Download TRANSITION METAL CHEMISTRY –PART 3 –class notes

TRANSITION METAL CHEMISTRY –PART 3 –class notes
Crystal Field Theory is a model that helps explain why some complexes are high spin and some are
low spin.
Crystal Field Theory views bonding in complexes as the result of electrostatic interactions and
considers the effect of ligand charges on energies of metal ion d orbitals.
d-electron to ligand-electron repulsions affect d-orbital energy levels.
dz2 and dx2 – y2 orbitals point at the ligands and have higher energies than other d-electrons.
This energy gap (_) is called crystal field splitting.
1. The crystal field splitting energy ( ) corresponds to light wavelengths in the visible region of the
spectrum. Remember
= Ephoton and
The color of any substance is related to the wavelength(s) of light absorbed by the substance.
2. Crystal field splitting energy depends on the ligand’s nature.
For Ni 2+ complexes increases as the ligand changes from H2O to NH3 to ethylenediamine (en).
In general, the crystal field splitting energy increases as the ligand varies in the following order.
I– < Br– < Cl– < OH– < F– < H2O < NH3 < en < CN– < CO
This is known as the spectrochemical series.
3. Ligands which lead to small values of are called weak-field ligands. Those which lead to large
values of are called strong-field ligands.
4. Weak-field ligands lead to high-spin paramagnetic systems. Strong-field ligands lead to low-spin and
sometimes diamagnetic systems.
Octahedral complexes of d4 through d7 metal ions
Color
When atoms or compounds absorb light of the proper frequency, their electrons are excited to higher energy
levels. Colored compounds absorb visible (colored) light and this absorption is responsible for their color.
An object is seen as black if it absorbs all colors of white light. A white object reflects all colors of white light
equally.
If an object absorbs all colors but one, we see the color it does not absorb. The yellow strip in the following
figure absorbs red, orange, green, blue, indigo and violet light. It reflects yellow light and we see it as yellow.
The eye also uses complementary colors in color vision. When a color is removed from white light we see the
complementary color. The following table shows the colors seen when a complementary color is removed.
The yellow strip in the following figure looks yellow because it absorbs indigo light from white light. Indigo is
the complementary color of yellow.
A solution containing the complex ion, [Cu(NH3)4]2+, is blue because the complex absorbs red and orange light,
the complementary colors of blue and blue-green as shown in the color wheel below:
EXERCISE:
1.
9
3.
4.
5.
Which of the following species could exist as a pair of isomers?
A)[Pt(NH3)3Cl]+ B) [Co(H2O)4Cl2]+ C) [Pt(en)Cl2] D) [Pt(NH3)Br3]- E) [Fe(CN) 6]3-
2. Which of the following ligands is most likely to form a high spin octahedral complex with cobalt(II)?
A) CNB) NO2C) I- D) CO E) en
3. In the complex ion ML6n+, Mn+ has four d electrons and L is a weak field ligand.
According to crystal field theory the magnetic properties of this complex ion correspond
to the presence of how many unpaired electrons?
A) 0 B) 1 C) 2 D) 3 E) 4
4. Which of the following octahedral complexes should have the largest crystal field splitting energy?
A) [Cr(NH3]3+ B) [CrF6]3- C) [Cr(H2O)]3+ D) [Cr(CN)6]3- E) [CrCl6]35. CN- is a strong field ligand, whereas Cl- is usually a weak field ligand. Which of the following octahedral
complexes has no unpaired electrons?
A) [NiCl6]4- B) [Zn(CN)6]4- C) [FeF6]4- D) [CuCl6]46. How many unpaired electrons are there in the complex ion [Cr(NH3) 6]3+?
7. If a complex ion absorbs blue light, what color is the complex ion?
8. Which of the following ions is least likely to form colored complex ions?
A) Zn2+ B) Mn2+ C) Fe3+ D) Cu2+ E) Cr3+
9. [Ti(H2O)6]3+ ion absorbs light at a wavelength of 498 nm. a) Calculate the crystal field splitting energy
b) Determine the color of the complex.
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