Chemistry - A Molecular Science

Chapter 14 Inorganic Chemistry

positive oxidation state and is only four-coordi

nate makes it a good Lewis acid. As such, it

is a good catalyst for the polymerization of alke

nes, which are weak Lewis bases. Again,

the activation energy for polymerizati

on is reduced by first weakening the

π

bond of the

alkene by coordination. As the ethene approaches the titanocene, the

π

electrons of the

double bond are shared with the titanium, and the double bond weakens. Upon coordination, the titanium adopts a

five-coordinate geometry, and the

original

Ti-C

H 2

(^5)
bond is weakened. As depicted by the curved arrows in Figure 14.16b, the
π
electrons on
the ethene move to form a Ti-C
bond. Simultaneously, the electrons that formed the σ
original Ti-C bond move to form a C-C
bond with the ethene (Cσ
H 2
) to produce a 4
coordinated C
H 4
group. The resulting structure is shown in Figure 14.16c. It should be 9
noted that the alkene is
inserted
into the original Ti-C bond

. After the insertion, the

titanocene reverts to a four-coordinate geometry

and reacts with another ethene molecule,

which inserts itself into the Ti-C bond in a similar manner to yield a coordinated C

H 6

(^13)
group. This process continues to cycle and, with each cycle, the length of the polymer chain increases by two CH
groups. As shown by the CH 2
-CH 2
in bold in Figure 14.16, 3
the initial organic group terminates the chai
n on the end away from the metal because
insertion is always into the Ti-C bond.
14.6
TRANSITION METALS AS ELEC
TRONIC AND MAGNETIC MATERIALS
Transition metal complexes are also useful in a variety of materials applications such as electronic and magnetic materials. The reactivity of transition metal complexes is important for both their catalytic and biological
function. However, the use of transition
metal coordination complexes for materials a
pplications requires systems in which the
ligands are strongly coordinated and thus l
ess reactive. Furthermore, to achieve such
properties as conductivity or magnetism, we must focus on transition metal complexes in their crystalline form. ELECTRONIC CONDUCTIVITY Consider the square planar ion Pt(CN)
2- 4
, which is colorless in aqueous solution. As
shown in Figure 14.17, th
e anions in crystalline K
[Pt(CN) 2
]·3H 4
O stack face-to-face to 2
form one-dimensional chains in the z-direction with Pt-Pt distances of 3.48 Å. The stacking of these square planar complexes aligns the z
2 orbitals of each platinum in the
chain so that they can overlap in a
σ-bonding fashion. Because the number of anions in a
Cl
Ti
CH
2
L L
CH
2
CH
2
Ti
L L
CH
3
Cl
HC^2
HC^3
Ti
L L
CH
Cl^2
H^2
C
CH
2
Ti
L L
Cl HC 2
HC^2
CH
2
HC^3
HC^3
Ti
L L
CH
Cl^2
HC^2
CH
2
HC^2
CH
2
H^3
C
a)
b)
c)
d)
e)
CH
2
CH
2
CH
2
CH
2

...
Figure 14.16 Polymerization of ethene to polyethylene by a titanium catalyst NC

CNCN

NC

CN

NCNC

CN CN

NC

CN

NC

3.48 A

o^

Figure 14.17 Linear chains for

med from the face-to-face

stacking of [Pt(CN)

2-] 4

ions

Note that the [Pt(CN)

2-] 4

units are staggered so that the cyanides of

adjacent units are not directly under one another.

© by

North

Carolina

State

University