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The synthesis, chemical behavior and single crystal X ray structure of two new CoNi bimetallic carbide clusters, namely [Co 2 Ni 10 (CO) 20 C] 2− and [Co 3 Ni 9 (CO) 20 C] 2− , are reported. The first complex was isolated in ca. 30% yield among the products of the reaction of Co 3 (CO) 9 CCl with [Ni 6 (CO) 12 ] 2− , while the second was obtained quantitatively by reaction with protonic acids of the previously reported [Co 3 Ni 9 (CO) 20 C] 3− . The two compounds are isostructural and show a metal frame which can be regarded as derived by distortion of a tetra-capped triangulated dodecahedron of D 2 d symmetry; the statistical (Co and Ni atoms being undistinguishable) idealized symmetry of the metal frame is only D 2 owing to progressive deformations toward a tetra-capped tetragonal antiprismatic geometry, which are probably related to the steric requirements of the interstitial carbide atom.