Chemistry Reference
In-Depth Information
g = 2.034
6.6%
dX"
d
B
g = 2.017
4.7%
g = 2.006
1.8%
320
330
340
B (mT)
350
360
Figure 8.14
Isotropic EPR spectra of 1mM dichloromethane solutions of: (a) [Ni
II
(L
9
)
•
]
+
;(b)[N
II
(L
9
OMe
)
•
]
+
;
and(c)[Ni
II
(L
9
NMe
)
•
]
+
.([90]ReproducedbypermissionoftheRoyalSocietyofChemistry.)Afull-colourversion
ofthisfigureappearsintheColourPlatesectionof this topic.
noteworthy that the kind of coupling depends on geometrical parameters, as the octahedral nickel complex
of the tetradentate ligand H
2
et,Me
L
10
in the presence of pyridine, namely [Ni
II
et,Me
L
10
)
•
(Py)
2
]
+
,isa
(
2
)system.
95
An important conclusion in this series of octahedral complexes is that the imine
nitrogens of classical Schiff bases tend to favor a metal-centered oxidation process, whereas amines favour
a ligand-centered process.
o
-Iminobenzosemiquinone radical ligands have been incorporated both in square planar and octahe-
dral nickel(II) complexes. The
trans
-[Ni
II
(L
14
ISQ
=
/
(S
t
3
)
••
2
] complex is planar within experimental error, and
the metal ion is consequently diamagnetic.
96
orbitals of the
radicals and one of the filled t
2g
metal orbital is significant (Figure 8.12), leading to a strong antiferro-
magnetic coupling. [Ni
II
(L
14
ISQ
In this geometry the overlap between the
π
)
••
2
] is therefore a singlet diradical. Its cyclic voltammetry curve exhibits
two cathodic waves attributed to the reduction of each of the
o
-iminobenzosemiquinone moieties, and a
single bielectronic anodic wave that corresponds to the simultaneous one-electron oxidation of the two
o
-iminobenzosemiquinone moieties according to Equation 8.11:
[Ni
II
L
14
AP
2
]
2
−
[Ni
II
L
14
AP
L
14
ISQ
)
•
]
−
[Ni
II
L
14
ISQ
)
••
2
]
[Ni
II
L
14
BQ
2
]
2
+
(
)
(
)(
(
(
)
(8.11)
)
••
2
] exhibits a remarkable feature in its UV-Vis spectrum, a very intense absorption at
890 nm (37 000 M
−
1
cm
−
1
Trans
-[Ni
II
(L
14
ISQ
assigned to the spin- and dipole-allowed ligand-to-ligand transition shown in
Figure 8.15. Similar bands have been also reported for square planar complexes of other metals (copper,
palladium). As expected the band loses most of its intensity in the monoradical [Ni
II
(L
14
AP
)
(L
14
ISQ
]
−
)
)
value of only 7500 M
−
1
cm
−
1
with an
ε
at 880 nm. Interestingly no redox process leading to a nickel(III)
ion has been reported in this series.
In the diradical complex [Ni
II
Pr
L
16
ISQ
)
••
], the
O,N o
-iminobenzosemiquinone moieties are forced to
(
be
cis
one to each other.
97
)
••
2
], this complex is a singlet diradical characterized by
an intense ligand-to-ligand transition at 854 nm (30 000 M
−
1
cm
−
1
Like [Ni
II
(L
14
ISQ
)
. The one-electron reduced form of
[Ni
II
Pr
L
16
ISQ
)
••
)
(
] has been characterized by EPR, revealing that it is a mixed-valent (S
=
1
/
2
) species
where the unpaired electron is delocalized over both ligands.
The [Ni
II
(L
14
ISQ
)
•
(tren)]
−
complex is different from the others of the series in the sense that the nickel
ion is octahedral, and thus paramagnetic. Consequently, the e
g
orbitals that host the metal unpaired electrons
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