Reaction of Na2[nido-7,8-Me2-7,8-C2B9H9] with [Ni2(mu-Br)2(eta3-C3H5)2] or [Pt2(mu-Br)2(eta-C3H5)2] in thf (tetrahydrofuran) gives the anionic species [M(eta3-C3H5)(eta 5-7,8-Me2-7,8-C2B9H9)]- (M = Ni or Pt) which have been isolated as their [NEt4]+ and [N(PPh3)2]+ salts 1a-1d. In the presence of suitable substrate molecules (L = CO or CNBu(t)), protonation (HBF4.Et2O) of the reagents 1 at low temperatures gives the neutral compounds [ML2-(eta5-7,8-Me2-7,8-C2B9H9)] (2a, M = Ni, L = CO; 2b, M = Ni, L = CNBu(t); 2c, M = Pt, L = CO; 2d, M = Pt, L = CNBu(t)). Dichloromethane solutions of the dicarbonylnickel complex 2a slowly decompose through loss of carbon monoxide to give a mixture of the isomeric dimetal species [Ni2(CO)2(eta5-7,8-Me2-7,8-C2B9H9)2] (3a), [Ni2(CO)2(eta5-7,8-Me2-7,8-C2B9H9)(eta5-2,7-Me2-2,7-C2B9H9)] (3b), and [Ni2(CO)2(eta5-2,7-Me2-2,7-C2B9H9)2] (3c). The molecular structure of 3c, in which the carborane CMe groups of both cages occupy vertices in different layers of the eta5-C2B9 nido-cages, was established by X-ray diffraction. Crystals are monoclinic, space group C2/c (No. 15), with a = 21.498(5) angstrom, b = 7.722(2) angstrom, c = 14.962(3) angstrom, beta = 98.21(2)degrees, and Z = 4. Each nickel atom is ligated by a terminal carbonyl group and by a 2,7-Me2-2,7-C2B9H9 carborane cage, the coordinated CBBBB ring of which displays pronounced nonplanarity. In addition, the metal-metal vector is spanned by two three-center, two-electron B-H half arrow pointing right Ni "agostic" interactions. Treatment of a CH2Cl2 solution of [N(PPh3)2][Pt(eta3-C3H5)(eta5-7,8-Me2-7,8-C2B9H9)] (1d) with HBF4.Et2O at ca. -78-degrees-C followed by addition of the alkyne PhC=CH affords the dimetal species [Pt2{mu-sigma,sigma’:eta4-C(H)C(Ph)C(H)C(Ph)}(eta5-7,8-Me2-7,8-C2B9H9)2] (4), the structure of which has been established by X-ray diffraction. Crystals are triclinic, space group P1BAR (No. 2) with a = 13.317(2) angstrom, b = 16.783(4) angstrom, c = 17.289(4) angstrom, alpha = 64.94(2(degrees, beta = 83.31(2)degrees, gamma = 78.58(2)degrees, and Z = 4. Two alkyne molecules have coupled in a "head-to-tail" manner to give a bridging "C4" moiety that is sigma-bonded to one of the metal centers through the two terminal carbon atoms and eta4-bound to the second metal center. In addition, each platinum atom is ligated by a 7,8-Me2-7,8-C2B9H9 carborane cage bound in a pentahapto manner, with neither cage involved in exopolyhedral bonding. Protonation (HBF4.Et2O) of 1a in CH2Cl2 followed by addition of [Co2(CO)8] affords the trinuclear metal complex [Co2Ni(mu-CO)(CO)5(eta5-7,8-Me2-7,8-C2B9H9)] (5), in which the NiC2B9 metallacarborane cage forms two exopolyhedral B-H half arrow point right Co "agostic" interactions. In addition to the X-ray diffraction studies, the new compounds have been characterized by microanalysis, H-1, C-13, and B-11 NMR spectroscopy, and infrared spectroscopy.