Three-coordinated phosphite form

Although there is no direct experimental evidence for the existence of the three-coordinated phosphite form, the tautomerization of dialkyl H-phosphonates to dialkyl phosphites has often been invoked to explain the mechanism of other reactions of dialkyl H-phosphonates, which result in a P-H bond cleavage. Dimethyl phosphite, although never experimentally observed, has been studied recently by ab-initio computations. Conformational analysis for this compound [6] results in four stable conformers (Figure 2.2). Conformer... 

Tri-ra-butylphosphite, tri-ra-butylarsine, and tri-ra-butylstibine complexes of platinum(O) have been prepared by the reaction of the ligand with pt(COD)2] or, in the case of the phosphite, by reaction with [Pt(PPh3)4].38 All form four-coordinate complexes when 4 mole-equivalents of ligand are used, but when 2 or 3 mole-equivalents of the phosphite ligand are used, mixtures of three- and four-coordinate complexes are observed.38... 

All three structural forms contain one terminal and one bridging hydride and ten terminal carbonyl ligands. The entering ligand occupies an equatorial site (la) when L is a phosphine or a phosphite whereas it coordinates axially (Ib or Ic) when L contains a nitrogen as the donor atom (nitriles, amines). The occurrenee of transformation (1) is accompanied by a net color change from the intense violet of (//-H)20s3(CO)io to the pale yellow of the saturated adducts. 

Under no conditions has it been found possible to replace more than one carbonyl group of acetylcobalt tetracarbonyl by triphenylphosphine. Even the chelating bis(phosphine), ethylenebis(diphenylphosphine) does not form stable chelated complexes with acetylcobalt tetracarbonyl (7). Phosphite esters, on the other hand, will replace two coordinated carbonyl groups at about 80° C, and three can be replaced by another method (8). 

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