Metal complex types spectrochemical series

The nagging question remains—why do these excellent ligands fail to coordinate The most convenient molecular explanation would be of a steric nature however, H2O and NH3 molecules are substantially smaller than Br so that spatial restrictions are not involved. It is tempting to seek some model based on specificity of the axial sites on nickel, perhaps for donors prefering B-type metals however, quantitative studies 12) show that the spectrochemical series for the X groups in Ni(CT)X2 is normal (CN- (> CT) > NCS > ONO" > F" > N3- > NO3- Cl > Br > I ). It therefore becomes necessary to seek the cause in factors external to the complexes in question. That such factors are readily recognized will be evident shortly. 

Since the heavier transition metals prodnce larger splitting of the d orbitals than their 3d congeners [i.e., they occupy systematically higher positions in the spectrochemical series of the metals, (1)] and also present lower pairing energies, the low-spin states are favored for 4d and 5d metals, specially when combined with high oxidation states. Hence, well-characterized low-spin tetrahedral d, d", and d complexes of the type [MR4] are known for Re(IV), Ru(IV), Os(IV), Ir(V), and Ir(IV). 

All of these criteria more-or-less follow simple common sense ideas about the characteristics of the two types of transition. Together, they enable distinctions to be drawn in most cases. Although the detailed study of the bands is in its infancy, it is interesting to note that f -> d transition energies seem to follow a spectrochemical series, just as do d -> d in the transition metal ions. So, the following 5f-> 6d transition energies, all xl0 cm" have been reported for complexes of U " and other similar ions ... 

---