Crystal Field Theory
Crystal field theory helps explain properties of complexes that are formed between transition metal ions and ligands. Ligands are ions, such as Cl–, or molecules, such as H2O, that surround a metal ion in solution. The combination of the ligand and metal ion is the complex. There are seven different types of shapes that can be formed by the bond between the metal and ligand that will determine the arrangement of the d orbitals:
- Trigonal bipyramidal
- Square planar
- Square pyramidal
- Pentagonal bipyramidal
- Square antiprismatic
For the basics of descriptive inorganic chemistry, the first four arrangements are the ones you will want to understand. The ligands that are attached to the metal have an effect on how the orbitals are spread out. The stronger the ligand, the greater the gap between the orbitals. This will effect if the complex favors low spin or high spin for its electron placement. Low spin is where the electrons fill the lowest energy orbitals first and high spin is when electrons fill every orbital with one electron before continuing to fill the lowest energy electron. Because of difference in electron placement, it may result in the complex being diamagnetic (no magnetic properties) or paramagnetic (has magnetic properties) just from changing the ligand that is attached to the metal.
In addition to potentially changing the complex’s electron positioning, the increase in the gap between orbitals due to a stronger ligand increases the energy required to excite an electron in a lower orbital. Since the gap determines the energy required to excite the electron, it determines the frequency of light that is absorbed resulting in the color of the complex. The exception to this is complexes using metals in the d10 column (column 12 on the periodic table) as they have full d orbitals and nowhere for electrons to easily jump to.
Inorganic chemistry is a very broad area of chemistry that encompasses compounds without a carbon to hydrogen bond. In the descriptive aspect it has an intricate system of organizing compounds by their bonds and structures. While there are many theories and systems to know in order to understand the classification of these molecules, you can obtain plenty of data about the compound or complex by using that knowledge.