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Experiment 3: Exploring Optical Isomers and Crystal Field Theory Using Molecular Modeling 1. Goals Build use molecular modeling various coordination...
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Experiment 3: Exploring Optical Isomers and Crystal Field Theory Using Molecular Modeling:






BEGIN HERE: Answer As Best You Can For the Following Questions.

(en) = Ethylenediammine





Experiment 3: Exploring Optical Isomers and Crystal Field Theory Using
Molecular Modeling
1. Goals
Build use molecular modeling various coordination compounds. Use the computations
environment to explore the concept of non-superimposable mirror images (optical
isomers). Calculate the Visualize the shapes of complex ions predicted by VSEPR
( valence shell electron pair repulsion). Explain how the colors of transition metal
complexes arise.
2. Background
2.1 Ligands and isomers
The Lewis bases in coordination compounds may be molecules, anions, or (rarely) cations. These
ligands have donor atoms that provide the electron pair that forms the coordinate covalent bond.
Ligands that can bond to a metal through only one donor atom at a time are monodentate or
unidentate (Latin, dent: tooth). Ligands that can bond simultaneously through more than one
donor atom are polydentate (Figure 1). Polydentates that bound through two, three, four, five, six
donor atoms are called bidentate, tridentate, tetradentate, pentadentate, and hexadentate,
cyanide ion
hydroxide lon
C :
thiocyanate lon
nitrite ion
ethylenediamine (en)
oxalate ion
C- H2C
:N- CH2 -CH2-N:
:0- C- Hac
ethylenediaminetetracetate (EDTA) lon
Figure 1. Common ligands found in coordination compounds.
As we have seen the structures of coordination compounds are governed largely by the
coordination number of the metal. Most can be predicted by VSEPR theory. The non-bonding
pairs of electrons in d-orbitals usually have only small influences on the geometry because they
are not in the outer shell. Transition metal complexes with coordination numbers as high as
seven, eight and nine are known, but they are very rare. Complexes with a coordination number
of 4 can have both tetrahedrond square planar geometries. In experiment one we look at
diamminedichloroplatinum( 10) which is a square planar complex ion Octahedral complexes like
the three cobalt complexes in experiments and and two are combination number h
We have alen seen that these square planar and octahedral geometries allow for the possibility of
isomers. Where isers are substances that have the same number and kinds of atoms but the
atoms are arranged differently. ('is-tetraamminedichlorecobalt(III) and trans-
tetraaminedichlorocobalt( II) are geometric (positional) isomers More specifically, these two
complex ions are stereoisomers. The not only have the same atoms but the same atom-to-atom
central atom.
bonding sequences, but they differ only in the spatial arrangements of the atoms relative to the
On the other hand (pun intended) optical isomers are geometric isomers that exist in two forms
that bear the same relationship to each other as you left and right hands do. They are non-
superimposable mirror images of each other. Optical isomers interact with polarized light in
different ways. One optical isomer (D or dextrorotary) rotate the polarized light to the right. The
other is levorotary (L), rotates plane polarized light to the left. Figure 2 shows cis and trans
[Co(en)2Cl2]'. Only the cis geometric isomer has an optical isomer.
cIs form (optical isomers)
trans form
.. CI
CHE-NH20.. 1.. NH2 - CH2
NH2- CH2
NH 2
Figure 2. Three isomeric forms of [Co(en)2Clz]' exist. The trans isomer, formed when the chlorines are
positioned at a 180" angle, has very different properties from the cis isomers. The mirror images of the cis
isomer form a pair of optical isomers, which have identical behavior except when reacting with other
enantiomers. https://courses.
Alfred Werner was the first person to demonstrate optical activity, the rotation of plane polarized
light, in coordination compounds. Tris(ethylenediamine)cobalt(III) is such a compound. It has
three identical ethylenediammine ligands all connected to the
central cobalt(III) ion. The nonbonding electrons on the nitrogen
atoms act as the Lewis bases in the complexes. Acetylacetonate
(acac) is also a bidentate ligand. Next week we will explore the
synthesis of two metal complexes with acetylacetonate. In the case
of acac, it uses its two oxygen atoms to form the coordinate covalent
. Figure 3. Acetylacetonate
2.2 Crystal Field Theory
Hans Bethe and J. H. van Vleck developed crystal field theory between 1919 and the early
