Transition Metal Ion Colors

Synonyms

d-Block elements

Definition

Transition metals are d-block elements with partially filled 3d, 4d, and 5d orbitals.

Color Production

The transition metals are (somewhat imprecisely) described as being colored because their compounds frequently take on a characteristic hue (Table 1). For example, Ni²⁺ cations tend to exhibit green coloration, while Cu²⁺ tends to show green-blue tones. Cobalt, as Co²⁺, has been used since medieval times to color glass deep blue, and small amounts of impurity Cr³⁺ in colorless Al₂O₃ produce the color in ruby gemstones (Fig. 1). All these colors arise from electronic transitions between the ionic ground state and energy levels lying between 1.77 and 3.10 eV above it, giving absorption maxima in the visible wavelength range (400–700 nm). These low-lying energy levels arise from interactions of the d orbitals on the cation with neighboring atoms in a material and are a function of the symmetry of the surroundings [1,2,3]. The energy levels that occur...