Question

Haemoglobin is a complex of
a) \[F{{e}^{3+}}\]
b)\[F{{e}^{2+}}\]
c) \[F{{e}^{4+}}\]
d) \[C{{u}^{2+}}\]

Answer 1

Hint: Haemoglobin is made up of four heme groups which is metalloporphyrin. Metalloporphyrin is a complex in which a metal ion is surrounded by four pyrrole rings or held by four nitrogens of four pyrrole rings (porphyrin). Now heme also( like metalloporphyrin) is a complex in which metal is held by four nitrogen of four pyrrole rings at the central hole. In this, metal Fe(iron) is present forming four coordinate bonds with four nitrogen of the pyrrole ring.

Complete step by step solution:
Haemoglobin is made up of four subunits and each of which contains one heme group. Thus, we can say Haemoglobin is formed by four heme groups (Porphyrin ring and metal at centre). The atomic number of iron is 26 so, its electronic configuration is
\[1{{s}^{2}},2{{s}^{2}},2{{p}^{6}},3{{s}^{2}},\text{ }3{{p}^{6}},\text{ }3{{d}^{6}},\text{ }4{{s}^{2}}\]

The heme group which is present in all four units of Haemoglobin is a complex of porphyrin ring and metal at the centre of the ring. The Porphyrin ring is made up of four pyrrole rings and each ring contains four nitrogen (two is tertiary and two are secondary). And all four nitrogen will act like lewis base or lone pair donors thus, the Porphyrin ring is a tetradentate ligand. All forms coordinate bonds with iron.

Now Haemoglobin is an oxygen carrier so it needs to combine oxygen with it so, in its excited state one electron out of a two-electrons of 4s jump to another outermost orbital like to 4p orbital and two unpaired electrons will form. Now with one electron of the iron in heme group, one protein (amino group chain) is attached and the other is an oxygen molecule. So, when it is not bonded with iron and protein, iron remains in a +2 state.
\[1{{s}^{2}},2{{s}^{2}},2{{p}^{6}},3{{s}^{2}},\text{ }3{{p}^{6}},\text{ }3{{d}^{6}},\text{ }4{{s}^{1}},4{{p}^{1}}\]

Haemoglobin is an oxygen carrier and when release oxygen into muscle tissue or other part of body and get deoxygenated, then actual oxidation state of iron in haemoglobin is +2 such as \[F{{e}^{2+}}\].
Thus, the correct option is B.

Note: Tertiary nitrogen (valency of +3) is the one that is bonded with three carbon next to itself. Even nitrogen which is double bonded to carbon will consider that nitrogen is bonded with two carbon. Whereas secondary nitrogen in the heme group is bonded with two carbon and one hydrogen. And all the nitrogen forms a coordinate bond which is formed when a donor atom (all four nitrogen) gives two electrons by itself to form a single bond with another atom like nitrogen donating both electrons of the bond by itself to form a coordinate bond with iron.