The d x 2-y 2 orbital is similarly repelled by the octahedral ligand field. That repulsive interaction will raise the energy of the atomic d z 2 orbital above that of the d xy, d yz, and d xz orbitals. For example, electrons in the atomic d z 2 orbital will repel localized electrons on the coordinating waters positioned along the positive and negative z-axis of the octahedral ligand field. The repulsive energy of interaction between the electrons in the water molecules will be different for different d-orbitals. In the case of the hexaaquacobalt(II) complex, the water molecules form an octahedral ligand field. Second, there is the repulsive interaction of the donated electron pairs of the ligands and the electrons of the metal ion.Ĭomplex ion formation and the spectrochemical seriesĮach surrounding ligand field will have a special geometry. That favorable interaction lowers the energies of the 3d orbitals. First, there is the attractive interaction of the charge and/or dipole moment of the coordinating ligand and the metal ion. The interaction between the coordinating ligands and the central metal ion can be understood in terms of two competing interactions. Hybrid atomic orbitals to accept the electron density donated by the coordinating water molecules. As such, the cobalt ion acts as a Lewis acid and the water molecule as a Lewis base. In that model, a lone pair of electrons on the water molecule are donated to an atomic orbital on the cobalt ion. The interaction of the water molecules with the cobalt ion can be understood using a localized electron model. Appealing to the rules of Valence Shell Electron Pair Repulsion Theory, we expect the water molecules to be arranged in an octahedral geometry. 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. That leads to an electron configuration for Co 2+ ofįor the isolated ion, the five 3d-orbitals will have identical energies. We can apply our rule of thumb that the ions of the first row transition metals have no 4s electrons. We can determine the electron configuration of the ion alone. The place to start is the central transition metal ion. 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. We would like to develop the same level of understanding of coordination compounds. 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. 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. 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. We have used quantum mechanical models to study the one electron atom, multielectron atoms, diatomic molecules, and polyatomic molecules. The quantum mechanics of complex ion formation The result is a solution with both pink and blue complex ions present, and a lavender color. 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 addition of chloride ion pushes the equilibrium to the right favoring the formation of the blue tetrachloro cobalt species product.Īddition of water to the solution pushes the equilibrium to the left favoring the pink hexaaqua cobalt species reactant. 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 drives the formation of the tetrachloro cobalt(II) complex ion. The addition of hydrochloric acid to the solution provides a high concentration of chloride ion. The color is due to the presence of the pink hexaaqua cobalt(II) complex ion. Add water to the solution and observe transformation in color of solution.Ī solution of cobalt(II)chloride is a lovely pink color. Add hydrochloric acid to the solution and observe change in color.ģ. Prepare a solution of cobalt chloride in 98% ethyl alcohol.Ģ. Ingredients: cobalt chloride, hydrochloric acid, waterġ. A light pink solution turns blue, and then turns pinkish-blue.
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