Modern inorganic chemistry

366 THE TRANSITION ELEMENTS

The ions Co2+ and Co^34 " have 7 and 6 d electrons respectively.

Where there are orbitals of the same (or nearly the same) energy,

the electrons remain unpaired as far as possible by distributing

themselves over all the orbitals. In the case of [Co(NH 3 ) 6 ]^2 ^, the

energy split in the d orbitals due to octahedral attachment of the six

Coordinated

Co^3 * ion

c/electrons, 4 unpaired Coordinated

Simple Co^2 +ion, Id

electrons 3 unpaired

Figure 13.2

ammonia ligands is small, and the electrons remain as in the Co^2 +
ion, i.e. 3 unpaired. For [Co(NH 3 ) 6 ]3+ the split is much larger, and
the electrons pair up in the lower energy orbitals as shown. Now
unpaired electrons in a substance give rise to paramagnetism—the
substance is weakly attracted to a magnet, and the larger the number
of unpaired electrons, the larger is the magnetic moment (which can
be determined by measuring the attraction). Hence it is found
that solids or solutions containing the [Co(NH 3 ) 6 ]^2 * ion are
paramagnetic, but those containing the [Co(NH 3 ) 6 ]3+ ion are not;
they are in fact very weakly repelled by a magnetic field and are
termed diamagnetic. Complexes with unpaired electrons are often
called 'spin-free' (because the electron spins are not 'paired-off)
and those with paired electrons 'spin-paired'. Measurement of the
magnetic moment of a complex can often tell us how many unpaired
electrons are present, and this is useful information when bonding
in the complex is considered.

Chemical properties

We have already seen that in the aquo-complex which is usually

formed when a simple transition metal salt dissolves in water, the