Modern inorganic chemistry

60 STRUCTURE AND BONDING

Energy

The 5 3d orbitals \

in the atom or ion V.

surrounded by ligands

The 5 3d orbitals The splitting of the

in the free atom 3d orbitals for six

or ion octahedral ligands

The magnitude of the energy split, A£, determines how the electrons
will be distributed between the d orbitals (and hence the magnetic
properties, p. 229). Moreover, electrons can be promoted from the
lower to the higher energy level by absorption of light; the frequency
of the absorbed light is directly related to A£; and hence this latter
quantity greatly influences the colour of the complex.
The detailed theory of bonding in transition metal complexes is
beyond the scope of this book, but further references will be made to
the effects of the energy splitting in the d orbitals in Chapter 13.

THE COLOUR OF INORGANIC COMPOUNDS

Many transition metal compounds owe their colour to absorption
of light which causes electrons to move between d orbitals of different
energy, these orbitals being essentially those of the central metal
atom or ion. However, colour is also seen in some main-group
elements (for example, iodine), some main-group compounds (e.g.
lead(II) oxide, yellow), and some transition metal complexes where
there are either no electrons initially in d orbitals (e.g. the manganate-
(VII) ion, MnO^), or the d orbitals are completely filled (and hence
electrons cannot move between them) (for example, copper(I) oxide,
yellow-red; mercury(II) oxide, red). A detailed discussion of the
causes of colour in these compounds is out of place in this book, but
essentially the colour is due to electrons moving between different
atoms or ions. In most compounds, the energy required for move-
ment of electrons (sometimes referred to as charge-transfer) is large,
and the frequency of light required is consequently in the ultra-violet
region of the spectrum. But in the coloured compounds already
mentioned, the energy is sufficiently low to cause absorption of light