The 10-vertex phosphadicarbaboranes, 6-R-arachno-6,8,9-PC2B7H11 (1) (R = Ph 1a or Me 1b) and 6-R-arachno-6,5,7-PC2B7H11 (2) (R = Ph 2a or Me 2b) have been synthesized using in situ dehydrohalogenation reactions of RPCl2 (R = Ph or Me) with the arachno-4,5-C2B7H13 and arachno-4,6-C2B7H13 carboranes, respectively. X-ray crystallographic determinations in conjunction with DFT/GIAO/NMR calculations and NMR spectroscopic studies have established that both 1 and 2 have open cage structures based on an icosahedron missing two vertexes. The two isomeric compounds differ in the positions of the carbons and bridging hydrogens on the open face. Studies of the reactions of 2a with BH3.THF, S-8, and hydrogen peroxide demonstrated that 2a shows strong donor properties yielding the compounds endo-6-H3B-exo-6-Ph-arachno-6,5,7-PC2B7H11 (3), endo-6-S-exo-6-Ph-arachno-6,5,7-PC2B7H11 (4), and endo-6-O-exo-6-Ph-arachno-6,5,7-PC2B7H11 (5) in which the BH3, S, and O substitutents are bonded to an electron lone pair localized at the phosphorus endo-position. The reaction of 2a with an excess of S-8 results in the loss of a framework boron to produce the unique open-cage compound mu(7,8)-{HS(Ph)P}-hypho-7,8-C2B6H11 (6). 2a also formed the donor complexes cis-(eta(1)-[6-Ph-arachno-6,5,7-PC2B7H11])(2)PtBr2 (7) and trans-(eta(1)-[6-Ph-arachno-6,5,7-PC2B7H11])(2)PdBr2 (8) in which the metal fragment is bonded in an eta(1)-fashion at the phosphorus endo-position. In these complexes, 2a is functioning as a two-electron sigma donor to the metals and can thus be considered as an analogue of the PR3 ligands in the classical cis-(PPh3)(2)PtBr2 and trans-(PPh3)(2)PdBr2 coordination complexes. Although la did not show the donor properties exhibited by 2a, its dianion 6-Ph-6,8,9-PC2B7H92- (1a(2-)) readily formed eta(4)-coordinated complexes with late transition metals including 8-Ph-7-(Ph3P)(2)-nido-7,8,10,11-PtPC2B7H9 (9), 7-Ph-11-(eta(5)-C5H5)-nido-11,7,9,10-COPC2B7H9 (10), and commo-Ni-(7-Ni-8'-Ph-nido-8',10',11'-PC2B7H9)(7-Ni-8-Ph-nido-8,10,11-PC2B7H9) (11).