Crystal Field Theory, d-Orbital Splitting and Colour

Calculate oxidation state. Write the d configuration of the metal ion. Identify the geometry as octahedral or tetrahedral. Draw the correct splitting order. Fill electrons according to ligand field strength and pairing energy. Use the arrangement to explain colour or magnetism.

A d-orbital splitting diagram is strongly recommended, showing octahedral t2g and eg levels, tetrahedral e and t2 levels, and electron promotion for colour.

A common wrong answer is reversing the octahedral levels by placing eg lower than t2g, which leads to incorrect electron filling and wrong magnetic predictions.

Crystal Field Theory explains coordination compounds by considering electrostatic interaction between metal d orbitals and ligands, causing the five d orbitals to split into sets of different energy in a ligand field.

Crystal Field Theory explains coordination compounds by considering electrostatic interaction between metal d orbitals and ligands, causing the five d orbitals to split into sets of different energy in a ligand field. Octahedral splitting: t2g lower and eg higher, separated by Delta_o. Tetrahedral splitting: e lower and t2 higher, separated by Delta_t. Approximate relation: Delta_t is about 4/9 of Delta_o for similar metal and ligands. [Ti(H2O)6]3+ is coloured because Ti₃⁺ has a d1 configuration. The d electron can absorb light and move from lower t2g to higher eg level in the octahedral field, so the transmitted or reflected light appears coloured.

In an octahedral field, ligands approach along the axes, so the dz2 and dx2-y2 orbitals experience greater repulsion and form the higher-energy eg set, while dxy, dxz, and dyz form the lower-energy t2g set. The energy gap is called crystal field splitting energy, written as Delta_o. In a tetrahedral field, the splitting pattern is reversed and the gap Delta_t is smaller than Delta_o. Electron arrangement depends on splitting energy and pairing energy, producing high-spin or low-spin complexes. Colour often arises when electrons absorb visible light and undergo d-d transitions between split d levels. Octahedral splitting: t2g lower and eg higher, separated by Delta_o. Tetrahedral splitting: e lower and t2 higher, separated by Delta_t. Approximate relation: Delta_t is about 4/9 of Delta_o for similar metal and ligands. Explain whether [Ti(H2O)6]3+ can show colour. H₂O is neutral, so Ti is +3. Titanium has atomic number 22, and Ti₃⁺ is d1. In an octahedral field, the one d electron occupies t2g and can absorb energy to move to eg. This d-d transition makes the complex coloured. Questions ask students to draw splitting diagrams, compare octahedral and tetrahedral splitting, explain colour, predict high-spin or low-spin behaviour, or relate unpaired electrons to magnetic properties. A common wrong answer is reversing the octahedral levels by placing eg lower than t2g, which leads to incorrect electron filling and wrong magnetic predictions.