Some hexadentate Ni(II)-edta-type complexes containing five-membered diamine rings. The molecular and crystal structure of the trans(O5) isomer of barium(ethylenediamine-N,N′-diacetato-N,N′-di-3-propionato) nickelate (II) hexahydrate, trans(O5)-Ba[Ni(eddadp)]·6H2O, and strain analysis of edta-type chelates in relation to their octahedral distortion