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Tetrapodal pentadentate ligands occupy five coordination positions in a coordination octahedron, thereby providing the metal ion with a square-pyramidal “coordination cap”: In such complexes, all reactivity is focused on a single coordination site. The review highlights recent advances in the coordination chemistry of iron. With a variety of NN4 ligands, the concept is being used to model non-heme active sites in biomolecules. Tetraphosphane ligands (donor set: NP4) undergo, depending on the solvent, remarkably specific P–C bond activation reactions, which may be reversed under suitable conditions.

Publisher Summary This chapter describes tetrapodal pentadentate nitrogen ligands. Tetrapodal pentadentate ligands have the ability to stabilize transition metal centers possessing a single labile coordination site, thereby, providing attractive platforms for reactivity studies in complexes of overall octahedral geometry. The chapter introduces a highly symmetrical aliphatic NN4 ligand, which consists of a “central” pyridine unit and four equivalent primary amino groups (“pyN4”, 1). This polyamine has predominantly σ donor character, is expected to render the coordinated metal ion electron-rich, imposes virtually no steric constraints, and may thus be construed as a “chelating analogue” of the pentaammine donor set (NH 3 ) 5 . The ligand may be construed as a chelating analogue of the pentaammine donor set has predominant σ-donor character, thereby, providing an electron-rich coordination environment for the metal center. This gives rise to unusual reactivity, such as the observed reduction of coordinated nitrite to NO.