Modern inorganic chemistry

THE TRANSITION ELEMENTS 393

reduces some cations to the metal (for example copper) in aqueous

solution, giving iron(II).

Iron absorbs hydrogen readily and is a hydrogenation catalyst.

In Mendeleef s form of the periodic table, iron (together with

cobalt and nickel) was placed in Group VIII and the three elements

together were called fca transitional triad'. Hence there was no

resemblance to any of the elements in the main Groups I-VII; these

triad elements have properties which are similar, and which show

some resemblances to the earlier transition metal properties.

However, unlike manganese and the preceding transition elements,

iron does not show the maximum possible oxidation state +8

corresponding to the removal of all its eight outer electrons (3d^6 4s^2 }:

the actual maximum oxidation state is +6, but oxidation states

above -1-3 are not very important, and +3 and + 2 are the pre

dominant and important states for iron. (Cobalt and nickel simi-

larly do not show high oxidation states.)

Oxidation states above + 3

As might be expected, these higher oxidation states are found almost

exclusively in anionic form, and are produced only under strongly

oxidising conditions.

Alkali metal ferrates(VI), for example K 2 FeO 4 , are obtained by

oxidation of a suspension of hydrous iron(III) oxide (assumed to be

Fe(OH) 3 in the equation below) by chlorate(I) in concentrated alkali:

2Fe(OH) 3 4- 3C1CT + 4OH~ -* 2FeOr + 3C1" + 5H 2 O

The deep red FeOj" is stable only in alkali; in acid, iron(III) is

produced :

2FeO*- + 1OH+ ^ 2Fe^3 + (aq) + 5H 2 O + |O 2

Ferrate(VI) has powerful oxidising properties, for example am-

monia is oxidised to nitrogen. Potassium ferrate(VI) is isomorphous

with potassium chromatefVT), and both anions are tetrahedral.

Decomposition of potassium ferrate(VI) at 1000 K gives a fer-

rate(V), K 3 FeO 4 , and several types of ferrate(IV), for example

FeO|~, FeOt' are known; these ferrates(IV) have no solution

chemistry and are probably best regarded as mixed oxides, since

the FeOl" ion has no identifiable structure.

Oxidation state + 3

In this state, iron has five d electrons, but does not show any strong