Pentacoordinate derivatives

Although very many pentacoordinated derivatives of phosphorus, arsenic, antimony and bismuth of all sorts have so far been synthesized, the fundamental hydrogenated skeletons PH5, AsH5, SbH5 and BiHs are still unknown materially. Theoreticians, on the other hand, have been familiarizing themselves for a long time with those prototypic species by virtue of more or less elaborate calculations 48,76,... 

As a general rule, tetra- and pentacoordinated derivatives of phosphirenes display a much higher stability than the corresponding derivatives of the phosphirane ring. Some kind of electronic stabilization is probably at work in phosphirenium salts (see the structural data of one such salt <88CC462 , which can either be synthesized directly (see below) or from tervalent phosphirenes (Equation (2)) <85JA4700>. Methyl triflate in chloroform at — 50°C has also been used successfully... 

Oae and co-workers have extensively examined this reaction. Nucleophilic attack of an heteroaryllithium or an organometallic reagent on the phosphorus atom produces an intermediate pentacoordinate derivative. Coupling between an equatorial and an axial substituent leads to the products.17,18 (Scheme 4.1)... 

Dibenzoarsoles <68CB121> and -stiboles react with chloramine-T to give their respective imines (Equation (6)), which are used in situ to prepare pentacoordinate derivatives. Dibenzobismoles do not react in this manner <64CB789>. 

While some reorganisation reactions are spontaneous and immediate, others such as the interchange of different ester groups on tetrahedral phosphates are extremely slow and have high activation energies. Reorganisation reactions occur in polyphosphate melts (Chapter 5.4), and they also occur with pentacoordinated derivatives (Chapter 13.4). 

Some alkylcobalt tetracarbonyl derivatives of general formula RCo(CO)4 have been obtained. These pentacoordinate derivatives are much less stable than the corresponding hexacoordinate alkylmanganese pentacarbonyl derivatives RMn(CO)5. Treatment of Na[Co(CO)4] with a deficiency of methyl iodide under carefully controlled conditions gives a 2 to 4% yield of the very unstable CH3Co(CO)4, a volatile material which decomposes above — 35° C (55). Similar unstable ethyl and benzyl RCo(CO)4 derivatives have been obtained by treatment of Na[Co(CO)4] with [(C2H5)30][Bp4] and benzyl bromide, respectively (231). 

Organosilanes bearing a phosphine imide moiety have been synthesized from phosphine imide in ethyl ether and characterized crystallographically. In ether solution, a dynamic equilibrium exists between the chelated pentacoordinated and unchelated tetra-coordinated silicon moieties. Reaction of the pentacoordinated derivative with water gives [2-(diphenylphosphino)phenyl]diphenylsilanol accompanied by both reduction of the phosphine imide moiety and the hydrolytic oxidation of the Si-H moiety." °... 

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