Weakly bound ligands

Further data on this reaction are summarised in Table 20. The role of complexing anions is clear from the kinetics and also from relative rates. It appears that strongly bound ligands are associated with second-order reduction but that weakly bound ligands such as H2O result in a third-order reaction. One view of the third-order term for dilute sulphuric acid (as for aqueous HCIO4) is that the active reductant is a bridged species of the type (Fe S04 Fe ) . 

One can prepare derivatives of 4 that contain a weakly-bound ligand L, such as a pyridine or a dialkyl sulfide. These compounds show no... 

CCR is easily measured by heteronuclear NMR experiments of isotopically labeled molecules. The information extracted from these experiments will significantly improve the resolution of NMR structures, especially bound conformations of weakly bound ligands, since /-couplings cannot be used in this case. The reason is the fact that the nonbound conformation significantly contributes to the averaged values of the coupling constant. 

Later, this study was extended to other tripodal tetraphosphines such as PP3 [PP3 = P(CH2CH2PPh2)3] and other transition metals (Fe, Ru, Os), as Ir was observed to be inactive in the presence of such ligands [46]. In the case of the latter system, a vacant site for substrate coordination was easily generated by the loss of a weakly bound ligand from the metal precursor, that is [(PP3)MH(H2)]L... 

Scheme 2. Synthetic routes to some fullerene organometallics by displacement of weakly bound ligands.

Theoretical smdies on unsaturated systems such as the d moieties IMf ri -CgMes) (CO)] and [tra -M(PH3)2X] (M = Rh or Ir) show that the thermodynamics of addition of NH3 to form MiHjNH, are more favorable for the third row element due to stronger M—H and M—NH, bonding. Experimental work available to date is in harmony with these calculations and has shown that it is possible to add NH3 to displace a weakly bound ligand and form an amido-hydride. The first such reactions under mild conditions involved an lr(l) complex, as shown in Equation (6.11)." ... 

Solid products require somewhat different treatment. In general, rapid expansion of SCFs [35] is the most effective approach, at least for small-scale reactions. RESS precipitation separates the product as fine particles, free for traces of the SCF and other volatile components of the reaction mixture. RESS is particularly useful for the precipitation of organometallic compounds with relatively weakly bound ligands (e.g., C2H4, T 2-H2) [12,13] see Figure 9.1-5. 

The product is separated without the use of either heat or vacuum, conditions which might otherwise lead to loss of the weakly-bound ligand. In addition, the product is cooled somewhat by expansion of the SCF, thereby reducing the rate of decomposition. In reactions where the SCF is also a reactant (i.e. SCC2H4 in the preparation of C2H4 complexes) the product, once precipitated is held under a flow of the reactant gas, again minimizing the chances of decomposition [12]. 

In other syntheses, a weakly bound ligand L is displaced from [AuCIL] (L Me, CO, alkene) by the new ligand, or direct combination of the gold(I) halide with the ligand is used (equations 44-46).329-331... 

Fligh reactivity may also result from easy displacement of weakly bound ligands. The zirconium compound [Cp2Zr(THF)(CH3)]+, shown in Fig. 2.1 and discussed in the previous section, is an example. Unlike RhCl(PPh3)3, which tries to form extra bonds in its reactions, the zirconium compound s reactivity in a catalytic reaction is due to the easy displacement of THF by the substrate. 

The term coordinative unsaturation includes this type of reactivity also. In other words, the ability to form extra bonds or facile displacement of weakly bound ligands, which in many cases may just be solvent molecules, are both manifestations of coordinative unsaturation. 

It is quite common to find the site trans an oxo ligand to be vacant or occupied by a weakly bound ligand due to this n interaction... 

Such a process was considered likely because in the quadruply bonded dimers the metal atoms have vacant coordination sites to accept the ligand atoms of X and Y, and the X-Y bond energy plus the weak 6-bond of the dimer are compensated by formation of M-X and M-Y bonds. On the further assumption that unfavorable steric crowding of ligands about the metal atoms account for the lack of addition chemistry previously observed for these dimers we sought to prepare compounds having weakly bound ligands. Dissociation in solution as in eqn. 2... 

The SnClj ion can generally displace Q", CO, and more weakly bound ligands, for example,... 

A useful synthetic route to specific carbonyl-PF3 transition metal complexes (method C) involves displacement of relatively weakly bound ligands such as ammonia, iodide ion, or unsaturated organic ligands (e.g., dienes, arenes) from suitable metal carbonyl complexes. 

---