To further illustrate brilliant Jan's answer, here are the experimental data for both structures: $\ce{[(CN)5CoO2Co(CN)5]^6-}$ \[1\]: [![enter image description here][1]][1] $\ce{[(CN)5CoO2Co(CN)5]^5-}$ \[2\]: [![enter image description here][2]][2] ###Bibliography 1. Fronczek, F. R.; Schaefer, W. P. *Inorganica Chimica Acta* **1974**, 9 (Supplement C), 143–151 DOI: 10.1016/S0020-1693(00)89896-5. 2. Fronczek, F. R.; Schaefer, W. P.; Marsh, R. E. *Inorg. Chem.* **1975**, 14 (3), 611–617 DOI: 10.1021/ic50145a035. [1]: https://i.sstatic.net/SPKv3.png [2]: https://i.sstatic.net/VCe1Y.png