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-(PBu2)-Bu-t (4d), PhobP-PPh2 (5a), PhobP-P(o-Tol)(2) (5b), PhobP-PCy2 (5c), PhobP-(PBu2)-Bu-t (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 1 is 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-1 is detected by variable temperature P-31 NMR spectroscopy. Diphosphane 3 reacts with BH3 to 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 1 and 2 did not react with DMAD. The CgP-containing heterodiphosphanes 4a-d all added to DMAD to generate the corresponding cis alkenes CgPCH(CO2Me)=CH(CO2Me)PR2 (6a-d) which have been used in situ to form chelate complexes of the type [MCl2(diphos)] (7a-d) where M = Pd or Pt. The PhobP-containing heterodiphosphanes 3 and 5a-d react 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 7c have been determined and their structures are discussed.