We are almost ready to begin learning about the kinds of chemical reactions
that transition-metal complexes will participate in. But first let’s learn one
additional topic that will be of relevance. We must understand hard and soft acids and bases,
and how they pertain to transition metals. We certainly will recall acid-base reactions
from our study of general chemistry, and we are certainly aware of the Bronsted-Lowry definition
for acidity and basicity that deals with proton exchange. But we should also remember the Lewis
model of acidity and basicity, where a Lewis base is an electron-pair donor, and a Lewis acid is an
electron-pair acceptor. This definition will be relevant for us because transition metal centers
often act as Lewis acids, and ligands act as Lewis bases when they coordinate to the metal
centers. So in the context of this tutorial, when we talk about acids we are referring to transition
metal ions, and bases will be the ligands that coordinate to them. In order to be able to predict
the strength of metal-ligand interactions, we need to understand the concept of hard and soft.
When we say that an acid or base is “soft”, we are referring to its polarizability. A highly
polarizable atom or molecule is very soft. This means that large metal ions are soft acids. They
have large electron clouds that can exhibit large momentary or induced dipoles and are
thus more polarizable. Smaller metal ions are hard acids. We can say the same for a base. If a lone
pair or negative charge is highly localized on a singular, small atom, this would be a hard base.
If the charge is diffuse, distributed around a larger molecular volume or an atom with
a large radius, this would be a soft base. Hard acids have a high charge density and form
bonds that are more ionic in nature. The greater the charge on a metal ion, the harder it is,
and the smaller the radius of the metal ion is, the harder it is. Early transition metals with
valence electrons in the 3d subshell are hard acids. Hard bases are small anions and neutral
molecules with lone pairs. Bases derived from electronegative elements in the n=2 shell will
be hard bases, like those of nitrogen, oxygen, and fluorine. This means that fluoride anions,
any kind of oxygen anion like an oxide or a hydroxide, and even neutral nitrogen atoms in
ammonia or other amines are classified as hard. Soft acids have a lower charge density and form
bonds that are more covalent in nature. Late 4d and 5d transition metals tend to be soft
due to their larger size and polarizability. These usually have charges of only 1+ or 2+.
Soft bases are larger anions and neutral molecules, which are more polarizable than their
hard counterparts. For example, while fluoride is hard, iodide is very soft, and the softness
increases going down the halogen group, as radius increases. Likewise, while an oxide ion is hard,
a sulfide ion is much softer, again due to the increased radius and polarizability.
There are also plenty of species that qualify as having intermediate hardness or softness,
not leaning heavily in either direction. These have intermediate size, charge density,
and polarizability. This is not always easy to predict, as even different oxidation states of
the same element can differ dramatically, so it is best to just learn through example
as we go through the series. But for those who like to get a glance at the bigger picture,
here is a table depicting some common hard and soft acids and bases.
The key thing we must understand when applying this framework to metal-ligand interactions
is that hard acids interact best with hard bases, and soft acids interact best with soft bases.
This means that the stability of a complex is enhanced by hard-hard
and soft-soft interactions. So a metal center that would be regarded as a hard acid
will bind better with fluoride than iodide, and for a soft acid that trend would be reversed.
This phenomenon can help us understand and predict the types of complexes a metal center will form.
Take a very soft acid like gold 1+. This will only form stable complexes with soft
bases like phosphines and similar ligands, and not those from the n=2 shell.
However, gold 3+ has a greater charge density than gold 1+, and is thus a much harder acid,
which means it is capable of interacting with harder bases like water and amines.
This concept, although qualitative, has widespread application, from the
industrial to the biological, and we will get to those in due time. But for now let’s
move forward and begin learning the very basics regarding organometallic reactions.