8 Electronic Properties of Transition Metal-Benzene Sandwich Clusters
315
These studies are intended to understand how the structures of metal-benzene
clusters are governed by the constituent metal atoms or molecules as well as the
cluster size. Then, we refer to joint experimental and theoretical studies, including
our recent works, for vanadium-benzene and manganese-benzene clusters and their
anions. A special emphasis is put on their electronic and magnetic properties, which
are characterized and understood from a nanoscopic point of view.
8.2 Preparation and Structures of Transition Metal-Benzene
Sandwich Clusters
8.2.1 Early Studies on Sandwich Complexes
The discovery of ferrocene, a typical metallocene consisting of a Fe(II) atom and
two cyclopentadienyl (C 5 H 5 = Cp) ligands [12], is counted as a milestone in
organometallic chemistry. Chemical, magnetic, and X-ray crystallographic characterizations revealed the sandwich structure of ferrocene, where the iron atom is
η 5 -bound to both the Cp ligands [13–14]. From the viewpoint of molecular orbital
theory, the interaction of the Fe atom and the Cp ligands is shown in Fig. 8.1. In the
following way, Lauher and Hoffmann [15] described the kinetic and thermodynamic
stability of ferrocene. The p orbitals of the two Cp ligands produce three sets of
approximately degenerate orbitals: a low-lying filled pair of a 1g and a 2u symmetry,
a filled set of e 1g and e 1u symmetry, and an unoccupied pair of antibonding orbitals
of e 2g and e 2u symmetry. When these orbitals interact with the Fe 3d orbitals, the
resultant molecular energy levels of ferrocene are understood in terms of symmetry
matching. The e 1g orbitals of the Cp ligands strongly interact with the Fe 3d yz and
3d zx orbitals, yielding a pair of bonding and antibonding orbitals, each of which is
doubly degenerate. The a 1g , a 2u , and e 1u orbitals could interact but are much less
combined with the Fe 4s or 4p orbitals that are too high in energy. The 3d z
2 , 3d xy ,
and 3d x
2 –y
2 orbitals of the Fe atom thus remain essentially nonbonding. Ferrocene
thus fulfills the 18-electron rule by filling its bonding and nonbonding orbitals. In
fact, other metallocenes that do not satisfy the 18-electron rule, such as cobaltocene,
CoCp 2 [16], are basically less stable than ferrocene.
Besides the metallocene family, bis(benzene)chromium, CrBz 2 (Bz = C 6 H 6 )
[17], and bis(cyclooctatetraene)uranium, U(COT) 2 (COT = C 8 H 8 ), called uranocene [18], are well-known sandwich compounds (Fig. 8.2). Their molecular
orbital interactions between the metal and ligands are described in the same manner
as that for ferrocene [19–20]. Bis(benzene)vanadium, VBz 2 , was also prepared by
the similar (Fischer-Hafner) method that was used for CrBz 2 , but it was much more
immediately oxidized in air than CrBz 2 [21]. Likewise, sandwich complexes that do
not satisfy the 18-electron rule are difficult to be synthesized in the condensed phase.
Another interesting derivative is a triple-decker sandwich complex of Ni atoms
and Cp ligands, [Ni 2 Cp 3 ] + (Fig. 8.2) [22]. Chemical synthesis of multiple-decker
sandwich compounds, however, often requires appropriate ligands to stabilize such
315
These studies are intended to understand how the structures of metal-benzene
clusters are governed by the constituent metal atoms or molecules as well as the
cluster size. Then, we refer to joint experimental and theoretical studies, including
our recent works, for vanadium-benzene and manganese-benzene clusters and their
anions. A special emphasis is put on their electronic and magnetic properties, which
are characterized and understood from a nanoscopic point of view.
8.2 Preparation and Structures of Transition Metal-Benzene
Sandwich Clusters
8.2.1 Early Studies on Sandwich Complexes
The discovery of ferrocene, a typical metallocene consisting of a Fe(II) atom and
two cyclopentadienyl (C 5 H 5 = Cp) ligands [12], is counted as a milestone in
organometallic chemistry. Chemical, magnetic, and X-ray crystallographic characterizations revealed the sandwich structure of ferrocene, where the iron atom is
η 5 -bound to both the Cp ligands [13–14]. From the viewpoint of molecular orbital
theory, the interaction of the Fe atom and the Cp ligands is shown in Fig. 8.1. In the
following way, Lauher and Hoffmann [15] described the kinetic and thermodynamic
stability of ferrocene. The p orbitals of the two Cp ligands produce three sets of
approximately degenerate orbitals: a low-lying filled pair of a 1g and a 2u symmetry,
a filled set of e 1g and e 1u symmetry, and an unoccupied pair of antibonding orbitals
of e 2g and e 2u symmetry. When these orbitals interact with the Fe 3d orbitals, the
resultant molecular energy levels of ferrocene are understood in terms of symmetry
matching. The e 1g orbitals of the Cp ligands strongly interact with the Fe 3d yz and
3d zx orbitals, yielding a pair of bonding and antibonding orbitals, each of which is
doubly degenerate. The a 1g , a 2u , and e 1u orbitals could interact but are much less
combined with the Fe 4s or 4p orbitals that are too high in energy. The 3d z
2 , 3d xy ,
and 3d x
2 –y
2 orbitals of the Fe atom thus remain essentially nonbonding. Ferrocene
thus fulfills the 18-electron rule by filling its bonding and nonbonding orbitals. In
fact, other metallocenes that do not satisfy the 18-electron rule, such as cobaltocene,
CoCp 2 [16], are basically less stable than ferrocene.
Besides the metallocene family, bis(benzene)chromium, CrBz 2 (Bz = C 6 H 6 )
[17], and bis(cyclooctatetraene)uranium, U(COT) 2 (COT = C 8 H 8 ), called uranocene [18], are well-known sandwich compounds (Fig. 8.2). Their molecular
orbital interactions between the metal and ligands are described in the same manner
as that for ferrocene [19–20]. Bis(benzene)vanadium, VBz 2 , was also prepared by
the similar (Fischer-Hafner) method that was used for CrBz 2 , but it was much more
immediately oxidized in air than CrBz 2 [21]. Likewise, sandwich complexes that do
not satisfy the 18-electron rule are difficult to be synthesized in the condensed phase.
Another interesting derivative is a triple-decker sandwich complex of Ni atoms
and Cp ligands, [Ni 2 Cp 3 ] + (Fig. 8.2) [22]. Chemical synthesis of multiple-decker
sandwich compounds, however, often requires appropriate ligands to stabilize such