Inorganica Chimica Acta 326 (2001) 89 – 100
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Metamagnetism in linear chain electron-transfer salts based on
decamethylferrocenium and metal – bis(dichalcogenate) acceptors
S. Rabac ¸a
a
, R. Meira
a
, L.C.J. Pereira
a
, M. Teresa Duarte
b
, J.J. Novoa
c
, V. Gama
a,
*
a
Departamento Quı ´mica, Instituto Tecnolo ´gico e Nuclear, P-2686 -953 Sacae ´m, Portugal
b
Departamento Engenharia Quı ´mica, Instituto Superior Te ´cnico, P-1049 -001 Lisbon, Portugal
c
Departamento Quı ´mica Fı ´sica and CER Quimica Teo ´rica, Uniersidad de Barcelona, A. Diagonal 647, 08028 Barcelona, Spain
Received 15 May 2001; accepted 12 September 2001
In memory of Professor Olivier Kahn
Abstract
The crystal structure of the electron-transfer (ET) salts [Fe(Cp*)
2
][M(tds)
2
], with M=Ni (1) and Pt (2), consists of an array of
parallel alternating donors, [Fe(Cp*)
2
]
+
, and acceptors, [M(tds)
2
]
-
, ···DADADA···, stacks. For T 20 K, the magnetic
susceptibility follows a Curie – Weiss behavior, with values of 8.9 and 9.3 K for 1 and 2, respectively. A metamagnetic behavior
was observed in 2, with T
N
=3.3 K and H
C
=3.95 kG at 1.7 K, resulting from a high magnetic anisotropy. A systematic study
of the intra and interchain magnetic interactions was performed on 1, 2 and other ET salts based on decamethylferrocenium and
on metal – bis(dichalcogenate) acceptors, with a similar crystal structure. The observed magnetic behavior of these compounds is
consistent with the presence of strong ferromagnetic intrachain DA interactions and weaker antiferromagnetic interchain
interactions, predicted by the McConnell I model. A variety of interionic interchain contacts were found in these ET salts (AA,
DD and DA) and these contacts were observed to give rise to antiferromagnetic interchain coupling. Although it was only
observed in the case of 2 and [Fe(Cp*)
2
][Ni(edt)
2
], metamagnetism is expected to occur at lower temperatures in the other ET salts
due to weaker intra and interchain coupling. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Magnetic molecular materials; Electron-transfer salts; Crystal structures; Magnetic anisotropy; Metamagnetism; Decamethylferroce-
nium
1. Introduction
The observation of metamagnetism in the electron-
transfer (ET) salt [Fe(Cp*)
2
]TCNQ [1] (Cp* =C
5
Me
5
,
TCNQ =7,7,8,8-tetracyano-p -quinodimethane), with a
crystal structure consisting of a parallel arrangement of
one-dimensional (1D) chains of alternating donors (D)
and acceptors (A), ···D
+
A
-
D
+
A
-
···, in 1979, lead to
strong efforts in the search of bulk ferromagnetism in
molecule-based ET salts, which was accomplished in
1986 with the report of ferromagnetic (FM) ordering at
4.8 K in [Fe(Cp*)
2
]TCNE [2] (TCNE =tetracya-
noethylene). During the following years, five other al-
ternating linear chain decamethylmetallocenium-based
ET salts with planar polynitrile acceptors, were re-
ported as ferromagnets, [M(Cp*)
2
]TCNE (M =Mn, [3]
Cr [4]) and [M(Cp*)
2
]TCNQ (M =Fe [5], Mn [6], Cr
[7]). The study of the relationship between the structure
and the magnetic properties in these compounds re-
vealed that the linear alternating chain structural motif
clearly favored the existence of FM DA coupling [8].
The first strategy for achieving FM coupling in
molecule-based materials was proposed in 1963 [9], the
so-called McConnell I mechanism, which, in spite of its
simplicity, has shown a good agreement with the exper-
imental observations in these ET salts [10]. However,
the magnetic coupling in these ET salts is still a subject
of controversy [11], and the validity of the McConnell I
mechanism has been put into question based on theo-
retical arguments [12], and also found not to work in
purely organic nitronyl nitroxide crystals [13].
* Corresponding author. Tel.: +351-21-9966 203; fax: +351-21-
9941 455.
E-mail address: vascog@itn.pt (V. Gama).
0020-1693/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
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