Question

Explain the following observations on the basis of the ligand field splitting of the d orbitals. Include the properties of the ligands in your explanation.

a) The complex [Ni(CN)4]2- is diamagnetic whereas [NiCl4]2- is paramagnetic with two unpaired electrons.

b) The octahedral complex [Fe(CN)6]3- has only one unpaired electron whereas the hexaaqua complex [Fe(H2O)6]3+ has five.

Answer 1

a) Ni general electronic configuration is [Ar] 3d8 4s2

  [Ni(CN)4]2- : Here Ni oxidation state is Ni+2

So electronic configuration is [Ar] 3d8 4s0

CN is strong ligand so electron get pair in d orbital, So in 3d (4 d orbtals fllied by 8 electrons as pair remaining one orbital empty). So no unpaired elct rons.So this is an Dia magnetic. Against to hunds rule in the presence of strong ligands.

[NiCl4]2- : chlorine weak ligand. So electrons can not pairup. So follow hunds rule. After filling 6 lectrons three d orbtal, 2 electrons as unpaired. So this is para magentic.

b) [Fe(CN)6]3- : Fe+2 : [Ar] 3d5 4s0

Same type: CN is strong ligand, against to hunds rule, so electrons get pair up. 2 pair of electrons in 2 d orbital and one electron in 3rd orbtal and remaing 2 orbital empty,

[Fe(H2O)6]3+; H2O is an weak ligand. Fe+3. All 5 electrons are unapired.