Phosphorus/metal interconversion compound

The trimetallic complex [Fe3(CO)8(p-PPh)2(PF3)] has been obtained from the corresponding [Fe3(CO)8(p-PPh)2(MeCN)J compound by displacement of MeCN (method G). The structure, which is based on a square-pyramidal array of three iron and two phosphorus atoms, consists of isomers whose interconversion has been studied by variable-temperature 19F and 31P NMR spectroscopy. A trimetallic PF3 complex containing mercury bonded to two metals results from the reactions... 

As described in Chapter 2, phosphines have sufficiently large barriers to inversion that phosphines with three different substituents can be resolved. In contrast, the inversion of configuration at phosphorus in metal-phosphido complexes tends to occur near room temperature. In imsaturated transition metal complexes, a vacant acceptor orbital stabilizes the planar transition state. This effect is shown by the interconversion between pyramidal structures through a planar transition state, like the one shown in Equation 4.101 and formed during inversion of related compounds. - In saturated middle and late transition metal complexes, inductive effects explained below destabilize the groimd state and lead to lower barriers to inversion. The presence of an ancillary planar phosphide or amide ligand can contribute to a low barrier by accepting the lone pair from a p)Tamidal phosphide, as in Equation 4.101. 

An important difference between P and As is the ease of interconversion between the (V) and (III) oxidation states. In nature almost all phosphorus compounds are in the (V) oxidation state (or 5 valent state). Arsenic compounds are usually in either the (V) or (III) state. Arsenic metal has (0) oxidation state assigned to it. To put some numbers on the relative ease of oxidation/reduction of arsenic over phosphorus, the following half cell potentials are instructive. Thus, whereas arsenate is easily reduced to arsenite in acid solution, the reverse is true, easy oxidation of arsenite to arsenate, in base. 

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