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Abstract:
The syntheses, structural characterizations and magnetic behaviors of
three new complexes, 1-(3', 4', 5'- trifluorobenzyl) pyridinium
[M(mnt)(2)](-) [M = Ni (1), Pd (2) or Pt (3)], are reported. These
complexes are isomorphous and their prominent structural character is
that the [ M( mnt) 2]- anions form columnar stacks, in which the
dimerization was observed. Complexes 2 and 3 are diamagnetic, while 1
possesses an energy gap of 2474 K. For crystal 4,
1-(4'-fluorobenzyl)pyridinium [Ni(mnt)(2)] (its structure and magnetic
susceptibility were briefly reported earlier), the magnetic behavior
can be divided into two regimes, namely, weakly ferromagnetic coupling
above 93 K and strongly antiferromagnetic coupling below 93 K. A
transition occurs at 93 K which switches the magnetic exchange nature
from ferromagnetic to antiferromagnetic. A sharp thermal abnormality
with lambda-shape, associated with the transition, appears from its
heat capacity measurement to indicate that the transition is first
order. The temperature dependences of the superlattice diffractions
revealed the existence of the pretransitional phenomena up to at least
140 K. The unusual magnetic behavior of 4, such as the origin of the
ferromagnetic interaction in the high temperature phase and what causes
the spin transition, are discussed further.
