page_title: Transition metal
section_title: Coloured compounds
hierarchical_section_title: Transition metal / Characteristic properties / Coloured compounds
caption_attribution_description: English: From left to right, aqueous solutions of: cobalt(II) nitrate, Co(NO3)2 (red); potassium dichromate, K2Cr2O7 (orange); potassium chromate, K2CrO4 (yellow); nickel(II) chloride, NiCl2 (green); copper(II) sulfate, CuSO4 (blue); potassium permanganate, KMnO4 (violet). Français&#160;: De gauche à droite, des solutions aqueuses de&#160;: nitrate de cobalt(II), Co(NO3)2 (rouge); dichromate de potassium, K2Cr2O7 (orange); chromate de potassium, K2CrO4 (jaune); chlorure de nickel(II), NiCl2 (vert); sulfate de cuivre(II), CuSO4 (bleu); permanganate de potassium, KMnO4 (violet).
context_page_description: In chemistry, the term transition metal has three possible definitions:
  The IUPAC definition defines a transition metal as "an element whose atom has a partially filled d sub-shell, or which can give rise to cations with an incomplete d sub-shell".
  Many scientists describe a "transition metal" as any element in the d-block of the periodic table, which includes groups 3 to 12 on the periodic table. In actual practice, the f-block lanthanide and actinide series are also considered transition metals and are called "inner transition metals".
  Cotton and Wilkinson expand the brief IUPAC definition by specifying which elements are included. As well as the elements of groups 4 to 11, they add scandium and yttrium in group 3, which have a partially filled d sub-shell in the metallic state. Lanthanum and actinium in group 3 are, however, classified as lanthanides and actinides respectively.
  English chemist Charles Bury first used the word transition in this context in 1921, when he referred to a transition series of elements during the change of an inner layer of electrons from a stable group of 8 to one of 18, or from 18 to 32. These elements are now known as the d-block.
context_section_description: Colour in transition-series metal compounds is generally due to electronic transitions of two principal types.
  charge transfer transitions. An electron may jump from a predominantly ligand orbital to a predominantly metal orbital, giving rise to a ligand-to-metal charge-transfer (LMCT) transition. These can most easily occur when the metal is in a high oxidation state. For example, the colour of chromate, dichromate and permanganate ions is due to LMCT transitions. Another example is that mercuric iodide, HgI₂, is red because of a LMCT transition.
  A metal-to-ligand charge transfer (MLCT) transition will be most likely when the metal is in a low oxidation state and the ligand is easily reduced.
  In general charge transfer transitions result in more intense colours than d-d transitions.
  d-d transitions. An electron jumps from one d-orbital to another. In complexes of the transition metals the d orbitals do not all have the same energy. The pattern of splitting of the d orbitals can be calculated using crystal field theory. The extent of the splitting depends on the particular metal, its oxidation state and the nature of the ligands. The actual energy levels are shown on Tanabe–Sugano diagrams.
  In centrosymmetric complexes, such as octahedral complexes, d-d transitions are forbidden by the Laporte rule and only occur because of vibronic coupling in which a molecular vibration occurs together with a d-d transition. Tetrahedral complexes have somewhat more intense colour because mixing d and p orbitals is possible when there is no centre of symmetry, so transitions are not pure d-d transitions. The molar absorptivity (ε) of bands caused by d-d transitions are relatively low, roughly in the range 5-500 M⁻¹cm⁻¹ (where M = mol dm⁻³). Some d-d transitions are spin forbidden. An example occurs in octahedral, high-spin complexes of manganese(II),
  which has a d⁵ configuration in which all five electron has parallel spins; the colour of such complexes is much weaker than in complexes with spin-allowed transitions. Many compounds of manganese(II) appear almost colourless. The spectrum of [Mn(H
  ₂O)
  ₆]²⁺
   shows a maximum molar absorptivity of about 0.04 M⁻¹cm⁻¹ in the visible spectrum.
caption: From left to right, aqueous solutions of: Co(NO
  3)
  2 (red); K
  2Cr
  2O
  7 (orange); K
  2CrO
  4 (yellow); NiCl
  2 (turquoise); CuSO
  4 (blue); KMnO
  4 (purple).
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