Modern inorganic chemistry

STRUCTURE AND BONDING 47

NOMENCLATURE FOR COMPLEX IONS

When naming complex ions the number and type of ligands is
written first, followed by the name of the central metal ion. If the
complex as a whole has a positive charge, i.e. a cation, the name of
the central metal is written unchanged and followed by the oxida-
tion state of the metal in brackets, for example [Cu(NH 3 ) 4 ]^2 +
becomes tetra-ammine copper(II). A similar procedure is followed
for anions but the suffix '-ate' is added to the central metal ion;
some examples are:
[Fe(CN) 6 ]^3 ~ hexacyanoferrate(III)
[HgI 4 ]^2 " tetraiodomercurate(II)
[Co(NO 2 ) 6 ]^3 ~ hexanitrocobaltate(III)

THE STRENGTH OF COVALENT BONDS: BOND ENERGIES

The energy required to break the bond between two covalently
bonded atoms is called the 'bond dissociation energy'. In polyatomic
molecules this quantity varies with environment. For example,
ammonia has three N—H bond dissociation energies:

NH 3 (g) -* NH 2 (g) + H(g) 448 kJ mol ~^1

NH 2 (g) -> NH(g) + H(g) 368 kJ mol"J

NH(g) -> N(g) + H(g) 356 kJ mol~^1

For many purposes, for example the estimation of approximate
heats of formation (p. 63), it is sufficient to have an average value.
This average of the bond dissociation energies is called the average
thermochemical bond energy or (more commonly) simply the bond
energy*.
Bond energy values can be obtained from thermochemical calcu-
lations (p. 72) and a number are included in Table 2.10 together with
the compound used in the calculation.
In most covalent compounds, the strong covalent bonds link
the atoms together into molecules, but the molecules themselves
are held together by much weaker forces, hence the low melting
points of molecular crystals and their inability to conduct electricity.
These weak intermolecular forces are called van der Waal's forces;
in general, they increase with increase in size of the molecule. Only

• Strictly, these values are bond enthalpies, but the term energies is commonly
  used. Other descriptions are: 'average standard bond energies', 'mean bond energies'.