THE TRANSITION ELEMENTS 389
the hydrated Mn3+ ion, which (as expected for a tripositive cation),
is strongly acidic.
The complexes of manganese(III) include [Mn(CN) 6 ]^3 ~ (formed
when manganese(II) salts are oxidised in presence of cyanide ions),
and [MnF 5 (H 2 O)]^2 ~, formed when a manganese(II) salt is oxidised
by a manganate(VII) in presence of hydrofluoric acid :
4Mn2+ + 8H+ + MnO4 -> 5Mn^3 + 4- 4H 2 O
Mn3+ + H 2 O + 5F" -» [MnF 5 (H 2 O)]^2 ~
Oxidation of manganese(II) hydroxide by air gives the brown
hydrated oxide Mn 2 O 3 .aq, and this on drying gives MnO(OH)
which occurs in nature as manganite. (The oxide Mn 2 O 3 also occurs
naturally as braunite.) Heating of the oxide Mn 2 O 3 gives the mixed
oxide Mn 3 O 4 [manganese(II) dimanganese(III) oxide].
In general, manganese(III) compounds are coloured, and the
complexes are octahedral in shape; with four d electrons, the colour
is attributable in part to d-d transitions.Oxidation state + 2This is the most common and stable state of manganese; the five d
electrons half fill the five d-orbitals, and hence any transition of d
electrons in a complex of manganese(II) must involve the pairing
of electrons, a process which requires energy. Hence electron
transitions between the split d-orbitals are weak for manganese(II),
and the colour is correspondingly pale (usually pink). The stability of
the d^5 configuration with respect to either loss or gain of electrons
also means that manganese(II) salts are not easily reduced or
oxidised. Indeed, in oxidation state 4-2, manganese shows fewer
'transition-like' characteristics than any other transition metal ion;
thus the aquo-ion [Mn(H 2 O) 6 ]2+ is barely acidic, allowing forma-
tion of a "normal' carbonate MnCO 3 which is insoluble in water
and occurs naturally as "manganese spar'. The aquo-ion forms
typical hydrated salts, for example MnSO 4 .7H 2 O, MnCl 2 .xH 2 O
and double salts, for example (NH 4 ) 2 Mn(SO 4 ) 2 .6H 2 O; dehydration
of the simple hydrated salts, by heating, produces the anhydrous
salt without decomposition. Addition of alkali precipitates the white
basic manganese(II) hydroxide Mn(OH) 2 ; if left in the alkaline
medium it is oxidised readily by air to brown Mn 3 O 3 .aq*.
- In water pollution studies, the oxygen content can be measured by making the
water alkaline and shaking a measured volume with an oxygen-free solution con-
taining Mn^2 ^(aq). The solution is acidified with sulphuric acid, potassium iodide
added and the liberated iodine titrated with sodium thiosulphate.