Computational Versus Experimental Spectroscopy …
177
The main conclusion of all these studies is that the origin of the magnetic
anisotropy, and even the trend in values depend strongly on the spin system and
analogies cannot be drawn a priori with systems that are yet to be investigated.
The dream of any coordination chemist is the rational design of transition-metal
complexes with desired properties. Studies by Ruamps et al. [71] have shown that
controlling the geometry of a transition-metal complex is the way to chemically
control their magnetic properties. This study shows that spin–orbit coupling can be
used to tune the magnetic anisotropy if symmetry considerations allow it. Knowing
the fact that both coordination of the ligands, and the partial quenching of the SOC
due to Jahn–Teller (JT) distortions reduce |D| from its ideal, the free ion value led
us to develop a simple model to define the main chemical factors that control the
magnetic anisotropy [72].
From that perspective, five coordinated Ni
II trigonal-bipyramidal complexes fulfill
all the above-mentioned requirements. We focused first on a case corresponding to
a D 3h point group, where the nickel ion is coordinated to three equivalent ligands
in the plane and two axial ligands, which differ from them. Since in Ni
II , the 3d
orbitals are populated with eight electrons in D 3h symmetry it can be either high (S
1), or low (S 0) spin, depending on the splitting between the d z2 orbital from
the degenerate d x2−y2 and d xy ones. Since low-spin complexes are diamagnetic, we
focused only on high-spin complexes that appear with ligands like Cl
− or F
− , with
3 E
as the ground state. As is well known, orbitally degenerate systems are subject
to the JT effect, which lowers the energy by reducing the symmetry to C 2v and
hence lifts the degeneracy of the e’ orbitals. In the case of Ni
II trigonal–bipyramidal
complexes [72], symmetry considerations indicate that as the degenerate orbitals
(d x2−y2 and d xy ) expand over the equatorial plane, the most important distortion is
described by the angle α controlling the distortion of the equilateral triangle formed
by equatorial ligands into an obtuse or acute one. The second important motion is
given by β, the bending of the axial ligands. It is important to note that the distortions
described by α and β are strongly coupled. The JT effect pushes the axial ligands
to relax into the plane, favoring β to be smaller than 180, which in turn pushes the
equatorial ligands toward smaller α values. Consequently, by choosing bulky axial
ligands, which cannot fit in between the equatorial ligands, the magnetic anisotropy
is expected to be enhanced by quenching the JT distortions (Fig. 11).
On predicting the |D|-values in theoretical models, we found that the maximum
value for magnetic anisotropy is always obtained for the high-symmetry configuration
but that JT distortions quickly reduce this value. In particular, we revealed that
complexes with small ligands (like F
− or NH 3 ) or ligands that break the symmetry
(like OH 2 ), are prone to large distortion, quenching the value of |D|. On the other
hand, complexes with large ligands are much less distorted leading to large magnetic
anisotropy, even in the low symmetry state.
We searched for an appropriate candidate in the Cambridge Structural Database
[73] with a structure that met our requirements. We found the experimentally
well-characterized complex [NiCl 3 (Hdabco) 2 ]
+ (dabco is 1,4-diazabicyclo[2.2.2]octane), with (dabco) as axial ligands and Cl
− as equatorial ligands (Fig. 11) [74,
75]. Our calculations both at LF-DFT and MRCI levels revealed that this complex
177
The main conclusion of all these studies is that the origin of the magnetic
anisotropy, and even the trend in values depend strongly on the spin system and
analogies cannot be drawn a priori with systems that are yet to be investigated.
The dream of any coordination chemist is the rational design of transition-metal
complexes with desired properties. Studies by Ruamps et al. [71] have shown that
controlling the geometry of a transition-metal complex is the way to chemically
control their magnetic properties. This study shows that spin–orbit coupling can be
used to tune the magnetic anisotropy if symmetry considerations allow it. Knowing
the fact that both coordination of the ligands, and the partial quenching of the SOC
due to Jahn–Teller (JT) distortions reduce |D| from its ideal, the free ion value led
us to develop a simple model to define the main chemical factors that control the
magnetic anisotropy [72].
From that perspective, five coordinated Ni
II trigonal-bipyramidal complexes fulfill
all the above-mentioned requirements. We focused first on a case corresponding to
a D 3h point group, where the nickel ion is coordinated to three equivalent ligands
in the plane and two axial ligands, which differ from them. Since in Ni
II , the 3d
orbitals are populated with eight electrons in D 3h symmetry it can be either high (S
1), or low (S 0) spin, depending on the splitting between the d z2 orbital from
the degenerate d x2−y2 and d xy ones. Since low-spin complexes are diamagnetic, we
focused only on high-spin complexes that appear with ligands like Cl
− or F
− , with
3 E
as the ground state. As is well known, orbitally degenerate systems are subject
to the JT effect, which lowers the energy by reducing the symmetry to C 2v and
hence lifts the degeneracy of the e’ orbitals. In the case of Ni
II trigonal–bipyramidal
complexes [72], symmetry considerations indicate that as the degenerate orbitals
(d x2−y2 and d xy ) expand over the equatorial plane, the most important distortion is
described by the angle α controlling the distortion of the equilateral triangle formed
by equatorial ligands into an obtuse or acute one. The second important motion is
given by β, the bending of the axial ligands. It is important to note that the distortions
described by α and β are strongly coupled. The JT effect pushes the axial ligands
to relax into the plane, favoring β to be smaller than 180, which in turn pushes the
equatorial ligands toward smaller α values. Consequently, by choosing bulky axial
ligands, which cannot fit in between the equatorial ligands, the magnetic anisotropy
is expected to be enhanced by quenching the JT distortions (Fig. 11).
On predicting the |D|-values in theoretical models, we found that the maximum
value for magnetic anisotropy is always obtained for the high-symmetry configuration
but that JT distortions quickly reduce this value. In particular, we revealed that
complexes with small ligands (like F
− or NH 3 ) or ligands that break the symmetry
(like OH 2 ), are prone to large distortion, quenching the value of |D|. On the other
hand, complexes with large ligands are much less distorted leading to large magnetic
anisotropy, even in the low symmetry state.
We searched for an appropriate candidate in the Cambridge Structural Database
[73] with a structure that met our requirements. We found the experimentally
well-characterized complex [NiCl 3 (Hdabco) 2 ]
+ (dabco is 1,4-diazabicyclo[2.2.2]octane), with (dabco) as axial ligands and Cl
− as equatorial ligands (Fig. 11) [74,
75]. Our calculations both at LF-DFT and MRCI levels revealed that this complex