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Lecture

CHEM 212 Lecture Notes - Delta Bond, Coordination Number, Pi Bond


Department
Chemistry
Course Code
CHEM 212
Professor
Richard Oakley

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1-1
If organic chemistry is defined as the chemistry of hydrocarbon compounds and their
WHAT IS
derivatives, inorganic chemistry can be described broadly as the chemistry of "every-
1
NORCAN lC
thing else." This includes all the remaining elements in the periodic table, as well as car-
CHEMISTRY?
bon, which plays a major role in many inorganic compounds. Organometallic
chemistry, a very large and rapidly growing field, bridges both areas by considering
compounds containing direct metal-carbon bonds, and includes catalysis of many or-
ganic reactions. Bioinorganic chemistry bridges biochemistry and inorganic chemistry,
and environmental chemistry includes the study of both inorganic and organic com-
pounds. As can be imagined, the inorganic realm is extremely broad, providing essen-
tially limitless areas for investigation.
1-2
Some comparisons between organic and inorganic compounds are in order. In both
CONTRASTS
areas, single, double, and triple covalent bonds are found, as shown in Figure 1-1; for
WITH ORGANIC
inorganic compounds, these include direct metal-metal bonds and metal-carbon bonds.
CHEMISTRY
However, although the maximum number of bonds between two carbon atoms is three,
there are many compounds containing quadruple bonds between metal atoms. In
addition to the sigma and pi bonds common in organic chemistry, quadruply bonded
metal atoms contain a delta
(6)
bond (Figure
1-2);
a combination of one sigma bond,
two pi bonds, and one delta bond makes up the quadruple bond. The delta bond is
possible in these cases because metal atoms have
d
orbitals to use in bonding, whereas
carbon has only
s
and
p
orbitals available.
In organic compounds, hydrogen is nearly always bonded to a single carbon. In
inorganic compounds, especially of the Group 13 (IIIA) elements, hydrogen is fre-
quently encountered as a bridging atom between two or more other atoms. Bridging hy-
drogen atoms can also occur in metal cluster compounds. In these clusters, hydrogen
atoms form bridges across edges or faces of polyhedra of metal atoms. Alkyl groups
may also act as bridges in inorganic compounds, a function rarely encountered in or-
ganic chemistry (except in reaction intermediates). Examples of terminal and bridging
hydrogen atoms and alkyl groups in inorganic compounds are shown in Figure 1-3.
1

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2
Chapter
1
Introduction to Inorganic Chemistry
Organic Inorganic Organometallic
0
,cO
I
,I,
OC-Mn- CH,
FIGURE
1-1
Single and Multiple Bonds in Organic and Inorganic Molecules
FIGURE
1-2
Examples of
Bonditig Interactions.
Some of the most striking differences between the chemistry of carbon and that of
many other elements are in coordination number and geometry. Although carbon is usu-
ally limited to a maximum coordination number of four (a maximum of four atoms
bonded to carbon, as in
CH4),
inorganic compounds having coordination numbers of
five, six, seven, and more are very common; the most common coordination geometry is
an octahedral arrangement around a central atom, as shown for [T~F~],- in Figure
1-4.

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1-2
Contrasts with Organic Chemistry
3
FIGURE
1-3
Examples of
Inorganic Compounds Containing
Terminal and Bridging Hydrogens
and AIkyl Groups.
FIGURE
1-4
Examples of
Geometries of Inorganic
Compounds.
Each CH3 bridges a face
of the Li4 tetrahedron
B,,H,;-
(not shown: one
hydrogen on each boron)
Furthermore, inorganic compounds present coordination geometries different from
those found for carbon. For example, although 4-coordinate carbon is nearly always
tetrahedral, both tetrahedral and square planar shapes occur for 4-coordinate compounds
of both metals and nonmetals. When metals are the central atoms, with anions or neutral
molecules bonded to them (frequently through
N,
0,
or
S),
these are called coordination
complexes; when carbon is the element directly bonded to metal atoms or ions, they are
called organometallic compounds.
The tetrahedral geometry usually found in 4-coordinate compounds of carbon
also occurs in a different form in some inorganic molecules. Methane contains four hy-
drogens in a regular tetrahedron around carbon. Elemental phosphorus is tetratomic
(P4)
and also is tetrahedral, but with no central atom. Examples of some of the geome-
tries found for inorganic compounds are shown in Figure 1-4.
Aromatic rings are common
in
organic chemistry, and aryl groups can also form
sigma bonds to metals. However, aromatic rings can also bond to metals in a dramati-
cally different fashion using their pi orbitals, as shown in Figure
1-5.
The result is a
metal atom bonded above the center of the ring, almost as if suspended in space. In
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