Treatment of a solution of excess PCl3 and PS (PS = "proton sponge" = 1,8-dimethylamino naphthalene) with arachno-4-CB8H14 (1) in CH2Cl2, followed by hydrolysis of the reaction mixture, resulted in the isolation of the eleven-vertex diphosphacarbaborane nido-7,8,9-P2CB8H10 (2) (yield 34%) as the main product. Other products isolated from this reaction were the phosphacarboranes nido-7,8,9,10-P3CB7H8 (3) (yield 5%) and closo-2,1-PCB8H9 (4) (yield 15%). Compound 2 can be deprotonated by PS in CH2O2 or NaH in diethyl ether to give the [nido-7,8,9-P2CB8H9](-) (2(-)) anion, which gives back the original compound, 2, upon re-protonation, Thermal rearrangement of anion 2(-) (Na+ salt) at 350 degrees C for 2 h produced the isomeric [nido-7,8,10-P2CB8H9](-) (5(-)) anion, which was isolated as a PPh4+ salt (yield 86%). Multinuclear (H-1(1) B-11, P-31, and C-13), two-dimensional [B-11-B-11] COSY, H-1-{B-11(selective)}, {B-11(selective)}, and gradient-enhanced ([H-1-C-13] HSQC) magnetic resonance measurements led to complete assignments of all resonances which are in excellent agreement with the structures proposed. Coupling constants, (1)J(P-31,C-13), (2)J(P-31,C,H-1), and (1)J(P-31,P-31), were calculated using the DFT method B3LYP/6-311 +G(d,p). The molecular geometries of all compounds were optimized ab initio at a correlated level of theory (RMP2(fc)) using the 6-31G* basis set, and their correctness was assessed by comparison of the experimental 1113 and 13C chemical shifts with those calculated by the GIAO-SCF/IIH/RMP2(fc)/6-31G* method, The computations also include the structures and chemical shieldings of the still unknown isomers [nido-7,10,8-P2CB8H9] (6) and [nido-7,9,8-P2CB8H9](-) (7(-)).