Modern inorganic chemistry

362 THE TRANSITION ELEMENTS

CHEMICAL PROPERTIES

OXIDATION STATES

The transition elements are often said to exhibit "variable valency'.

Because they so readily form complex compounds, it is better to use

the term 'variety of oxidation states'. The states usually found for the

elements Sc-Zn are:

Sc +3

Ti + 2, + 3, + 4

V + 2, + 3, + 4. + 5

Cr 0 + 2. + 3, + 6

Mn 0 + 2, + 3, + 4, + 6, + 7

Fe 0 + 2, + 3

Co 0 +1 + 3

Ni 0 +2

Cu 0 -hi, + 2

Zn +2

(The states which are most stable with respect to decomposition or

oxidation/reduction are underlined.)

We may note (a) the common occurrence of oxidation state 4- 2

where the 4s^2 electrons have been formally lost, (b) the increase in

the number of oxidation states from scandium to manganese; in

the latter element, the oxidation state + 7 corresponds to the formal

loss of the 3s^2 and 3d^5 electrons, (c) the sharp decrease in the number

of oxidation states after manganese—suggesting that removal of

the paired 3d electrons is less easy; (d) the oxidation state 0, occurring

for many of the later elements in the series*.

Some of the oxidation states given above, especially the higher

oxidation states (7,6) and oxidation state 0, are found only when the

metal atom or ion has attached to it certain groups or ligands.

Indeed the chemistry of the transition elements is so dominated by

their tendency to form coordination complexes that this aspect of

their behaviour must be considered in some detail.

COORDINATION COMPLEXES

Complexes have already received some discussion; it will be
recalled that they are defined (and named) in terms of (a) the central

• Some transition metal atoms combined with uncharged molecules as ligands
  (notably carbon monoxide. CO) have a formal oxidation state of 0. for example
  Ni + 4CO ^ Ni°(CO) 4.