Three COORDINATION COMPOUNDS

Thus it has so far proved possible to isolate stable derivatives of monomeric metaphosphoric acid and of metathio- and metaselenophosphoric acid, which, understandably, generally bear tert-butyl and/or trimethylsilyl substituents u. Specifically, we know aminobisiminophosphoranes (3, Z = NR2, X = Y = NR)2,3,4), aminoiminothio (or seleno)phosphoranes (5, Z = NR2, X = NR, Y = S or Se)5), and aminoiminomethylenephosphoranes (1, R = NR2, X = NR)6>. Conspicuously, no stable phosphorus(V) three-coordinate compounds have been synthesized with oxygen as divalent ligand. 

So far, the amides described have all been in the -1-4 state, but one rare three coordinate compound is U N(SiMe3)2 3(pyramidal, like the lanthanide analogues). Unusual amides... 

Furthermore, these bulky aryloxide ligands played a crucial role in making the crystallographically characterized less-familiar three-coordinate compounds of scandium, yttrium, and the lanthanides (Ln), such as Ln(OC6H2-r-Bu2-2,6-R-4)3 (28) more accessible. 

By far the most important type of reaction displayed by halophosphines is nucleophilic substitution. This is pivotal to the preparation of many other three-coordinate compounds containing either solely P—C, P—O, P—N bonds, or mixed combinations. These reactions are often exothermic and frequently carried out at low temperatures. For the synthesis of phosphorus(III) compounds containing a P—O or P—N bond it is often necessary to add a base (triethylamine or pyridine are frequently used) to capture the hydrogen halide eliminated from these condensation reactions. In the case of P—C bond formation, a variety of routes are possible using various carbon-derived nucleophiles. 

A student in 1895 prepared three coordination compounds containing chromium, with the following properties ... 

Burger and co-workers 24, 25, 26) prepared a number of binary metal compounds with the very bulky silazane ligand N(SiMe3)2, e,g, MLa (M = Al, Ga, Cr, and Fe), ML2 (M = Be, Mn, Co, Ni, Zn, Cd, and Hg), ML (M = Li, Na, and Cu). It is reasonably certain that the ML2 compounds contain two-coordinated metals and that the metals are three-coordinated in ML3 compounds. Recently Bradley and co-workers isolated some additional three-coordinated species of the transition metals ML3 (M = Sc, Ti, and V) 27, 28, 29) and the lanthanides 30, 31), X-ray single-crystal diflFraction analysis revealed that the three-coordinated compounds exhibit either trigonal planar MN3 units (Ti, V, Cr, Fe, Al, and Ga) or pyramidal MN3 (Sc, Eu, and Yb) 32, 33, 34, 35), The reason for this structural difference is not yet clear, but it does appear that the more covalently bonded compounds have the trigonal planar configuration. 

Figure 1 shows typical organophosphorus substrates, which usually act directly, or after activation at a metal center, as nucleophiles. Most include a reactive P-H bond, which, for P(V) substrates, is involved in an important tautomerization equilibrium which interconverts four- and three-coordinate compounds 1 and 2. The source of electrophilic carbon for C-P bond formation is usually unsaturated (alkynes, alkenes, aldehydes, etc), or contains a good leaving group (aryl and alkyl halides, allyl acetates). 

They readily form Lewis acids, three coordinate compounds that are capable of accepting an electron pair and increasing the coordination number. 

---