Acyclic ligands, conformational flexibility

In the available literature, only six PBP structures of Ni(II) have been reported so far (34,38,40,45,48,50) and four of them include acyclic pentadentates (XVII-XX). These pentadentates are equato-rially coordinated in their neutral form with weak conformational flexibility. Apical positions are always occupied by water molecules, while nitrate anions always represent counter ions in the structure. The change of any of the mentioned structural elements (polydentate, axial ligands, or counter ion) affords complexes of which the structure has not been determined (52,54,56,58). These are complexes with the same type of ligands XX-XXIII, containing Cl- ions and water molecules, but it is still unknown which of these two monodentate ligands occupies the apical positions. It is questioned whether both monodentates actually occur in the coordination sphere, and because of that it is impossible to discuss the geometry of these complexes in more detail. 

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