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Polyhedron 19 (2000) 2297 – 2302
Influence of L-type ligands on the relative stability and
interconversion of cis – trans -[Ru(phen)
2
L
2
]
n +
type complexes.
A theoretical study
J. Concepcio ´n, B. Loeb *, Y. Simo ´ n-Manso, F. Zuloaga*
Facultad de Quı ´mica, Pontificia Uniersidad Cato ´lica de Chile, Casilla 306, Santiago, Chile
Received 1 October 1999; accepted 10 January 2000
Abstract
Polipyridine complexes of type [Ru(N-N)
2
L
2
]
2 +
are useful for the conversion of light into usable energy for which the trans
geometry would be preferable. In this paper, the effect of distortion on trans structure stability was studied theoretically. A formal
separation of the contributions was carried out, and it explained why the hydrogen repulsions are an important factor. A study
of the main aspects related to the cis – trans isomers and also their mutual interconversion was analyzed. Calculations were carried
out at the PM3, ZINDO and ab initio level. Energy differences between trans and cis isomers after full geometry optimizations
for different L-ligands show a dependence on the L-ligand Jo ¨ rgensen field strength parameter f. A driving coordinate was
performed in order to determine the kinetic preferences for isomer formation. The analysis showed that the entrance of the second
phenanthroline is favoured for the cis geometry in regard to the trans geometry. The activation barrier is also dependent on the
L-ligand strength. The thermal trans – cis interconversion seems to be very improbable adiabatically (considering only one
potential energy surface). © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Ruthenium; Phenanthroline; Cis – trans ; MO calculations; Barriers; Field strength
1. Introduction
Ruthenium polypyridyl complexes of the type [Ru(N-
N)
2
L
1
L
2
]
n +
(N-N =2,2-bipyridine, 1,10-phenanthrol-
ine; L
1
and L
2
other ligands) have been widely studied
in search of potential solar energy conversion systems
[1]. Most of the work reported up until now is related
to complexes having cis geometry, in spite of the fact
that to exploit charge- and energy-transfer phenomena
to their greatest effect, the trans geometry is more
suitable [2]. Thus, the reported experimental work
shows that there is a preferential formation for the cis
isomer, and the interconversion to the trans isomer
appears difficult to achieve.
Regarding the trans complexes, a significant distor-
tion of the coordination sphere in the N
4
equatorial
plane is generally observed. Two main types of distor-
tion have been found experimentally [2c,3 – 6]: the
twisted and the bowed configurations, as shown in
Scheme 1 [3]. In the twisted configuration, each N-N
ligand is planar, but twisted out of the equatorial plane
of the octahedrically coordinated metal. In the bowed
geometry the octahedral coordination is maintained but
each N-N ligand is bent (for example non-planar bpy’s)
and the equatorial plane is distorted in such a way as to
minimize the opposing hydrogen interactions [7]. A
third geometry, called the tilt configuration, has also
been reported [7] and it can be visualized as a special
case of the bowed configuration where the degree of
distortion of the equatorial N-N ligands changes from a
minimum value (tilt configuration) to higher values of
distortion (bowed configuration) depending upon the
axial L
1
,L
2
ligands. To our knowledge, there is no
ruthenium [Ru(N-N)
2
L
1
L
2
]
n +
type complex reported in
a flattened configuration, presumably because it is not a
ground state geometry.
* Corresponding authors. Tel.: +56-2-686-4404; fax: +56-2-686-
4744; e-mail: bloeb@puc.cl; fzuloaga@puc.cl.
0277-5387/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.
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