Chemistry Reference
In-Depth Information
whereas tetramers, such as [Cu
4
Cl
16
]
4
−
adopt a slightly distorted box-like shape. These
may also involve metal-metal bonding; for example, [Re
3
Cl
12
] has bonds between the Re
atoms additional to those formed by bridging chloride ions.
The above examples are relatively simple, since they contain just two types of atom and
adopt symmetrical shapes. It should be anticipated that many examples will prove much
more complicated in structure. Although polymeric coordination compounds are growing
in number and importance, we shall limit our exploration of these, leaving this to more
advanced textbooks.
3.5
Making Choices
3.5.1
Selectivity - Of all the Molecules in all the World, Why This One?
The heading above takes us back to a movie analogy again, but we're not going to take it
too far. Suffice to say that, in the same way there is a vast number of bars or cafes in the
world to choose from, there is an even vaster range of potential ligands for a metal ion to
select. Why, then, does selection happen (or choice arise) at all?
What we do know from experimental evidence is that metal ions do not interact with
potential ligands purely on a statistical basis. A simple example will suffice. Water, the
most common solvent, is
∼
55 M. If ammonia is dissolved in water to a concentration of
∼
0.5 M, there is a 100-fold excess of water molecules over ammonia molecules. If a small
concentration of copper ion is introduced into the solution, the vast majority of the copper
ions bind to the ammonia - even though each cation 'meets' many more water molecules.
Why is there this clear preference for ammonia over water as a ligand in this case? This is
but one example of a general observation.
3.5.2
Preferences - Do You Like What I Like?
Ligand preference for and affinities of metal ions present themselves as experimentally
observable behaviour that is not easily reconciled. As a general rule, when a metal (M)
is mixed with equimolar amounts of ligands A and B, the result is
not
usually equimolar
amounts of MA and MB. Both metal ions (
Lewis acids
) and ligands (
Lewis bases
)show
preferences.
In seeking an understanding of this phenomenon, we can sort ligands according to their
preference for forming coordinate bonds to metal ions that exhibit more ionic rather than
purely covalent character. Every coordinate covalent bond between a metal ion and a
donor atom will display some polarity, since the two atoms joined are not equivalent. The
extreme case of a polar bond is the ionic bond, where formal electron separation rather
than electron sharing occurs; coordinate bonds show differing amounts of ionic character.
Small, highly charged entities will have a high surface charge density and a tendency
towards ionic character. In a sense, we can think of these ions or atoms as 'hard' spheres,
since their electron clouds tend to be drawn inward more towards the core and thus are
more compressed and less efficient at orbital overlap. A large, low-charged entity tends to
have more diffuse and expanded electron clouds, making it less dense or 'soft', and better
suited to orbital overlap. This concept, applied to donor atoms and groups, leads to us
defining 'hard' donors as those with a preference for ionic bonding, and 'soft' donors as
those preferring covalent bonding. An important observation is that the 'hard' donor atoms
