Chelate Phosphanes Influence of Ring Size

In the series mer- MLfl (M=Rh, Ir), we see that A is large (in magnitude) and B is small, indicative of a much smaller influence of o donicity on AS. There is no coligand of appreciable 7r-acceptor strength present, and thus all of the. r-electron density from the metal will be taken by the phosphorus ligands. The difference between the two non-equivalent phosphorus atoms is small, both in A and in B, and we may snspect that the high oxidation state may lower the tran -influence on the T-donicity. 

We can verify that statement by looking at a pair of iridium complexes, trans-[Ir(CO)L2Cl] with an Ir(I)-center, and mer- h CO)L C with an Ir(III)-center. The two complexes differ in that the latter is derived by oxidizing the square planar Ir(I)-complex adding chloride ligands to the axial positions (see Fig. 7.1). Whereas the A value is virtually unaffected by this oxidation (-0.357 and -0.377, respectively), the B value decreases from 29.11 to 0.70 upon going from Ir(I) to Ir(III). In other words, the influence of r-donicity and thus the fran -influence diminishes significantly when the oxidation state of the central metal is increased. 

We have seen that the P chemical shifts in cyclic phosphinidenes are influenced by their position within the ring (and thus by their position with respect to other donble bonds), bnt not necessarily by the ring size as such. Observations from other organophosphorus compounds show that deviations between acyclic and closely-related cyclic systems do exist, but they cannot be explained merely by ring size. 

If a bidentate phosphane ligand is coordinated to a transition metal, an additional chemical shift attribntable to the size of the resulting metallacycle can be observed for rings containing three, four, five, and six members. 

Definition The ring chemical shift is dehned as the valne observed for the metallacycle compared to the chemical shift fonnd for a complex where the chelate ligand is replaced by two eqnivalent monodentate ligands. 

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