The compounds [Co-2(CO)(8)] and nido-7,8-C2B9H13 react in CH2Cl2 to give a complex mixture of products consisting primarily of two isomers of the dicobalt species [Co-2(CO)(2)(eta(5)-7,8-C2B9H11)(2)] (1), together with small amounts of a mononuclear cobalt compound [Co(CO)(2)(eta(5)-10-CO-7,8-C2B9H10)] (5) and a charge-compensated carborane nido-9-CO-7,8-C2B9H11 (6). In solution, isomers 1a and 1b slowly equilibrate. However, column chromatography allows a clean separation of 1a from the mixture, and a single-crystal X-ray diffraction study revealed that each metal atom is ligated by a terminal CO molecule and in a pentahapto manner by a nido-C2B9H11 cage framework. The two Co(CO)(eta(5)-7,8-C2B9H11) units are linked by a Co-Co bond [2.503(2) Angstrom], which is supported by two three-center two-electron B-H half arrow pointing right Co bonds. The latter employ B-H vertices in each cage which lie in alpha-sites with respect to the carbons in the CCBBB rings bonded to cobalt. Addition of PMe(2)Ph to a CH2Cl2 solution of a mixture of the isomers 1, enriched in 1b, gave isomers of formulation [Co-2(CO)(PMe(2)Ph)(eta(5)-7,8-C2B9H11)(2)] (2). Crystals of one isomer were suitable for X-ray diffraction. The molecule 2a has a structure similar to that of 1a but differs in that whereas one B-H half arrow pointing right Co bridge involves a boron atom in an alpha-site of a CCBBB ring coordinated to cobalt, the other uses a boron atom in the beta-site. Reaction between 1b and an excess of PMe(2)Ph in CH2Cl2 gave the complex [CoCl(PMe(2)Ph)(2)(eta(5)-7,8-C2B9H11)] (3), the structure of which was established by X-ray diffraction. Experiments indicated that 3 was formed through a paramagnetic Co-II species of formulation [Co(PMe(2)Ph)(2)(eta(5)-7,8-C2B9H11)]. Addition of 2 molar equiv of CNBu(t) to solutions of either 1a or 1b gave a mixture of two isomers of the complex [Co-2(CNBu(t))(2)(eta(5)-7,8-C2B9H11)(2)] (4). MMR data for the new compounds are reported and discussed.