The tetrahedral carbonylcobalt clusters (CO)9Co3(CC6H2Me3-2,4,6) (I), (CO)8(Ph3P)Co3Co(CO)3 (II) and (CO)7(PhMe2P)2Co3Fe(H)(CO)2PMe2Ph (III) were synthesized, and investigated by single-crystal x-ray diffraction methods together with the known compd. (CO)9Co3CCl (IV). The results of the structure detns. show that the mesityl-substituted methylidyne-cluster I can be arranged structurally between the non-bridged cluster IV and the two CO-bridged clusters II and III. The o-Me substituents of the mesityl group act as pseudo ligands of the apical carbon atom and push the neighboring CO ligands into the Co3-plane, one condition for the formation of bridges as they exist in II and III.