Stability - Introduction to Coordination Chemistry - page 129

appears more important than that of ionic radius, from the fall in log K values for very
small but lower-charged ions (Al 3 + and Be 2 + ). Of course, our simple model of the cations
as hard spheres applied in this analysis is also imperfect; overall, nevertheless, it is possible
to predict stability adequately if not finitely from the charge/radius ratio relationship. As a
result, a large K is favoured by a large charge/radius ratio, or the smaller the ion and the
larger its charge, the more stable will be its metal complexes.
Although our focus currently is on the metal, it should be recognized that the size (or
molar volume) of the ligand plays a role in the electrostatic effect on stability of a complex,
particularly for anionic ligands. This is sensible, since the ligand can be assigned a surface
charge density (when anionic) in the same way that we have done for the cation, and
obviously an anion with a high surface charge density should form stronger complexes
from an electrostatic perspective. This is best illustrated by examining halide monatomic
anions, where spherical surfaces have some meaning. With F − (radius 133 pm) and Cl −
(radius 181 pm), Fe 3 + forms complexes with log K of 6.0 and 1.3 respectively, reflecting
the greater charged/surface ratio for the former. The concept of ion radius becomes diffuse
when we move to molecular anions, however, whose shape may not be anywhere near
spherical. However, at least for large reasonably symmetrically-shaped and thus pseudo-
spherical anions like ClO 4 − , the very low stability of its complexes is fairly consistent with
an electrostatic model, since this ion has a much greater radius than simple halide ions of
the same charge.

5.1.2.1.2 Metal Class and Ligand Preference
We have examined the hard/soft acid/base 'like prefers like' concept as it applies to
metal-ligand binding already in Chapter 3, so it is simply necessary to revise aspects here.
Electropositive metals (lighter and/or more highly charged ones from the s, d and f block
such as Mg 2 + ,Ti 4 + and Eu 3 + , belonging to Class A ) tend to prefer lighter p-block donors
(such as N, O and F donors). Less electropositive metals (heavier and/or lower-charged
ones such as Ag + and Pt 2 + belonging to Class B ) prefer heavier p-block donors from the
same families (such as P, S and I donors). A more significant M L covalent contribution
is asserted to apply in the latter case, along with other effects such as

back-bonding.
Of course, in any situation where there are only two categories, there is a 'grey' area of
metals and ligands who do not sit easily in either set. A summary is given in Table 5.2
below, in which a large number of the metal ions and simple ligands you are likely to meet
appear.

Table 5.2 Examples for both ligands and metals of 'hard' and 'soft' character, with some less
clearly defined intermediate cases also included.
Hard Intermediate Soft

Ligands
F − ,O 2 − , − OH, OH 2 , OHR, RCOO − ,NH 3 ,
NR 3 , RCN, Cl − ,NO 3 − ,CO 3 2 − ,SO 4 2 − ,
PO 4 3 −

Br − , − SR, NO 2 − ,N 3 − ,
SCN − ,H 5 C 5 N

PR 3 ,SR 2 ,SeR 2 ,AsR 3 ,
CNR, CN − ,SCN − ,CO,
I − ,H − ,R −

Metal Ions
Mo 5 + ,Ti 4 + ,V 4 + ,Sc 3 + ,Cr 3 + ,Fe 3 + ,Co 3 + ,
Al 3 + ,Eu 3 + ,Cr 2 + ,Mn 2 + ,Ca 2 + ,Mg 2 + ,
Be 2 + ,K + ,Na + ,Li + ,H +

Fe 2 + ,Co 2 + ,Ni 2 + ,
Cu 2 + ,Zn 2 + ,Pb 2 +

Cu + ,Rh + ,Ag + ,Au + ,
Pd 2 + ,Pt 2 + ,Hg 2 + ,Cd 2 +