Engineering Chemistry S Datta

TRANSITION METAL CHEMISTRY 301
Octahedral complexes result from d 2 sp 3 or sp 3 d 2 hybridizations.
Let us site an example of an octahedral complex [Fe(CN) 6
] 4– . The complex formation is
as follows:
(a) Electronic configuration of Iron in
6 2
ground state (...3d .4s )
(b) Electronic configuration of Fe
6 0
ion in ground state (...3d .4s )
(c) Electronic configuration of Fe
2 3
d sp hybridised state
(d) Electronic configuration of Fe
[Fe(CN) ]
4–
6
2+
2+
2+
ion in
ion in
3d 4s 4p
×× ×× ×× ×× ×× ××
CN – CN – CN – CN – CN – CN –
The ground state electronic configuration of Fe is ... 3d 6 4s 2 . In Fe 2+ ion, the two 4s
electrons move out leaving 4s orbital vacant. In d 2 sp 3 hybridisation, six d 2 sp 3 hybrid orbitals
are generated which are filled by electron pairs donated by six CN – ions acting as ligands,
hence [Fe(CN) 6
] 4– complex is formed.
The shape of the complex molecule is octahedral and due to the absence of unpaired
electrons, the complex is diamagnetic. This complex uses the inner d-orbitals. So, this is often
called inner orbital complex. This is often referred to as spin paired complex.
Another example of sp 3 d 2 configuration is [FeF 6
] 3– complex. The formation of this ion is
as follows:
Electronic configuration of
3+ 5 2
Fe (... 3d 4s )
3d 4s 4p 4d
Electronic configuration of
3+ 3 2
Fe in sp d configuration
3d
sp d
3 2 4d
4d
Electronic configuration of
3+ 3–
Fe ion in [FeF 6]
configuration
sp 3 d
2
×× ×× ×× ×× ×× ××
F – F – F – F – F – F –
3 2
Six sp d hybridised orbitals
having lone pairs of electrons
–
donated by six F ligands.
Such complex is octahedral in shape and since outer d-orbitals are involved, it is called
outer orbital complex. Because of presence of five lone electrons in 3d level the compound is
highly paramagnetic and often referred to as sp high spin complex.