The homodiphosphanes CgP–PCg (1) and PhobP–PPhob (2) and the heterodiphosphanes CgP–PPhob (3), CgP–PPh2(4a), CgP–P(o-Tol)2(4b), CgP–PCy2(4c), CgP–PtBu2(4d), PhobP–PPh2(5a), PhobP–P(o-Tol)2(5b), PhobP–PCy2(5c), PhobP–PtBu2(5d) where CgP = 6-phospha-2,4,8-trioxa-1,3,5,7-tetramethyladamant-9-yl and PhobP = 9-phosphabicyclo[3.3.1]nonan-9-yl have been prepared from CgP(BH3)Li or PhobP(BH3)Li and the appropriate halophosphine. The formation of 1is remarkably diastereoselective, with the major isomer (97% of the product) assigned to rac-1. Restricted rotation about the P–P bond of the bulky meso-1is detected by variable temperature 31P NMR spectroscopy. Diphosphane 3reacts with BH3to give a mixture of CgP(BH3)–PPhob and CgP–PPhob(BH3) which was unexpected in view of the predicted much greater electron-richness of the PhobP site. Each of the diphosphanes was treated with dimethylacetylene dicarboxylate (DMAD) in order to determine their propensity for diphosphination. The homodiphosphanes 1and 2did not react with DMAD. The CgP-containing heterodiphosphanes 4a–dall added to DMAD to generate the corresponding cisalkenes CgPCH(CO2Me)CH(CO2Me)PR2(6a–d) which have been used in situto form chelate complexes of the type [MCl2(diphos)] (7a–d) where M = Pd or Pt. The PhobP-containing heterodiphosphanes 3and 5a–dreact anomalously with DMAD and do not give the products of diphosphination. The X-ray crystal structures of the diphosphanes 2, 3, 4a, and 5a, the monoxide and dioxide of diphosphane 1, and the platinum chelate complex 7chave been determined and their structures are discussed. [ABSTRACT FROM AUTHOR]