Mixed valent phosphorus compounds synthesis characterization and reactivity The objectives of this work were the synthesis of mixed valent phosphorus compounds including related precursor reagents and investigations of their reactivity. The synthetic approach of salt metathesis of phosphinic chlorides with alkali metal phosphanides which was believed to be straightforward to achieve mixed valent compounds resulted in unexpected reaction sequences and products. Optimizing the composition of the reagents and altering the substitution on the $\\lambda^3$ phosphorus made it possible to observe the target molecules in solution and some derivates in the solid state. However the stability of a PP double bond is inferior compared to a CC double bond which could be observed by degradation of target molecules in solution. Recently published results in this field suggest the possibility to utilize mixed valent phosphorus compounds as phosphinidene transfer reagents. Thus the degradation of target molecules was intensively studied by kinetic measurements using phosphorus NMR techniques.