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kkkjkjklkbkbj,bjhbnhj kjepare a solution of cobalt chloride in 98% ethyl alcohol.
2. Add hydrochloric acid to the solution and observe change in color.
3. Add water to the solution and observe transformation in color of solution.
Understanding: A solution of cobalt(II)chloride is a lovely pink color. The color is due to the presence of the pink hexaaqua cobalt(II) complex ion.
Co2+ + 6 H2O → [Co(H2O)6]2+
The addition of hydrochloric acid to the solution provides a high concentration of chloride ion. The addition of chloride ion drives the formation of the tetrachloro cobalt(II) complex ion.
[Co(H2O)6]2+ + 4 Cl- → [CoCl4]2- + 6 H2O
Using our understanding of LeChatelier's Principle, we can push this equilibrium to the right, by the addition of more reactant, or the left, by the addition of more product.
The addition of chloride ion pushes the equilibrium to the right favoring the formation of the blue tetrachloro cobalt species product. Addition of water to the solution pushes the equilibrium to the left favoring the pink hexaaqua cobalt species reactant.
If we add a sufficient amount of water to the blue solution dominated by the blue tetrachloro cobalt(II) complex ion, we can convert some of the tetrachloro cobalt(II) complex ion to hexaaqua cobalt(II) complex ion. The result is a solution with both pink and blue complex ions present, and a lavender color.
The quantum mechanics of complex ion formation
We have used quantum mechanical models to study the one electron atom, multielectron atoms, diatomic molecules, and polyatomic molecules. In each case, we have determined the discrete allowed energies of the system, and the one-electron wave functions or orbitals corresponding to each of the allowed energies. Using the Aufbau Principle, Pauli Principle, and Hund's Rule, we have were able to build the lowest energy ground state electron configurations for those systems. Through the electron configuration, we are able to develop a fundamental quantum mechanical understanding of the structure, ionization energy, bond strengths, magnetism, and spectroscopic properties of the atom or molecule. Quite powerful!
We would like to develop the same level of understanding of coordination compounds. We will find that a few simple rules provide us with a powerful means of understanding the detailed structure, magnetism, and spectroscopic properties of transition metal complexes.
The place to start is the central transition metal ion. We can determine the electron configuration of the ion alone. We can apply our rule of thumb that the ions of the first row transition metals have no 4s electrons. That leads to an electron configuration for Co2+ of
For the isolated ion, the five 3d-orbitals will have identical energies. However, when the ion is surrounded by the coordinating ligands, things change! In the case of the hexaaquacobalt(II) complex ion, the central cobalt ion is surrounded by six coordinating water molecules. Appealing to the rules of Valence Shell Electron Pair Repulsion Theory, we expect the water molecules to be arranged in an octahedral geometry. And they are!
The interaction of the water molecules with the cobalt ion can be understood using a localized electron model. In that model, a lone pair of electrons on the water molecule are donated to an atomic orbital on the cobalt ion. As such, the cobalt ion acts as…