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Three new C,C-dichlorophosphaalkenes RP=CC12 18-20 have been synthesized from the corresponding dichlorophosphines RPCl 11-13. Compounds 18-20 are stabilized owing to [2,6-bis(2-methoxyphenyl)-4-methylphenyl] and the known 2,6-dimesityl-4-methylphenyl (R2). The compounds with a R3 group (R31: 4, R'PCI,: 13, R3PH2: 17 and R3P=CC12: 20) exist in the form of two conformational isomers on the NMR time scale. A 'H and 3'P NMR dynamic study (for 4 and 13 respectively) allowed the determination of the AG* of this phenomenon, respectively 18.5 and 18.2 kcal/mol. the use of the very bulky new groups R T [2,6-bis(4-methylphenyI)-4-methylphenyI] and R3