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-d(5) 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 ω(0) and 'chemical pressure' resulted from the substitution of pyridine by pyridine-d(5).