Modern inorganic chemistry

46 STRUCTURE AND BONDING

solution, and only in strongly acidic solutions is the hexaaquo ion

[A1(H 2 O) 6 ]^3 + found. The solid hydrate, often written A1C1 3. 6H 2 O

and more correctly [A1(H 2 O) 6 ]C1 3 can, therefore, only be obtained

from a strongly acidic solution. The reaction with water resulting in

the liberation of a proton is again known as hydrolysis and occurs

whenever the central metal ion is small and highly charged (i.e.

having a high surface density of charge), for example in salts of

iron(III), chrornium(III)*.

There are many ligands in addition to water, for example Cl~,

NH 3 , CN~, NO^, and transition metal ions, in particular, form a

large number of complex ions with different ligands. The number

of ligands surrounding the central atom, or ion, is called the co-

ordination number. The numerical value of the co-ordination number

depends on a number of factors, but one important factor is the

sizes of both the ligands and central atom, or ion. A number of

complex ions are given below in Table 2,9. The shape of complex

Table 2.9

CHARACTERISTICS OF SOME COMPLEX IONS

Central

unit

Be^2 +

Co^3 +

A1^3 +

Ni°

Fe^2 +

Co^3 *

f "iga nd Co-ordination Ligand

number type

H 20

NH 3

F-

CO

CN-

NOJ

4 6 6 4 6 6

Molecule

Molecule

Ion

Molecule

Ion

Ion

Complex ion

[Be(H 2 0) 4 ]^2 +

[Co(NH 3 ) 6 ]^3 +

[A1F 6 ]^3 -

Ni(CO) 4

[Fe(CN) 6 ]^4 -

[Co(N0 2 ) 6 ]^3 -

Shape

Tetrahedral

Octahedral

Octahedral

Tetrahedral

Octahedral

Octahedral

ions formed by typical elements can be determined by assuming
each ligand to be covalently bonded to the central ion and applying
the theory of electron pair repulsion which gives the structures
summarised in Table 2,9. The shape of transition metal complexes,
however, cannot always be deduced by this method. The develop-
ment of the theory of bonding in transition metal complexes is
beyond the scope of this book but a brief outline of the main
features is given at the end of this chapter.

• The species resulting from the 'hydrolysis' of hydrated cations such as those
  mentioned here are often highly complex, containing more than one metal atom (i.e.
  they may be polynuclear). The description here is simplified to show the essentials
  of the processes.