Transition metals spectrochemical series

Ligands coordinated to transition metal ions may also be arranged in order of increasing A. This order is called the spectrochemical series, reflecting colour variations in chemical compounds of individual cations with different ligands. Thus, for Cr3+ and Co3+ cations in octahedral coordination with different ligands, the order of increasing A0 is... 

Transition metal ions with partially filled d orbitals will show the expected d-d spectra which will depend on the degree of perturbation of these orbitals by the coordinating ligand. It has been well established that coordination through sulfur causes less perturbation than coordination through the nitrogen of the thiocyanate ion, or —SON occurs lower in the spectrochemical series than — NCS (see, e.g.. Ref. 664), and a similar distinction may be made between —SeCN and —NCSe (663). 

The position of acac ion in the spectrochemical series involves weaker metal ligand jT-bonding (close to water) than the relatively high jt-jt transition energies would suggest. This assessment is based on Hiickel s LCAO theory. In MO levels of the acac ion the jt3 orbital is occupied and the 7t4 unoccupied antibonding MO is energetically suitable for overlap with the metal rf-orbital. The reason is that both donor-acceptor... 

---