Basic Coordination Chemistry for Biologists - Biological Inorganic Chemistry - page 23

Coordination 3 compounds consist of a central atom or ion, like Co 3 þ , surrounded by electron-rich groups
(ligands), like NH 3 . The ligands are directly bound (coordinated to) to the central atom or ion, they are usually
between 2 and 9 in number and may be single atoms, ions, or molecules. The ligands directly bound to the metal
are said to be in the inner coordination sphere, and the counter-ions that balance out the charge are said to be outer
sphere ions. Coordination compounds are usually referred to as complexes, they can be charged or uncharged and
their structure is defined by the coordination number (the number of ligand atoms bonded to the central atom) and
their coordination geometry (the geometrical arrangement of the ligands and the symmetry of the entire complex).
The central ion can be in any oxidation state, which remains unchanged in the coordination complex. We shall
endeavour in what follows to explain some of the concepts of coordination chemistry and their relevance to
biological inorganic chemistry.

HARD AND SOFT LIGANDS
In 1923, the American chemist G.N. Lewis provided a broad definition of acids and bases which covered
acid base reactions not involving the traditional proton transfer: an acid is an electron pair acceptor (Lewis acid)

and a base is an electron pair donor (Lewis base). The concept was extended to metal

e

e ligand interactions with the

ligand acting as donor, or Lewis base, and the metal ion as acceptor, or Lewis acid.
The metal ions can be empirically sorted into two groups on the basis of their preference for various ligands:
the large and polarisable ions which prefer large, polarisable ligands and the smaller, compact, and less polarisable
ones which prefer compact, less polarisable ligands. Such a correlation, coupled to the broader definition of
acid

e

base, led to the concept of “hard” and “soft” acids and bases which can be useful in classifying and to some
extent predicting the strength of metal

e

ligand bonds, and hence the stability of complexes.
The general characteristics of each group are summarised in Table 2.1 along with a classification of metal
ions and ligands of importance in biological inorganic chemistry. In general, “hard” acids prefer “hard”

TABLE 2.1 Classification of Biologically Important Metal Ions and Ligands According to the

'Hard e Soft Acid e Base' Concept and their General Characteristics

Acid/Acceptor (Metal ions) Base/Donor (LIGANDS)

Hard High charge density

Small ionic radius
No easily excited outer shell electrons
Na þ ,K þ ,
Mg 2þ ,Ca 2þ ,
Cr 3þ ,Fe 3þ ,Co 3þ

Low polarisability
High electronegativity
Vacant, high-energy orbitals
Hard to oxidise
H 2 O, OH ,CO 2 ,CO 2 3 ,NO 3 ,PO 3 4 , ROPO 2 3
PO 3 4 , ROPO 2 3 , ðROÞ 2 PO 2 ,
ROH, RO ,R 2 O, NH 3 , RNH 2 ,Cl

Fe 2þ ,Co 2þ ,Ni 2þ ,Cu 2þ ,Zn 2þ NO 2 ,SO 2 3 ,Br ,N 3 , imidazole

Intermediate

Soft Low charge density

Large ionic radius
Easily excited outer shell electrons
Cu þ

High polarisability
Low electronegativity
Low energy vacant orbitals
Easily oxidised
RSH, RS ,CN ,CO

3. Although there is no real reason for treating coordination compounds separately from molecular ones, the historic convention will be used
here for reasons of convenience.