Monometaphosphate Anions

Pentavalent 3-coordinated derivatives of phosphorus (Table 3.8) are generally short-lived species which are difficult to isolate, although in a few cases compounds have been prepared (7.529) and (6.620). 

Most interest has centred on the monometaphosphate anion which, up to about 60 years ago, was erroneously thought by some to have been prepared in the form of simple inorganic salts such as NaP03. 

Over 50 years ago Westheimer and Bunton [26] proposed that the hydrolyses of phosphate esters might proceed via an intermediate monometaphosphate anion (13.147a). Since that time the monometaphosphate anion has been postulated as an intermediate in numerous phosphorylation reactions and hydrolyses of phosphate esters (Chapter 5.6). 

The anion has been observed in the gas phase by MS and there is IR spectroscopic evidence that it exists in the high-temperature vapour evolved from molten phosphate salts (Chapter 5.3). Although the POj anion has been observed, it has never been isolated from solution because of its high reactivity. It is a powerful electrophile which is unstable in water with respect to H2PO4. 

Monomethyl metaphosphate (methyl phosphenate) Me0P02 (13.147b) and the PO3 anion (13.147a) are formed by electron impact and can be observed in the mass spectra of pesticides such as Monocrotophos and Mevinphos, (Me0)2P(0)0C(Me)=CH C(0)0Me (Table 12.29). 

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While (5.1h) is sometimes used to represent the elusive monometaphosphate anion and (5.1i) may exist as a spectroscopic species, the remaining (5.1j-l) are hypothetical structures. 

The monometaphosphate anion is believed to have a transient existence during some high-temperature transformations, phosphorylation or hydrolysis reactions (Chapter 13.4). Although there is evidence that the PO3 anion is stable and unreactive in the gaseous phase, its existence in aqueous media remains controversial [55-60]. 

Dimethyl methylphosphonate can be used as a methylating agent (6.279). In aqueous alkaline solutions, p-halophosphonates are decomposed with rupture of the P-C bond (Conant-Swan fragmentation) (6.280) [23]. This reaction is believed to involve the monometaphosphate anion as an intermediate species (13.147). 

Isolated in 1994 was (7.155a), but simple salts of the monometaphosphate anion, M POj have not been isolated. Some salts are polymerised and are expressed with covalent formulae, for example, (7.155b). Many presently known compounds are based on polyhedral frameworks formed from P,N and metal atoms (often Li) sitnated at the corners as in (7.155b,c) (cf. Chapter 8.8). 

Unlike the monometaphosphate anion PO3, the monometaphosphorothioate anion PS3 forms stable salts such as (Ph4As)PS3 in which it appears to have a pyramidal configuration [23]. 

The monometaphosphate anion is believed to be formed initially in the hydrolysis of phosphate monoesters of the type R0P(0)(0H)2, but it is rapidly attacked by nucleophiles and this prevents its isolation [27]. 

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