Unexpected isotopic effect of a deuteriumed countercation on the spin-Peierls-type transitions in quasi-one-dimensional bis(maleonitriledithiolato)nickelate monoanion spin systems

Two deuteriumed quasi-one-dimensional (quasi-1D) spin-Peierls-type compounds, 4-X-benzylpyridinium-d₅ bis(maleo-nitriledithiolato) nickelate (the substituent X = Br or Cl), were structurally characterized. Compared with the corresponding non-deuteration compounds, the transition temperature T_C shifts to higher temperature. The isotopic effect of countercations on T_C is probably related to the change of phonon frequency ω₀ and 'chemical pressure' resulted from the substitution of pyridine by pyridine-d₅.