C. The Stereochemical Properties of Metal Tris Chelates
1. Two definitions for Chiral Molecules
We have two definitions to determine if molecule is chiral (handed):
We can use one definition or the other (or both!)
a. A molecule is chiral if it has a non-superimposable mirror image. Two non-superimposable mirror image isomers are called enantiomers.
b. A molecule is chiral if the ONLY symmetry operations (and elements) of the molecule are PROPER ROTATIONS (other than E, the identity).
There are also several ways to rephrase this rule for chirality. So for example you can say that a molecule is chiral if it has NO IMPROPER ROTATIONS. Since a
reflection plane (σ plane) can also be called an S1 operation, and an inversion operation can also be referred to as an S2 operation, this exclusion eliminates
all types of symmetry operations other than proper rotations.
So by definition b:
Molecules with C3 Symmetry (having only the E, C3 and C32 operations) are chiral
and Molecules with D3 Symmetry (having only the E, C3 and C32 and 3 C2 operations) are chiral
Many Metal Tris Chelates have either C3 or D3 Symmetry and are therefore chiral
On the next few pages we illustrate the stereochemical properties and show you how to assign the absolute
configuration to enantiomers of Metal Tris Chelates.
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