STRUCTUREAND BONDING 59
each metal atom of sodium is surrounded by (and therefore bonded
to) eight other atoms, and each atom contributes one valency
electron; clearly the number of electrons per "bond' is §. For a larger
atom with the same co-ordination number and the same number of
valency electrons, for example, caesium, the electron/bond ratio is
still |, but the interatomic distance is necessarily larger and the 'bond
strength' would be expected to be weaker. In fact, the heats of
atomisation at 298 K for solid sodium and caesium are 109 and 79
kJ mol~^1 respectively. The atoms of sodium in metallic sodium, and
calcium in metallic calcium, have almost identical sizes (calculated
for the same co-ordination number); but since calcium has two
valency electrons, the heat of atomisation is increased to 177
kJ mol~^1. Many transition metals have high heats of atomisation;
these elements have d electrons and a larger number of electrons is
available for interatomic bonds in the metals; examples of heats of
atomisation are: iron, 416 kJ mol"^1 , tungsten 837 kJ mol"^1. The
stronger bonds in transition metals give rise not only to higher m.p.
but also to greater tensile strength and hardness—hence the many
uses of these metals for practical purposes.BONDING IN TRANSITION METAL COMPLEXES
We have already noted that transition metals can readily form com-
plexes with a variety of ligands. We have also noted that, in complexes
of the main group metals, the metal-ligand bonds can be electrostatic
(i.e. ion-ion or ion-dipole), or covalent, or intermediate between
these two extremes. In transition metal complexes, the bonding can
be described on the basis of either an 'electrostatic' or a 'covalent'
model; again, the actual bonding may well be intermediate in
character. But an important feature of either descriptions must be
to take account of the d orbitals. When a transition metal ion forms
a complex with ligands, two important changes often occur; a
change of colour, and a change in magnetic properties', any theory
of bonding must account for these changes. Briefly, this is done by
postulating a split in the d orbital energy levels. In the free atom or
ion of a first series transition metal, there are five d orbitals all
having the same energy. If the metal ion is surrounded by ligands,
all the d orbital energies are raised; when there are six ligands
arranged octahedrally (or six ions of opposite change in an ionic
lattice) the d orbitals undergo an energy split as follows: