The synthesis, chemical behavior and single crystal X ray structure of two new CoNi bimetallic carbide clusters, namely [Co 2Ni 10(CO) 20C] 2− and [Co 3Ni 9(CO) 20C] 2−, are reported. The first complex was isolated in ca. 30% yield among the products of the reaction of Co 3(CO) 9CCl with [Ni 6(CO) 12] 2−, while the second was obtained quantitatively by reaction with protonic acids of the previously reported [Co 3Ni 9(CO) 20C] 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.