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 TC shifts to higher temperature. The isotopic effect of countercations on TC is probably related to the change of phonon frequency ω0 and ‘chemical pressure’ resulted from the substitution of
