The reaction of 9-bromo-9-borafluorene (1) with Li[PtBu2] in toluene gave quantitatively the corresponding di-tert-butyl-phosphaboradibenzofulvene (9-di-tert-butylphosphanyl-9-borafluorene, 2). Degradation of thf occurred by treatment of 2 with thf in toluene at room temperature. In this paper the reaction of 2 with gaseous H2 in toluene solution at room temperature is described, by which the corresponding H2 addition product 4 was formed. The hydrogen addition product 4 crystallizes from benzene in the monoclinic space group P21/n. Addition reactions of 2 with acetonitrile, benzophenone, and 2,3-dimethylbutadiene were also investigated. Treatment of 2 with a 20-fold excess of acetonitrile afforded the corresponding adduct, which itself dimerized to a mixture of cis and trans isomers of the corresponding cycloiminoborane 6. Cocrystals of cis-6 and trans-6 (ratio 2:1) were obtained from toluene in the presence of 20 equiv of acetonitrile at 6 °C (monoclinic space group P21/c). The isolation of the pure tra...