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 0,0'-donor ligands. 

(b) [EDTA]⁴‾ forms very stable complexes with first row d-block metal ions M²⁺ (e.g. log K = 18.62 for the complex with Ni²⁺); where the M³⁺ ion is accessible, complexes between M³⁺ and [EDTA] are more stable than between the corresponding M²⁺ and [EDTA]⁴‾ (e.g. log K for the complex with Cr²⁺ is 13.6, and for Cr³⁺ is 23.4).