Suggest reasons for the following observations. a) Although Pd(II) complexes wit

Question

Suggest reasons for the following observations.

a) Although Pd(II) complexes with monodentate O-donor ligands are not as plentiful as those with P-, S- and As-donor ligands, Pd(II) forms many stable complexes with bidentate O,O-donor ligands.

b) EDTA4- forms very stable complexes with first row d-block metal ion M(II)(e.g. log K = 18.62 for the complex with Ni (II)); where the M (III) is accessible, complexes between M (III) and EDTA4- are more stable than between the corresponding M (II) and EDTA4- (e.g. log K for the complex with Cr (II) is 13.6 and for Cr (III)is 23.4).

Explanation / Answer

a) Pd (II) is a soft metal and from Hard-Soft acid base principle, soft metals form more stable complexes with soft ligands. Monodentate O-donor ligand, eg. R2O, is a hard ligand compared to P,S and As donor ligands like R3P, R2S and R3As respectively, which are soft ligands. So the Pd(II) complexes with monodentate O-donor ligands are not as plentiful. On, the other hand, due to the chelation effect, for a given metal ion, the thermodynamic stability of a complex with a bidentate ligand is higher than a complex involving the corresponding number of monodentate ligands. Thus, Pd(II) forms many stable complexes with the bidentate O,O-donor ligands compared to the monodentate O-donor ligands.

b) EDTA4- is a hexadentate ligand which forms very stable complexes with the first row d-block metal ions due to the chelation effect mentioned earlier. Also, a complex is more stable if it has less overall charge, so with M(III), the overall charge on the complex is -1 which makes them stable at high pH compared to the complexes with M(II) with overall charge of -2 which are less stable over a wide range of pH.

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