Tetrahedral phosphorous compounds

Phosphoric acids and the phosphates maybe defined as derivatives of phosphoms oxides where the phosphoms atom is in the +5 oxidation state. These are compounds formed in the M2O—P20 system, where M represents one cation equivalent, eg, H", Na", 0.5 Ca ", etc. The molecular formula of the phosphoms(V) oxide [1314-56-3] is actually P O q, but this oxide is commonly referred to in terms of its empirical formula, P2O5. StmcturaHy, four phosphoms—oxygen (P—O) linkages are arranged in an approximate tetrahedral configuration about the phosphoms atom in the phosphate anion. Compounds containing discrete, monomeric PO ions are known as orthophosphates or simply as phosphates. 

This very common coordination number usually leads to a tetrahedral configuration for phosphoric acid, phosphates, and many phosphorus(V) compounds. In these compounds, the P(V) atom forms one double bond and three single bonds with other atoms. Figure 15.4.2(e) shows the structure of P4O10, whose symmetry is Td. The length of the terminal P=0 bond is 143 pm, and the bridging P-O bond is 160 pm. The bond angle O-P-O is 102°, and P-O-P is 123°. 

Some trivalent pyramidal molecules exist in tautomeric equilibrium with tetrahedral forms. Phosphorous acid, for example, may be written as (3.37). This compound exists in tetrahedral form in the solid state or in aqueous solution, although in many of its reactions it behaves as a molecule containing trivalent P. Derivatives obtained by replacing the hydrogen by varions atoms or groups R, can usually be isolated only in one form, depending on the nature of R. Tri-esters with 3 H atoms replaced, exist only in pyramidal form (Chapter 6.8). 

The trioxides can all be obtained by reacting the elements or their sulphides in air (Table 4.13). They show increasing thermal stability and basic character, but reluctance to oxidise to the pentava-lent state, on progressing from P to Bi. Although less soluble in water than its phosphorus analogue (Table 4.11), AS4O6 eventually produces arsenous acid As(OH)3 Unlike phosphorous acid, however, the latter compound does not exist in tetrahedral form with an As-H linkage. 

Since the distribution of bonds around carbon (1) is tetrahedral, attachment of the phosphate group to either carbon (1) or carbon (3) results in the same compound if optical activity is ignored (glycero-l-phosphoric acid is equivalent to glycero-3-phosphoric acid). If nomenclature distinguishes between C(l) and C(3), however, this will make L-3-glycerophosphoric acid equivalent to D-l-glycerophosphoric acid, and the d-3 isomer equivalent to the L-1 isomer (Chapter 13.2). 

Enantiomers are compounds that have the same chemical structure but different conformations, whose molecular structures are not superimposable on their mirror images, and, because of their molecular asymmetry, these compounds are optically active. The most common cause of optical activity is the presence of one or more chiral centers, which are usually related to tetrahedral structures formed by four different groups around carbon, silicon, tin, nitrogen, phosphorous, or sulfur. 

Bonding in species with the unit ()U3-M)E3, e.g. those derived from P4 and AS4, can be regarded for the purpose of electron-counting either as complexes in which the triphosphorous cycle P3 behaves as a tri-hapto three-electron donor towards the transition metal moiety or as a tetrahedral cluster in which one phosphor in P4 has been substituted by a three-electron donor. Counting 30 valence electrons corresponds to an electron-precise tetrahedron with six two-electron/two center bonds. In such kind of compound with stoichiometries ME3 and M2E3 (see Figs. 3.13 and 3.14) the E-E distances are shorter than those in the pure element E4. This has been attributed to electron delocalization toward... 

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