveal or protect the active site at a metal centre.28... 

This difference in behavior is felt to be due to the fact that Cr(co)e must first dissociatively lose a CO ligand, a fairly energetic process with an activation enthalpy of 40 kcal. (25.), prior to binding the formate ion. The once formed Cr(C0)502CH proceeds to Cr(C0)sH with subsequent production of H2 and C02 as described in the Scheme. Support for this interpretation was obtained by replacing Cr(C0)e with a chromium carbonyl species containing a labile ligand,... 

In aqueous solution the labile ligands (X") are stepwise replaced by water molecules. Under acidic conditions the hydrolysis reactions are strongly reversible, which gives an equilibrium mixture according to Eq. 1. Much effort has been given to quantify the rate and equilibrium... 

See Section IV.B.2 above for a recent example of the balance between dissociative (D) and associative mechanisms, in the case of ring opening of platinum(II) chelates of hemi-labile ligands. 

C (23). While these results would be consistent with a mechanism whereby a labile ligand has to be replaced by benzene, no inhibition of the reaction was observed with added trifluoroacetate. Instead a slight enhancement occurred. Since the data do not exclude either mechanism, the authors explicitly left open the question of whether this reaction occurs by replacement of trifluoroacetate by benzene or via the direct pathway. 

Facile C-H bond activation by Pt(II) metal centers seems to require at least one labile ligand in the coordination sphere of platinum. One of the earliest intermolecular examples of this is the activation of C-D bonds in benzene-f/, by 0 an.S -(PAIe .) Pt(neopentyl)(OTf) at 133 °C, where trifluoromethanesulfonate (triflate, OTf) provides the labile group (Scheme 7, A) (26). 

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