Four-coordinate phosphorus

This intramolecular interaction of heteroatoms may be distorted by introducing different substituents onto the 4,6-positions of the ring. In the case of acceptor substituents, (CC13) dioxaboraphosphorinane (97) is not alkylated at the phosphorus atom and is inert to sulfur at the same time it forms a stable complex with pyridine [Eq. (96)]. In the case of donor substituents (Aik) (93)—(94), by contrast, the corresponding sulfides have been obtained and the pyridine complex is formed only with four-coordinated phosphorus derivatives. 

The general synthetic route for the preparation of bridged phosphorus cavitand is outlined in Scheme 5. From the resorc[4]arene, bearing various substituents at the lower rim, the cyclization step, which leads to the formation of the four fused eight-membered rings, was performed with three-co-ordinated and four-coordinated phosphorus reagent to give respectively tet-ra-phosphite, tetra-phosphonite, and tetra-phosphate or tetra-phosphonate derivatives. 

Fig.2 Solid-state P H MAS-NMR spectrum of l[Brj. The asterisk and cross denote the isotropic lines attributable to the two- and four-coordinate phosphorus atoms, respectively (according to [37])...

Haake et al. (228) postulate that the structures of both the [(C6H5)2PO] and [(CeH5)2P] ions involve cyclized systems with four-coordinate phosphorus. Rake and Miller (176-177) have suggested that the loss of MFg from [(CeP6)2M)+ (M = P, As, Sb) occurred via similar structures, as shown for ions (IV) and (V), also consistent with the structures proposed by Williams (175), Bowie and Nussey (179), and Chow and McAuliife... 

Addition of electronegative ligands (fluorine) to three- and four-coordinate phosphorus... 

Another system, which is also allowed by IUPAC and is sometimes very convenient (but which should be avoided when official names are established by IUPAC and cited in Chemical Abstracts), is the so-called a -nomenclature, in which the term phospha indicates replacement of carbon by phosphorus, e.g. phosphacyclohexane = phosphorinane, and phosphabenzene = A3- phosphorin. Five-coordinate phosphorus compounds are called phos-phoranes. However, this name is also used for phosphorus ylides with a four-coordinate phosphorus and a more or less polar R3f—CX moiety or a d -p R3P=CX double bond, e.g. methylenetriphenylphosphorane = triphenylphosphorus methylene ylide or tri-phenylphosphonium methyl ylide, Ph3P=CH2 = Ph3P—CH2. 

Mechanistic Problems Phosphoranes as Intermediates or Transition States in Reactions of Four-coordinate Phosphorus Compounds... 

The most interesting properties of phosphoranes, i.e. their role as intermediates or transition states of nucleophilic addition reactions of four-coordinate phosphorus compounds and their intramolecular rearrangements according to BPR or TR, have already been fully considered. The synthetic potential of stable phosphoranes has been reviewed by Burger in great detail (B-79MI11702), and only some special aspects need be mentioned in this chapter. 

In addition, we will present the mechanistic aspects concerning the electron-entry on four-coordinated phosphorus compounds. There are indications promoting the concept that a TBP is involved with the odd electron in an equatorial position. This presumption is based on the temperature-dependent ESR spectra of the radicals derived from 5. X-irradiation of a single crystal of 5 at liquid nitrogen temperature (77 K) yields the spectrum of radical 13 exclusively with ap//= 1120 G, api = 930 G, while,splitting is not resolved. From these values one calculates apls° = 993 G, indicating the phosphorus 3s spin density is 0.27, while the anisotropic contribution places 0.61 of the spin density in its 3p orbital, giving a total spin density of 0.88 on phosphorus. 

In this way the irreversible retro-arrangement of 13 to 12 is also consistent. These experiments strongly suggest that the initial state for the capture of an electron by a four-coordinated phosphorus may lead to a TBP configuration via an equatorial entry of the electron. 

An important aspect of this paper is the demonstrated facile formation of the P-O-P system by nucleophilic attack of phosphoryl oxygen on the four-coordinated phosphorus center. This result confirms the reaction schemes proposed earlier for a number of reactions, important from the synthetic point of view of organo-phosphorus compounds (9-13) and polymers (1 A). 

There remain no X-ray crystallographically determined structures of 2H- and J //-phospholes as a result of their low stability. A number of structures of 2,3- and 2,5-dihydrophospholes have been determined since 1995 and are collected in Table 14, with most possessing a tetrahedral four-coordinate phosphorus center. 

Proton NMR studies of aqueous arsenious acid solutions show only a single proton resonance attributable to arsenious acid. This result together with Raman spectroscopy suggest the species having a pyramidal structure which can be formulated as As(OH)3. This is to be contrasted with H3PO3, which is HP(0)(0H)2 with four-coordinate phosphorus and which exhibits two distinct proton NMR resonances. Spectroscopic evidence supports the formation of the three anions [H2ASO3], [HAsOs] ", and [AsOs] when solutions of As(OH)3 are treated with hydroxide ion. This observation agrees with a three-step acid dissociation. The pA a for the dissociation of the first proton is 9.2, which means that H3 AsOs is a very weak acid. 

The four-coordinate phosphorus(V) nitrogen compounds are extremely numerous and detailed data about phosphonic and phosphinic acid derivatives and others can be found in the handbook literature. 

The 3 Ip chemical shifts of most these compounds are in the normal range expected for four-coordinate phosphorus. Compound 42 deserves some comment. 

Pudovik, A.N., and Nazmutdinova, V.N., Reactions of three- and four-coordinate phosphorus cyanides with a,P-unsaturated aldehydes and ketones, Zh. Obshch. Khim., 58, 2215, 1988 J. Gen. Chem. USSR (Engi. Transl.), 58, 1973, 1988. 

The AX spin systems (A = 31P, X = 109Ag) in the silver complexes (18-20a) associated with the AgP core and A2X spin system in 20b have been revealed. The 31P and 109 Ag NMR data in Table 3 have been obtained for 18-20 and 21a, b compounds in CH2CI2. These data suggested the presence of four-coordinate phosphorus centres and the attachment of the trifluoroacetate as the fourth substituent to phosphorus in 18, 20a and 20b. 

The infrared spectrum shows a strong band at 2370 cm. - characteristic of BH3 coordinated to a donor group, and a band at 1110 cm.-1, characteristic of four-coordinate phosphorus. The entire infrared spectrum is given in Table I. 

We have pointed out that compounds containing four-coordinate phosphorus are characterized by shorter bonds and wider valence angles than the three-coordinate phosphorus derivatives. The shortening of the PN bond on passing from Me2NPCl2 to Me2NP(=0) CI2 serves an example. This shortening is associated with a decrease of On to 116°, as mentioned already. 

Multiple oxygen-phosphorus bonds are characteristic only of four-coordinate phosphorus. These bonds are considerably shorter (by about 0.15 A) than the single PO bonds. The nature of double P=0 bonding is widely discussed in terms of the more general problem of chemical bonding involving four-coordinate phosphoms " ... 

Note In the amine substituted phosphino-phosphenium cations, the four-coordinate phosphorus atom carrying the cationic charge resonates upfield from the tricoordinate phosphorus atom, while the reverse is the case in the phenyl substituted phosphino-phosphenium cations. The effect of the rr-bonding interaction of the amino groups is clearly visible. 

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