Transition-group complexes, forbidden transitions

In the transition group complexes containing a partly filled d shell, it is possible that a reminiscent Gillespie-effect contributes to the intensity of Laporte-forbidden transitions. Thus, the absorption band of Co(NH3)6 has roughly the same oscillator strength as -that of Co(NH3)5C1+2, and there is some evidence (54) that Cu is above the plane of the four ammonia molecules, not only in the tetragonal-pyramidic Cu(NH3)5+2, but also in an instantaneous picture in Cu(NH3)4 2 which would constitute essentially a Gillespie effect. 

In the 4d and 5d groups, the first spin-forbidden transition to a3Tig has a rather high intensity, some 2% and 15%, respectively, of the spin-allowed ones. It has even been observed in cobalt (III) complexes and would be expected at... 

Dipole transitions magnetically allowed and electrically forbidden are, e.g., the nn transitions of the carbonyl group in aldehydes or ketones or the d-d transition of transition metal complexes. For an allowed magnetic dipole transition, is 10 (cgs). For a forbidden electric dipole... 

In the point-group Td, p-orbitals transform in the same way as dxy, dyt, and dtx, owing to the absence of a center of symmetry. This has the important consequence that all six orbitals are mixed together and the electronic transitions have some d-p character they are therefore not forbidden and have higher intensities than the nearly pure d-d transitions of octahedral complexes. 

The d-d bands are usually relatively low in intensity compared to CT bands contrast the palish hues of familiar salts of Cr, Mn, Fe, Co, Ni, and Cu with the intense purple of Mn04. This can be explained within the CF model. The probability of a transition is governed by selection rules see Group Theory). In atomic spectra, transitions between states having the same / quantum number are forbidden if this rule were strictly obeyed, d-d transitions should not be observable. Moreover, if there is a center of symmetry, as in an octahedral or square coplanar complex, d-d transitions are forbidden although they can be observed, albeit relatively weakly. Molecular vibration can disturb the center of synunetry, and Vibronic Coupling lends intensity to d d absorption. A tetrahedral complex has no center of synunetry and the... 

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