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),