A ditantalum hydride complex undergoes head-to-head C-C coupling of six CO molecules under mild conditions to produce an octaanionic C6O6 unit. During the reaction, eight-electron reduction of six CO molecules takes place. In the product, the C6O6 unit is coordinated to four tantalum metals through six oxygen and two carbon atoms. The geometrical parameters within the Ta4C6O6 core indicate significant contribution from a hexatriene-hexaolate form.