Chemical properties of dibismuthines

Dibismuthines are very labile they react readily with free radicals, electrophilic and nucleophilic reagents, which all cleave the Bi-Bi bond. Some dibismuthines are thermolabile tetramethyldibismuthine decomposes at 25°C quantitatively into trimethylbismuthine and bismuth metal. The half-life of this dibismuthine is approximately 6 h in a dilute benzene solution [82OM1408]. Tetraphenyldibismuthine [83CC507] and 2,2, 5,5 -tetramethyl-bibismole are stable up to 100°C [920M2743]. 

Thermal decomposition occurs more rapidly under ultraviolet irradiation or in a halogenated hydrocarbon such as chloroform or carbon tetrachloride. Even the thermostable tetraphenyldibismuthine does decompose in chloroform at 25°C [830M1859]. These interesting findings are explained best by a free radical mechanism. 

Me2BiBr + Me2BiCCl3 + MesBi + C2CI0 + Cl2C=CCl2 

the thermal decomposition of tetramethyldibismuthine in the presence of bromotrichloromethane may take place via the following chain mechanism, (i) Tetramethyldibismuthine decomposes first into two dimethyl-bismuth radicals, (ii) These radicals react with bromotrichloromethane to form bromodimethylbismuthine bromide and a trichloromethyl radical, (iii) The trichloromethyl radical reacts with the dibismuthine to form dimethyl(trichlor-omethyl)bismuthine and dimethylbismuth radical. The termination step is the self-combination of the trichloromethyl radicals. In fact, the major product of the reaction is bromodimethylbismuthine bromide and the second product is dimethyl(trichloromethyl)bismuthine. Trimethylbismuthine, hexachloro-ethane and tetrachloroethylene are also formed in smaller quantities [830M1859]. 

The reaction of tetramethyldibismuthine with hydrogen chloride affords hydrogen gas, bismuth metal, trimethylbismuthine and chlorodimethylbis-muthine. These products are likely to be formed by the decomposition of thermolabile dimethylbismuthine, which can be detected at — 78°C by NMR measurement [830M1859]. 

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