From neutral iminophosphoranes to multianionic phosphazenates. The coordination chemistry of imino-aza-P(V) ligands

This review deals with the chemistry and coordination behaviour of imino-aza phosphorus(V) ligands focussing on s- and p-block as well as Group 11 and 12 metal complexes, Imino phosphorus(V) ligands contain one or more terminal R-N=P-units, which include iminophosphoranes R3PNR', monoanionic diiminophosphinates [R2P(NR')(2)]-, dianionic triiminophosphonates [RP(NR')(3)](2)- and trianionic tetraiminophosphates [P(NR')(4)](3)-. Aza-phosphorus(V) ligands feature bridging P-N=P units, which include cyclic and polymeric phosphazenes [RPN](eta). Imino-aza- phosphorus(V) ligands containing both imino and aza functions include linear diiminodiphosphazenates [N{R2P(NR')(2)}(2)](-) and multianionic poly(imino) cyclophosphazeantes such as [N-4{RP(NR')}(4)](4-) and [N-3{P(NR')(2)}(3)](6-). Imino-aza phosphorus(V) ligands are assembled of three basic building blocks: the cationic tetravalent phosphonium centre (P), the anionic divalent amido function (N) and the terminally arranged R-group. The overall negative charge Z of the resulting ligand system is equal to the difference between the number of P and the number of N-centres: Z = n(P)-n (N). Imino-aza phosphorus(V) ligands are electron rich N-donor ligands which co-ordinate via both N(imino) and N(aza) functions and have been applied in numerous metal complexes in order to stabilise low coordination numbers, unusual oxidation states and bonding modes or serve as ligands in homogeneous catalysis. The R-group provides both steric bulk and solubility in non-polar solvents, Multianionic phosphazenates feature a polydentate ligand surface, which facilitates an extremely high metal load. P = N units of iminophosphoranes and phosphazenes have acceptor properties and enhance the acidity of alpha-alkyl and ortho-aryl protons. Deprotonation of P-alkyl and P-aryl iminophosphoranes give ligand systems featuring C,N chelating sites, which are also discussed. (C) 2002 Published by Elsevier Science B.V.