Ab Initio Calculations of the Trigonal and Zero-Field Splittings in Trischelated Diketonato
Complexes of Trivalent Chromium
Carl Ribbing, Kristine Pierloot, and Arnout Ceulemans*
Division of Quantum Chemistry, University of Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium
ReceiVed February 12, 1998
The ground and excited states of the neutral tris(1,3-propanedionato)chromium(III) d
3
complex are investigated
with quantum chemical methods. Trigonal splittings are calculated and compared with experiment for the first
two excited quartet states
4
T
2g
and
4
T
1g
. The effect of spin-orbit coupling is also introduced, and a reassignment
is proposed for three absorption bands in the doublet region. Trigonal symmetry-induced mixing between the e
g
and t
2g
shells is found to be responsible for the unusually large zero-field splitting of the
4
T
2g
ground state.
Orbitals pictures are presented which illustrate the role of the phase-coupling effect from the unsaturated ligands.
1. Introduction
The tris(acetylacetonato)chromium(III) complex, Cr(acac)
3
,
has been the subject of numerous spectroscopic, magnetooptical,
and theoretical studies.
1
Especially the π -bonding role of the
conjugated bidentate chain and the possible occurrence of a
phase-coupling effect have attracted attention.
2
On the basis of a detailed analysis in the framework of the
angular overlap model (AOM) Atanasov et al. concluded that
the trigonal splitting of the first spin-allowed band,
4
T
2g
r
4
A
2g
,
by some 800 cm
-1
was indeed due to the phase-coupling effect.
3
In a further study
4
it was argued that the unusually large zero-
field splitting (ZFS) of the ground state
5
by 1.2 cm
-1
could
only be explained by assuming an anisotropic spin-orbit
coupling mechanism. The low-temperature emission spectrum
was assigned to phosphorescense from the 2A h (
3
/
2
) spin-orbit
component of a trigonal
2
E state with predominant
2
E
g
octahedral
parentage.
1,4
The extremely large ZFS
6
and unusual g tensor
7,8
of the doublet state remained poorly understood.
Progress in correlated methods of computational chemistry
currently allows us to perform electronic structure calculations
that can throw a new light on these issues. In this paper we
report the first investigation of a trischelated Cr(III) complex
with the unsubstituted 1,3-propanedionato ligand, PDO
-
, using
the complete active space methods CASSCF and CASPT2. The
conclusions corroborate the results by Atanasov et al. concerning
the phase-coupling origin of the trigonal splitting.
3,4
In addition
a new assignment for the doublet bands is suggested.
2. Computational Details
The calculations were performed with the MOLCAS program
system,
9
and the effective symmetry was limited to the C
2
subgroup of the actual D
3
point group. Optimized orbitals were
obtained with the CASSCF method for each state. The active
space was restricted to the five d orbitals. It was noticed during
optimization that the orbitals acquired some ligand character,
but only to a minor extent. Orbitals of e, a
1
, and a
2
type were
prevented from mixing by using the supersymmetry option in
the CASSCF program.
To introduce further correlation into the calculations a
CASPT2
9,10
calculation was performed for each state, using the
CASSCF wave function as reference. In the CASPT2 calcula-
tions all electrons, except 1s, 2s, and 2p on chromium and 1s
on carbon and oxygen, were correlated (95 electrons). ANO
type basis sets
11
were chosen as follows: Cr(17s12p9d4f/
6s4p3d1f), O(10s6p/3s2p), C(10s6p/3s2p), and H(7s/2s).
The geometry of the Cr(PDO)
3
complex was determined in
the following way. First we optimized the free PDO
-
ligand
with the constraint that the O-O distance is 2.77 Å (see below).
This produced a flat C
2V
geometry for the PDO
-
ligand. If the
numbering of the carbons along PDO is O-C
1
-C
2
-C
3
-O, the
geometry is the following: distances (Å), O-O ) 2.77, O-C
1
) 1.33, C
1
-C
2
) 1.39, C
1
-H
1
) 1.10, C
2
-H
2
) 1.08; angles
(deg), O-C
1
-C
2
) 124.6, O-C
1
-H
1
) 119.1, C
1
-C
2
-H
2
)
117.8. If the PDO
-
ligands are placed around the Cr
3+
ion so
that the oxygens are in exact octahedral positions, an O-O
distance of 2.77 Å corresponds to a Cr-O distance of 2.77/ 2
) 1.96 Å. For the closely related Ga(acac)
3
and Cr(acac)
3
complexes the geometry is available from X-ray diffraction
data
12-14
and the coordination of the oxygens is very close to
(1) Scho ¨nherr, T. Top. Curr. Chem. 1997, 191, 87.
(2) Ceulemans, A.; Vanquickenborne, L. G. Pure Appl. Chem. 1990, 62,
1081.
(3) Atanasov, M. A.; Scho ¨nherr, T.; Schmidtke, H. H. Theor. Chim. Acta
1987, 71, 59.
(4) Atanasov, M.; Scho ¨nherr, T. Inorg. Chem. 1990, 29, 4545.
(5) Elbers, G.; Remme, S.; Lehmann, G. Inorg. Chem. 1986, 25, 896.
(6) Scho ¨nherr, T.; Eyring, G.; Linder, R. Z. Naturforsch. 1983, 38, 736.
(7) Fields, R. A.; Haindl, E.; Winscom, C. J.; Khan, Z. H.; Plato, M.;
Mo ¨bius, K. J. Chem. Phys. 1984, 80, 3082.
(8) Fields, R. A.; Winscom, C. J.; Haindl, E.; Plato, M.; Mo ¨bius, K. Chem.
Phys. Lett. 1986, 124, 121.
(9) Andersson, K.; Fu ¨ lscher, M. P.; Karlstro ¨ m, G.; Lindh, R.; Malmqvist,
P.-Å.; Olsen, J.; Roos, B. O.; Sadlej, A. J.; Blomberg, M. R. A.;
Siegbahn, P. E. M.; Kello ¨, V.; Noga, J.; Urban, M.; Widmark, P.-O.
MOLCAS, Version 3; Department of Theoretical Chemistry: Chemistry
Center, University of Lund, P.O.B. 124, S-221 00 Lund, Sweden, 1994.
(10) Andersson, K.; Malmqvist, P.-Å.; Roos, B. O.; Sadlej, A. J.; Wolinski,
K. J. Phys. Chem. 1990, 94, 5483.
(11) Pierloot, K.; Dumez, B.; Widmark, P.-O.; Roos, B. O. Theor. Chim.
Acta 1995, 90, 87.
(12) Dymock, K.; Palenik, G. J. Acta Crystallogr. 1974, 30, 1364.
(13) Morosin, B. Acta Crystallogr. 1965, 19, 131.
5227 Inorg. Chem. 1998, 37, 5227-5232
S0020-1669(98)00161-X CCC: $15.00 © 1998 American Chemical Society
Published on Web 09/01/1998