Donor atom sets tetradentate ligands

Other tetradentate SB ligands having either N2P2 or N3P donor atom set containing both hard and soft donors and their complexes with some soft metals have been investigated.116 117... 

As for V(III), trigonal prismatic complexes, and trigonally distorted octahedral complexes are not uncommon for V(IV) [56]. The ligand set exerts a stronger influence on the redox potentials, whether the coordination geometry is octahedral or trigonal prismatic, at least to a first approximation. A series of compounds made with tridentate and tetradentate ligands with N- and O-donor atoms such as the azophenol derivative shown in Fig. 8 has both reversible oxidations and reductions. 

Nearly all iron complexes of synthetic macrocyclic ligands contain nitrogen either as the only ligand atom or as the major donor present. Moreover, most macrocyclic ligands are tetradentate usually presenting a roughly planar N4 donor set to the centrally complexed metal ion. The comprehensive review by Melson390 of the coordination chemistry of macrocyclic compounds should be consulted for work published up until 1978. 

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