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When the stability constants of the high-spin octahedral [ML 6 ] 2+ complex
cations are studied, an increase in complex stability is found as follows:
Ba 2+ < Sr 2+ < Ca 2+ < Mg 2+ < Fe 2+ < Co 2+ < Ni 2+ < Cu 2+ > Zn 2+
This series refers to the Irving-Williams series, and basically it is the
stability sequence of high-spin octahedral metal complexes for the re-
placement of water by other ligands.
The series can be explained in terms of (i) the ionic radius of the metal
cations, (ii) the crystal fi eld stabilization energy (CFSE) values, and (iii)
the John-Teller effect [18].
We may point out, from the series of the stability constants of the metal
ions in octahedral environment, that the ionic radius decreases from left
to right, that is, the ratio “charge/radius” or simply the polarizing power
of the cations increases. Similarly for ions of different charge and similar
radius, the polarizing power is essential.
However, if we study the Irving-Williams series for different ligand
types, we can see that the stability of complexes cannot be explained ex-
clusively as a charge density effect. If we consider the fi rst transition se-
ries, for example, we can observe that beyond Mn 2+ ion there is an abrupt
increase in the fi rst step-wise stability constant (K 1 ) for the rest of the
divalent cations and a sharp decrease for Zn 2+ ion. We know that these ions
possess additional ligand fi eld stabilization energy (LFSE) when moving
from d 5 to d 9 . If we consider the complexes that are obtained from the
aqueous solutions, we can easily understand that to have a positive LFSE,
the ligand (L) needs to have a greater octahedral fi eld stabilization energy
0 ) than that created by H 2 O, as occurs with ethylenediamine and oxalate.
If the situation is reversed, the LFSE would be negative with regard to
water as occurs with F .Cu 2+ complexes with chelate ligands present an ad-
ditional stabilization energy when compared with those of Ni 2+ despite the
supplementary electron placed in an antibonding orbital. This anomaly is
due to the stabilizing infl uence of the John-Teller effect, which increases
the fi rst step-wise stability constant, K 1 value. In a tetragonally distorted
complex, there is a strong bond with the four ligands in the plane. For this
reason, the value of K 1 and second step-wise stability constant K 2 follow
this order. By contrast, the third step-wise stability constant, K 3 for Cu 2+
with three chelate ligands is much smaller, because it is formally impos-
sible to place the three ligands in C is positions.
 
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