Now that you have seen the demonstration of Co⁺² heated with chloride, bromide and iodide, the following questions will be answered:
1.  Why is heat needed to obtain colors?
2.  Why different colors for chloride, bromide, and iodide?
3.  Why does anion concentration affect color?

Thermochromism refers to color change by means of heat.  Explanations require Crystal field theory--although a bit strange, the essentials were covered in 2045C.

Electron absorbs energy and is elevated to higher energy level. When the electron returns to original state, the absorbed energy is released in the form of electromagnetic radiation.

Wavelengths in the range of 4000-7000A are visible; the higher energy in violet/blue region (4000A) while lower energy in red region (7000A).
 

Coordination Compounds
Consider: Y  +  :X ® Y:X

:X provides electron pair and forms coordinate covalent bond with Y. The entity supplying electron pair is called Ligand.  Coordination compounds are distinct chemical species--properties and behavior are different from the metal atom/ion and ligands from which they are composed.  The coordination number is the number of ligands bonded to the central metal atom/ion.

Co⁺² can form coordinate covalent bonds with different ligands such as  Cl^-1  Br^-1  I^-1  H₂O
     ^-1Cl:   Co⁺²   :Cl^-1  ®  Cl:Co:Cl   

Co⁺² generally bonds with 2, 4, or 6 ligands. The following cobalt chlorides exist:

Octahedral Complex
According to crystal field theory, Co⁺² and its ligands () exhibit electrostatic attractions along X,Y,Z axes. However, electrons in d-orbitals of Co⁺² are repelled by negative charges of the ligands and therefore experience increase in potential energy. The degree of repulsion between ligands and d-orbital electrons depend on the orientation of the d-orbitals. d-Orbitals with lobes directed between ligands feel weaker repulsion while d-orbitals with lobes directed at ligands feel stronger repulsion.  For octahedral arrangement, the 6 ligands split d-orbitals into 3 lower energy levels (dxy/dyz/dxz) and 2 higher energy levels (dx²-y²/dz²).  This is called Crystal Field Splitting.


 

Square Planar Complex
The Co⁺² square planar complex also undergoes crystal field splitting but since lobes of dx²-y² orbital point directly at the four ligands, this orbital has the greatest potential energy.

Different ligands cause different field splitting!
Relative Ligand Field Strength (see top page 862):
I^-1<Br^-1<Cl^-1<F^-1<H₂O<NH₃<ethyl diamine<CN^-1

When mixing Co⁺² with:
Cl^-1   (blue)    Greatest splitting
Br^-1  (blue/green)  Moderate splitting
I^-1    (green®yellow)  Least splitting

Answers to Questions:
1.  Heat required to excite electrons between the split d-orbitals.
2.  Different ligands (anions) cause varying degrees of crystal field splitting.
3.  Ligand concentration determines complex formed (CoCl₂/CoCl₄/CoCl₆)

Internet References: