The homogeneous gas phase reaction between diborane and phosphine at 0°C, yielding a solid product of composition BH_3PH_3, has been observed to give a linear relation between the initial rate of reaction and the pressure of either reactant, provided a certain pressure of each reactant, determined by the equilibrium constant and the pressure of the other gas, is exceeded. The reaction rate is independent of the amount of surface of the solid product or of the wall. When either reagent is in appreciable excess, the rate‐time data obey a pseudo first‐order equation.
These facts are consistent with a rate‐controlling reaction of diborane with phosphine to give BH_3PH_3 and BH_3, the borine reacting further with phosphine, and the BH_3PH_3 condensing rapidly to a solid. This mechanism leads to a rate equation which is in good agreement with the experimental evidence. The activation energy of the rate‐controlling step is 11.4±2.0 kcal mole^(-1)