1930's. In coordination number six complexes like the ones we are and have been exploring, the
geometry is octahedral. The dx2-y2, and dz2 orbitals are directed along the set of mutually
perpendicular x, y, and z axes. The orbitals are called the eg orbitals. The dxy, dxz, and dyz obitals
are collectively known as the tag orbitals and they lie in between the axes. According to VSEPR
theory the ligand donor atoms approach the metal ion along the axes. There are greater
repulsions between the ligand electrons and the metal ion electrons in the eg orbitals than with
the metal electrons in the tag orbitals. Crystal field theory proposes that the approach of the six
donor atoms along the axes sets up an electric field. This removes the degeneracy of the d-
With bound ligands
orbitals and splits them into two sets,
the lower energy be and the higher
energy co. The energy separation
between the two sets is as we say the
Free metal ion
last couple of weeks the origin of the
sometimes intense color that
octahedral coordination compounds
can have.
Here we are going to explore two
different metals (Al and C'o) as wells
Figure 3. Energies of the d-orbitals in an octahedral field.
different ligands. What we want is to
gain a better understanding of optical
isomers and crystal field theory.
3. Procedure
3.1 Exploring optical isomers of coordination compounds
There is a short video under files on Canvas, "Building tris(ethylenediammine)cobalt(III)".
a. [Co(en)3]3+
Tris(ethylenediammine)cobalt(III) has two optical isomer or non-superimposable mirror
images. Build both of them side-by-side in Spartan and verify that they are different. You
cannot simply rotate one in any direction and get the other.
Are there any optical isomers of the dichlorobis (ethylenediamine)cobalt(III) complex ion?
c. What about if we use monodentate ligands; can you build optical isomers of complexes with a
coordination number of 4? Try, using only monodentate ligands, to build an octahedral
coordination compound.
3.2 Orbitals and energies of metals and coordination complexes
a. Co, Cost, Al and Al3+
1. In your notebook draw the orbital diagram for the neutral cobalt atom. You need
only include the 4s and 3d orbitals. Determine the number of unpaired electrons.
2. Place a cobalt atom in the build screen. It doesn't matter what the bond valence is
(tetrahedronahedral etc.) we are going to erase the bond.
3. Click on the eraser and remove any bonds.
4. Now go to Setup and then Calculations.
5. Change the Calculate drop down to read Energy.
6. Make sure the with drop down has Density Functional selected.
7. In the drop down next to that on select B3LYP.
8. Now change the unpaired electrons to match what you determined in step I.
9. If you are using Spartan Student Version 7. don't forget to check Orbitals and
10. Submit your calculations.
1 1. When the calculations are complete bring up Orbital Energies and click on the
energy levels. For the Energy levels HOMO and below click on the lines
representing them and record their energies and whether they are s, p. or d
12. Repeat what you did for Co for Co3*, Al and All*. Don't forget to change the
charge and the number of unpaired electrons where appropriate. Remember we
are focusing on the HOMO and energy levels below it.
b. Co(OHz)63* and AI(OH2)<3+
1. Build these two hexaaqua complex ions.
2. Repeat the process you did in part a.
3. Take care to identify any energy levels that show d-orbitals pointing towards the
ligands. If you can't find any d-orbitals look for p-orbitals instead. Remember we
are focusing on the HOMO and energy levels below it.
c. [Co(OH2)4(acac)|2 + and [AI(OH2)4(acac)|2+
1. Build these two complex ions.
2. Repeat the process you did in part b. Remember we are focusing on the HOMO
and energy levels below it.
3. Of the d-orbitals or p-orbitals you found in this lower energy group identify the
one with the highest energy.
4. Now, look at the LUMO energy levels. Find the lowest energy level that is up in
the group at or near the LUMO that has d-orbital or p-orbital character.
X Worksheet: Exploring Optical Isomers and Crystal Field Theory Using
Molecular Modeling
1. Exploring optical isomers of coordination compounds
Draw or copy-paste from Spartan any optical isomers for the coordination complexes
a. [Co(en);]'
b. [CoCl(eny]"
c. Using monodentate ligands
2. Orbitals and energies of metals and coordination complexes
Make a list of the orbital types and energies you found for each metal or complex ion
a. Co
b. Al
c. Co(OH2)63+
d. [Co(OH2)4(acac)]2 +

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