Chemistry Reference
In-Depth Information
There are a range of ligand systems without carbon chains known, but they need no
special consideration. They can be treated at an elementary level just like the more common
carbon-backboned ligands. Herein, it is this latter and most common type upon which we
shall concentrate.
2.3.2
More Teeth, Stronger Bite - Polydentates
As a general observation, the more donor atoms by which a molecule is bound to a metal
ion, the stronger will be the assembly. There is basic common sense in this, of course, since
more donors bound means more coordinate bonds linking the ligand to the metal ion. To
separate metal and ligand, all of these bonds would need to be broken, which one would
obviously anticipate costing more energy than breaking just a single bond to separate a
metal ion from a monodentate ligand. A case of more is better, or greed is good. There is a
conventional nomenclature to identify the number of bonds formed between a ligand and
a central metal ion, shown in Table 2.1. Here,
denticity
defines the number of bound donor
groups; this can equal the number of potential donor groups available (as in examples in
the table), or be a lesser number if not all groups available are coordinated. See Appendix
1 for some other notes on polydenticity.
Table 2.1 identifies molecules that can bind at up to six coordination sites. This is by
no means the limit, as inherently a molecule (such as a peptide polymer) may offer a vast
number of potential donors. It is not uncommon for polydentate ligands to use only some
and not all of their donors in forming complexes; this may be driven by the relative location
of the potential donors on the ligand (and the overall shape and folding of the ligand),
which affects their capacity to all bind at once, and the number of donors, as more may
be offered than can be accommodated around the metal ion. As an example, a ligand like
−
O
OC CH
2
N
H CH
2
CO
O
−
has one amine and two carboxylate groups, all of which
are capable of coordination. However, it may bind through one, two or all three of these
donors, acting, in turn, as a mono-, di- and tri-dentate ligand. Where more are available than
required, the set that eventually bind are usually those that produce the thermodynamically
most stable assembly. Some molecules may easily accommodate more than one metal ion;
a polymer with an array of donors would be an obvious example, with different parts of the
chain binding to different metal ions - we shall look at this prospect soon.
Table 2.1
Naming of different numbers of donor groups bound to a single metal ion, with
examples of typical simple linear ligands of each class.
No. bound donors
Ligand denticity
Examples of ligands (donor atoms highlighted in bold)
F
−
One
Monodentate
N
H
3
O
H
2
O
−
Two
Didentate
H
2
N
CH
2
CH
2
N
H
2
H
2
N
CH
2
CO
Three
Tridentate
H
2
N
C
H
2
C
H
2
N
H
C
H
2
C
H
2
N
H
2
−
O
O
−
C
O
C
H
2
N
H
CH
2
CO
H
2
N
CH
2
CH
2
S
C
O
CH
2
P
(C
H
3
)
2
Four
Tetradentate
H
2
N
C
H
2
C
H
2
N
H
CH
2
CH
2
N
H
C
H
2
C
H
2
N
H
2
−
O
N
−
N
−
C
O
C
H
2
CO
C
H
2
C
O
C
H
2
N
H
2
Five
Pentadentate
H
2
N
C
H
2
C
H
2
N
H
CH
2
CH
2
N
H
CH
2
CH
2
N
H
CH
2
C
H
2
N
H
2
−
O
C
O
C
H
2
N
H
CH
2
CH
2
S
CH
2
CH
2
N
H
O
−
CH
2
C
H
2
−
O
Six
Hexadentate
C
O
C
H
2
N
H
C
H
2
C
H
2
N
H
CH
2
CH
2
N
H
O
−
CH
2
CH
2
N
H
CH
2
CO
