The title compound has been isolated from the reaction of Gel4 with Na[Co(CO)₄] in the presence of Hg and has been characterised by X-ray analysis. Crystals are triclinic, a= 12.065(1), b= 17.334(2), c= 9.222(1)Å, α= 95.17(3), β= 82.72(3), γ= 96.95(3)°, space group P , and Z= 2. The structure has been solved by direct methods and refined to R= 0.049 for 3 136 reflections [I 5σ(I)]. The anion contains a common Ge atom bridging the Co–Co bonds both of a Co₂(CO)₇ unit and of a Co₂(CO)₆ unit which is also bridged by a HgCo(CO)₄ group.