Reduction of Cs[Co(1,2-C2B9H11)(2)] (Cs[I]) with 1 equiv of Na(Hg) amalgam or Cs metal forms the cobalt(II) dianion, [Co(1,2-C2B9H11)(2)](2-) (II), which reverts to I upon exposure to 0.5 equiv of I-2. The molecular structure of [Cs-2(DME)(4)[Co(1,2-C2B9H11)(2)] shows the carbon atoms of dianion U[ occupy transoid rotational geometry about the cobalt center, consistent with isoelectronic nickel(III) bis(dicarbollide). Reaction of THF solutions of I with 1.0 and 2.0 equiv of n-butyllithium forms purple Cs[Co(LiC2B9H10)(C2B9H11)] (Cs[I-Li]) and blue Cs-[CO(LiC2B9H10)(2)] (Cs[I-Li-2]), respectively. Treating I-Li and I-Li-2 with water causes reversion to I; reaction with D2O forms a dicarbollide C-D bond (IR : v(CD/CH) = 2275/3040 cm(-1)). Mono- and dilithio(cobalt dicarbollide) react with alkyl halides (R-X) to form the corresponding alkyl(cobalt dicarbollide) complexes, [Co(RC(2)B(9)H(10))(C2B9H11)](-) and [Co(RC(2)B(9)H(10))(2)](-) (I-R and I-R(2); R = CH3, C6H13) Alkylation of I-Li-2 using methyl iodide gives a mixture of meso and rac isomers of [Co(1-Me-1,2-C2B9H10)(2)](-) (I-1,1’-Me(2)). In contrast, treating I-Li,Me (generated via deprotonation of I-Me) with methyl iodide forms [Co(1,2-Me(2)-1,2-C2B9H9)- (C2B9H11)](-) (I-1,2-Me(2); 30%), plus meso- and rac-I-1,1’-Me(2) (70% total). The stereochemistry of alkylation is kinetically determined by the sites of deprotonation. Reaction of I-Li-2 with 2 equiv of CH3OCH2CH2OCH2Cl (MEM-CI) produces only the structurally characterized rac isomer of Cs[I-1,1’-MEM(2)]. Crystal data for [Cs-2(DME)(4)][Co(C2B9H11)(2)] : triclinic , a = 9.7930(10) Angstrom, b = 10.1300(10) Angstrom, c = 12.1010(10) Angstrom, alpha = 101.13 degrees, beta = 98.16 degrees, gamma = 114.09 degrees, V = 1042.1(2) Angstrom(3), Z = 1. Crystal data for Cs[Co(1-MEM-1,2-C2B9H10)(2)] : monoclinic P2(1)/c, a = 11.5780(10) Angstrom, b = 14.0880(10) Angstrom, c = 16.998(2) Angstrom, beta = 96.21 degrees, V = 2756.3(4) Angstrom(3), Z = 4.