Octacyanometallate qubit candidatesDedicated to Prof. Kim R. Dunbar in honour of her 60th birthday.Electronic supplementary information (ESI) available: Synthetic and experimental details from EPR experiments, fitting parameters and discussion, additional figures and fits. See DOI: 10.1039/c8dt02312c

We report the temperature dependence of the spin dynamics of the octacyanometallates [Mo(CN) 8 ] 3− and [W(CN) 8 ] 3− . At 5 K, these complexes display remarkably long spin-lattice relaxation times of 1.05 s, and 0.63 s, respectively. We probe the contributing factors to the spin relaxation and demonstrate the impact of spin-orbit coupling as a handle to tune vibrationally mediated spin-lattice relaxation. Cyanometallates are excellent building blocks for networks, we demonstrate counterintuitively long spin-lattice relaxation times in two heavy octacyanometalltes.