Chemistry Reference
In-Depth Information
Fig. 8 The changes occurring to [Ni(CH
2
NH
2
NH
2
CH
2
)
3
][NO
3
]
2
from ambient conditions (
left
)to
low temperature (
right
). The molecular geometry remains almost unchanged, but the molecule
moves away from a crystallographic threefold axis producing a 3
1
helix. The phase change occurs
abruptly and at (
T
c
,
P
c
) the two structures are different and in equilibrium
axis of the hexagonal lattice doubles below a critical temperature. At molecular level,
the molecular symmetry is reduced from
3 to 3 and therefore the perfectly
staggered
conformation. Free rotation is now possible for the two Co(CO)
3
As(C
6
H
5
)
3
moieties
about the Co-Co axis. As temperature is lowered, the molecular conformation further
changes, maintaining the threefold symmetry and without reaching the
6m
2
of a
perfectly
eclipsed
conformation. It is interesting that the group-subgroup relation-
ship at the critical temperature is such that no low temperature phase is actually
possible above
T
c
, because the two phases are simply identical. Therefore this is an
example of second-order type phase transition, described in detail by Landau [
99
].
No discontinuity of molar volume, enthalpy, and entropy are expected, as in fact
demonstrated by X-ray diffraction and DSC measurements. Sometime these
transitions are called
continuous
to emphasize this soft nature, but one should
remember that expansivity and specific heat are discontinuous. The high tempera-
ture phase could exist below
T
c
, but no hysteresis is observed. Notably the same kind
of phase transition is observed for the isomorphic species Co
2
(CO)
6
(P(C
6
H
5
)
3
)
2
(3)
[
108
] and both 1 and 3 undergo similar transitions at high pressure.
Ni(C
2
H
8
N
2
)
3
(NO
3
)
2
is quite different - the space group type and the lattice
change at
T
c
(ca. 106 K). The transition show discontinuity of the cell volume and,
as expected, there is a latent heat of transition. Notably at the critical temperature
the two phases are structurally different and therefore they are in equilibrium at that
temperature. A minor hysteresis is observed.
As mentioned above, sometimes two phases are recognized even though no
space group type change is actually observed. Often these transformations are
associated with competition between two different electronic configurations, or
otherwise between two molecular conformations. The latter are, for example,
order/disorder transitions caused by a group in a molecule subjected to weakly
bounding potential surface, and therefore showing multiple minima. Below a
critical temperature the higher energy conformer may be inaccessible. More inter-
esting is instead the competition between different electronic configurations. This
implies different bonding in a molecule (or supramolecular synthon). Notable
examples are
spin crossover
materials, i.e., molecular compounds with the ability
