additional electrons then pair with electrons in the t 2 gset. Such a complex would have
the same number of unpairedelectrons on the metal atom or ion as when the metal is
uncomplexed; this is called a high spin complex.But if the splitting energy is greater
than the pairing energy, the electrons will be at lower energy if they pair in the t 2 gorbital
before any electrons occupy the higher energy egorbitals. Such a complex could have
fewer unpaired electrons on the metal atom than when the metal is uncomplexed, so it is
called a low spin complex.
Let us now describe the hexafluorocobaltate(III) ion, [CoF 6 ]^3 , and the hexa-
cyanocobalt(III) ion, [Co(CN) 6 ]^3 . Both contain the d^6 Co^3 ion. The [CoF 6 ]^3 ion is a
paramagnetic complex, whereas [Co(CN) 6 ]^3 is a diamagnetic complex. We will focus our
attention on the delectrons.
The Co^3 ion has six electrons (four unpaired) in its 3dorbitals.
3 dCo^3 [Ar] __hg __h__h__h__hMagnetic measurements indicate that [CoF 6 ]^3 also has four unpaired electrons per ion.
So there must be four electrons in t 2 gorbitals and two in egorbitals. Fluoride, F, is a
weak field ligand, so the crystal field splitting energy is very small ( (^) oct P) and electron
pairing is unfavorable. Thus [CoF 6 ]^3 is a high spincomplex.
On the other hand, [Co(CN) 6 ]^3 is diamagnetic, so all six delectrons must be paired
in the t 2 gorbitals. Cyanide ion, CN, is a strong field ligand, which generates a large
crystal field splitting energy ( (^) oct P), making electron pairing more favorable;
[Co(CN) 6 ]^3 is a low spin complex.
The difference in configurations between [CoF 6 ]^3 and [Co(CN) 6 ]^3 is due to the rela-
tive magnitudes of the crystal field splitting, (^) oct, caused by the different crystal field
strengths of Fand CN. The CNion interacts with vacant metal orbitals more strongly
than the Fion does. As a result, the crystal field splitting generated by the close approach
of six CNions (strong field ligands) to the metal ion is greater than that generated by
the approach of six Fions (weak field ligands).
A diamagnetic species has no unpaired
electrons. A paramagnetic species has
unpaired electrons, and the strength
of the paramagnetism depends on the
number of unpaired electrons. See
Chapter 5.
25-8 Crystal Field Theory 993
[Co(OH 2 ) 6 ]^2 ions are pink (bottom). A limited amount of aqueous ammonia produces
Co(OH) 2 , a blue compound that quickly turns gray (middle). Co(OH) 2 dissolves in excess
aqueous ammonia to form [Co(NH 3 ) 6 ]^2 ions, which are yellow-orange (top).
[CoF 6 ]^3 is a high spincomplex.
[Co(CN) 6 ]^3 is a low spincomplex.
Co^3 ion in
spherical
crystal field
}eg
[CoF 6 ]^3
}t 2 g
(^) oct small; weak crystal
field due to F
(not to scale)
3 d, free Co^3 ion
Co^3 ion in
spherical
crystal field
}eg
[Co(CN) 6 ]^3
}t 2 g
(^) oct large; strong crystal
field due to CN
(not to scale)
3 d, free Co^3 ion