Metallocenes are a class of organometallic compounds that have two cyclopentadienyl anions (C5H5-) bound to a metal center, typically a transition metal. One of the most well-known examples is ferrocene, where the metal center is iron (Fe). Here's an overview of metallocenes:
1. Structure:
-...
Chem 59-651 Main Group Metallocenes
A strict definition of “metallocene” insists that such compounds must
contain a metal that is sandwiched between two cyclopentadienyl (C5H5,
Cp) groups, however I will use the term to describe any compounds
containing a “metal” that is bonded to the π-cloud of a cyclopentadienyl
(or similar) substituent. I will use the term for convenience because:
(a) some of the important p-block elements are not metals
(b) many of the main group compounds are necessarily sandwiched
between two Cp substituents
(c) other substituents analogous to Cp are also of interest
X X X
M M M X M X
X X X
M = Ga, In, Tl; X = Cl, Br (See: Schmidbaur, Angew. Chem. Int. Ed., 1985, 24, 893).
, Chem 59-651 Cyclopentadienyl Ligand Basics
The structural features and bonding interactions between
cyclopentadienyl groups and metal atoms shows considerable variation
and is described using a few different conventions (See Jutzi and Burford,
Chem. Rev., 1999, 99, 969 and references therein). In most main group
metallocenes, even those that are σ-bonded in the solid state, the rings
rotate rapidly through a series of 1,2-shifts.
A Cp group can be considered to
be p-bonded to an element if the Hapticity:
element sits inside the cylinder
defined by the 5 carbon atoms of M M M
the Cp ligand. The center of mass
of the 5 C atoms is known as the
centroid.
η1-Cp η2-Cp η3-Cp
M M
M
A σ-bonded Cp can be identified η4-Cp η5-Cp ≡ η5-Cp
by the angle at the ipso carbon
M
and bond lengths should indicate
single bonds to the ipso carbon
and a localized diene structure for
the α and β carbon fragment. σ-Cp
, Chem 59-651 Cyclopentadienyl Ligand Basics
To understand the nature of the covalent bonding between a
cyclopentadienyl ligand and an element, one must examine the
important molecular orbitals of the ligand. Note that the orbital
interactions are generally more important for the elements in the p-block
(and d-block) because the elements of the s-block are almost purely
ionic.
The SALCs of the Cp ligand can
≡ interact with appropriate AOs on the
metal.
, Chem 59-651 Cyclopentadienyl Ligand Basics
The metrical parameters of the Cp substituent can also provide
significant information about the nature of the Cp-E interaction. In
particular, the groups attached to the carbon atoms of the Cp ring often
bend out of the plane of the ring and the amount and direction of the
bending can be used to determine whether the interaction is
predominantly covalent or ionic.
R R R R
For covalent compounds, the
direction of the bending is
For ionic compounds, the
variable and depends on the
substituents bend away from
size of the p orbitals on the R R
the metal because this
the metal. The π orbitals on
increases the amount of
the Cp ring distort to create
negative charge on the face
the most effective overlap.
of the ring adjacent to the
cation. Similar bending is
The nature of the bending can R R
observed if a positive charge
prove the nature of the Cp-E
is used instead of a cation in
bonding (Macdonald, JACS,
computational investigations.
1999, 121, 12113, and refs.).
B B
R R R R
ca.7° ca.11°
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