Page 82 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

2 RNDAMENTAL THEORETICAL PRINCIPLES OF REACTIONS IN SOLUTION

between small ions of high charge are particularly strong and lead to stable
complexes.
2. Cations with completely filled d sub-shells. Typical of this group are copper(I),
silver(1) and gold(1) which exhibit Class B acceptor properties. These ions
have high polarising power and the bonds formed in their complexes have
appreciable covalent character.
3. Transition metal ions with incomplete d sub-shells. In this group both
Class A and Class B tendencies can be distinguished. The elements with
Class B characteristics form a roughly triangular group within the Periodic
Table, with the apex at copper and the base extending from rhenium to
bismuth. To the left of this group, elements in their higher oxidation states
tend to exhibit Class A properties, while to the right of the group, the higher
oxidation states of a given element have a greater Class B character.
The concept of 'hard' and 'soft' acids and bases is useful in characterising
the behaviour of Class A and Class B acceptors. A soft base may be defined
as one in which the donor atom is of high polarisability and of low
electronegativity, is easily oxidised, or is associated with vacant, low-lying
orbitals. These terms describe, in different ways, a base in which the donor
atom electrons are not tightly held, but are easily distorted or removed. Hard
bases have the opposite properties, i.e. the donor atom is of low polarisability
and high electronegativity, is difficult to reduce, and is associated with vacant
orbitals of high energy which are inaccessible.
On this basis, it is seen that Class A acceptors prefer to bind to hard bases,
e.g. with nitrogen, oxygen and fluorine donor atoms, whilst Class B acceptors
prefer to bind to the softer bases, e.g. P, As, S, Se, Cl, Br, 1 donor atoms.
Examination of the Class A acceptors shows them to have the following
distinguishing features; small size, high positive oxidation state, and the
absence of outer electrons which are easily excited to higher states. These
are al1 factors which lead to low polarisability, and such acceptors are called
hard acids. Class B acceptors, however, have one or more of the following
properties: low positive or zero oxidation state, large size, and several easily
excited outer electrons (for metals these are the d electrons). These are al1
factors which lead to high polarisability, and Class B acids may be called
soft acids.
A general principle may now be stated which permits correlation of the
complexing ability of metals: 'Hard acids tend to associate with hard bases
and soft acids with soft bases'. This statement must not, however, be regarded
as exclusive, i.e. under appropriate conditions soft acids may complex with
hard bases or hard acids with soft bases.
(b) Characteristics of the ligand. Among the characteristics of the ligand
which are generally recognised as influencing the stability of complexes in
which it is involved are (i) the basic strength of the ligand, (ii) its chelating
properties (if any), and (iii) steric effects. From the point of view of the
analytical applications of complexes, the chelating effect is of paramount
importance and therefore merits particular attention.
The term chelate effect refers to the fact that a chelated complex, i.e. one
formed by a bidentate or a multidenate ligand, is more stable than the
corresponding complex with monodentate ligands: the greater the number of
points of attachment of ligand to the metal ion, the greater the stability of