Modern inorganic chemistry

THE TRANSITION ELEMENTS 363

metal atom or ion and its oxidation state, (b) the number of surround

ing ligands which may be ions, atoms or polar molecules, (c) the

overall charge on the complex, determined by the oxidation state

of the central atom and the charges (if any) on the ligands. Some

examples are:

Oxidation state

of central atom

or ion

Example

MnO 4

,2-

3

2

CrOT

TiCl 4

[Fe(CN) 6 ]

[Ni(NH 3 ) 6 ]^2 +

Fe(CO) 5

(^3) ~
Name
f 'permanganate', but better
4 manganate(VII)
[ (strictly, tetraoxomanganate(VII))
f bchromate\ better
J chromate(VI)
[ (strictly, tetraoxochromate (VI))
titanium tetrachloride
or tetrachlorotitanium(IV)
hexacyanoferrate(III)
hexamminonickel(II)
iron pentacarbonyl
or pentacarbonyliron(O)
Note that complexes can have negative, positive or zero overall
charge. The examples MnO^, CrO4~ are usually considered to be
oxoacid anions (p. 44); but there is no essential difference between
these and other complexes. For example, the anion MnO^ can be
regarded formally as a manganese ion in oxidation state + 7 sur-
rounded by four oxide ion (O^2 ~) ligands (in fact of course there is
covalent bonding between the oxide ligands and the Mnvu ion,
leading to partial transfer of the oxide negative charges to the
manganese). In general, high oxidation states (for example those of
manganese 4- 7 and chromium -f 6) are only found in oxides (for
example Mn 2 O 7 , CrO 3 ), oxoacid anions (MnO^, CrO^, Cr 2 O|~)
and sometimes fluorides (there is no MnF 7 known, but CrF 6 is
known). Hence the number of complexes in high oxidation states is
very limited. At lower oxidation states, a variety of ligands can form
complexes—some common ligands are:
H 2 O NH 3 CN" Cl~
[Fe(H 2 O) 6 ]^2 + [Co(NH 3 ) 6 ]3+ [Ni(CN) 4 ]^2 ' [CuCl 4 ]^2 -
However, stable complexes where the oxidation state i6f the central
metal atom is 0 are only formed with a very few ligands, notably