Nucleophilic reactions tetradentate ligands

The tetradentate ligand forms monomeric square planar complexes. Synthetic and kinetic studies reveal that the coordinated mercapto group may be converted into the coordinated thioether function without breaking the metal-sulfur bond. The nucleophilic power of the coordinated mercapto group exceeds that of RSH, but depends on the metal atom. Bridging protects the sulfur atom from alkylation. In the case of nickel(ll), alkylation is accompanied by expansion of the coordination number of the nickel from 4 to 6. Ligand reactions have led to the synthesis of planar ligands completely cydized about the metal ion. 

Binuclear zinc complexes wherein each metal center is supported by a tripodal tetradentate ligand environment involving a bridging phenolate donor are reactive toward tris(4-nitrophenyl) phosphate and bis(4-nitrophenyl) phosphate in ethanol/ water.234 The pH-rate profile studies of these reactions are consistent with a mechanism that involves the formation of a reactive mononuclear zinc hydroxide species via cleavage of the binuclear solid-state structure. This mononuclear Zn-OH species is proposed to act as the nucleophile toward the phosphate ester substrate. 

Nb and Ta derivatives are hard acids and then-complexes with P- or As-donors are limited. Tertiary phosphines, especially PMes, have been widely used to stabilize low-valent derivatives. C-H activation reactions, promoted by the formation of thermodynamically stable Ta-H, Ta-C, and Ta=C bonds have resulted in metallacycles based on unusual anionic phosphorus donors. Nucleophilic Ta phosphinidene complexes could be stabilized by a tripodal tetradentate [NN3] amido ligand. The terminal PR ligand reacts smoothly with aldehydes, providing a general synthesis of phosphaalkenes RP=C(H)R and act thus as a phospha-Wittig reactant see Phosphorus Organophosphorus Chemistry). 

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