A d4 ion refers to an ion that has four electrons in its d orbital, which is one of the five orbitals found in the outermost energy level (or shell) of an atom. These ions typically belong to the transition metal elements in the periodic table.
Crystal field terms refer to the different energy levels that are created in a transition metal ion when it is placed in a crystal lattice (a repeating pattern of atoms or molecules). These energy levels are a result of the interactions between the electrons in the d orbital and the surrounding ligands (molecules or ions attached to the central metal ion).
The Crystal field terms affect the electronic structure and properties of a d4 ion by splitting the d orbital into different energy levels. This splitting leads to changes in the color, magnetic properties, and reactivity of the ion.
The five Crystal field terms for a d4 ion are:
- Tetrahedral: when the surrounding ligands are arranged in a tetrahedral shape, the d orbital is split into two energy levels
- Square planar: when the ligands are arranged in a square planar shape, the d orbital is split into three energy levels
- Trigonal bipyramidal: with this shape, the d orbital is split into four energy levels
- Octahedral: when the ligands are arranged in an octahedral shape, the d orbital is split into five energy levels
- Linear: this shape results in the d orbital being split into six energy levels.
The Crystal field term for a d4 ion can be determined by looking at the geometry of the surrounding ligands. Depending on the shape of the ligands, the d orbital will split into a certain number of energy levels, as described in the previous question. The term with the lowest energy will be the dominant one for that particular ion.