Treatment of [Pd-2(mu-Cl)(2)(eta(3)-C3H5)(2)] with Na-2[nido-7,8-Me(2)-7,8-C2B9H9] in thf (tetrahydrofuran), followed by addition of [NEt(4)]Cl; gives the complex [NEt(4)][Pd(eta(3)-C3H5)(eta 5-7,8-Me(2)-7,8-C2B9H9)] (1c). Protonation of CH2Cl2 solutions of this species with HBF4.Et(2)O in the presence of CO at low temperatures affords a neutral but very unstable dicarbonyl complex [Pd(CO)(2)(eta(5)-Me(2)C(2)B(9)H(9))] (2c). Reaction Between [PdCl2(cod)] (cod = cydoocta-1,5-diene) and T1[closo-1,2-Me(2)-3,1,2-T1C(2)B(9)H(9)] in thf yields as the principal product (75%) [Pd(cod)(eta(5)-7,8-Met(2)-7,8-C2B9H9)] (3a), together with small quantities of the polytopal isomer [Pd(cod)(eta(5)-2,7-Me(2)-2,7-C2B9H9)] (3b) and the sandwich compound [Pd(eta(5)-2,7-Me(2)-2,7-C2B9H9)(2)] (4). The structure of 3b was established by X-ray diffraction. Crystals are monoclinic, space group P2(1)/c (No. 14), with a = 10.030(1) Angstrom, b = 14.197(2) Angstrom, c = 12.524(3) Angstrom, and beta = 103.98(1)degrees, Z = 4. In this molecule one of the CMe groups has migrated from the open pentagonal bonding face of the carborane ligand td the next pentagonal belt, while still occupying a vertex adjacent to the other CMe group. Treatment of CH2Cl2 solutions of 3a with CO displaces the cod group, giving 2c, but attempts to isolate the latter in the presence of cod afforded 3b. A similar polytopal rearrangement of 3a into 3b occurs either on heating solutions of the former, or in room-temperature reactions with PhC=CH or [W(=CC(6)H(3)Me(2)-2,6)(CO)(2)(eta(5)-C(5)Me(5))]. Protonation of 1c in CH2Cl2 in the presence of CNBu(t) gave [Pd(CNBu(t))(2)(eta 5-7,8-Me(2)-7,8-C2B9H9)] (5a), which was also prepared from [PdCl2(CNBu(t))(2)] and T1[closo-1,2-Me(2)-3,1,2-T1C(2)B(9)H(9)]. The tmen (tetramethylethylenediamine) complex [Pd(tmen)(eta(5)-7,8-Me(2)-7,8-C2B9H9)] (6) is obtained from [PdCl2(tmen)] and Ti[closo-1,2-Me(2)-3,1,2-TlC2B9H9] and reacts with CO to give initially 2c followed by 4. The latter is also the product from the reaction between 6 and butadiene in the presence of BF3.Et(2)O, this reagent being used to remove tmen as the adduct (BF3)(2).tmen. A similar reaction of 6 with cod gives a mixture of the two isomers 3a and 3b. The complex [Pd(PMe(2)Ph)(2)(eta(5)-7,8-Me(2)-7,8-C2B9H9)] (7) is formed from [PdCl2(PMe(2)Ph)(2)] and Tl[closo-1,2-Me(2)-3,1,2-TlC2B9H9], and by displacement of cod from 3a with PMe(2)Ph. In contrast with 2c or 3a, the complexes 5a, 6, and 7, having a closo-3,1,2-PdC2B9 cage topology, show no tendency at room temperature or below to convert to products having the closo-2,1,4-PdC2B9 architecture. In addition to the X-ray diffraction study on 3b, the new compounds were characterized by microanalysis, H-1, C-13{H-1}, and B-11{H-1} NMR spectroscopy, and infrared spectroscopy.d b