Characterization of the two geometrical isomers of (1,3-propanediamine-N,N′-diacetato-N,N′- di-3-propionato)-nickelate(II). X-ray structure of the binuclear complex trans(O5)-[Ni2(1,3-pddadp) (H2O)4]·4H2O and octahedral distortion of edta-type chelates