Linus Pauling: I should like to discuss one of the standard coordination complexes in this respect.
Here, we have a model representing the complex CoNH3 six times, triple plus, cobalt three hexammine. Six ammonia molecules attached to
a central cobaltic ion. This group of atoms, this complex ion, has a total charge
plus three, but this charge is not to be considered as located on the cobalt atom.
If I draw the regular structure for the complex, represented as involving a cobalt
ion, I can say cobalt three-plus, NH3 out here, NH3. I might draw this showing covalent bonds. Then N, H, H, H, N, H, H, H, and then
out in front here, N, H, H, H, and behind, N, H, H, H. Now with the charge plus three,
if these were normal covalent bonds, a pair of electrons on the nitrogen would be
shared with the cobalt, six electrons would be transferred to cobalt, and the charge
would become minus three. But, in fact, the position of cobalt in the electronegativity
scale is such that we expect the cobalt-nitrogen bonds to have about fifty percent
covalent character, fifty percent ionic character.
That is just enough then. Six half-electrons transferred, half of our covalent bonds,
on, in each of the six directions, to neutralize the three plus charges, leave the
cobalt atom with the zero charge, and each nitrogen atom has then a charge of plus
one-half. But this isn’t the end of the story. The nitrogen-hydrogen bonds, as indicated
by the difference in electronegativity of nitrogen and hydrogen, hydrogen at 2.1,
the nitrogen-hydrogen bonds have about one-sixth partial ionic character, so that
there is a charge of plus one-sixth of an electronic charge, of magnitude of electronic
charge on each hydrogen atom, and this neutralizes the charge on the nitrogen, leaving
it zero. Consequently, the total charge of plus three for this complex ion is divided
up into eighteen little charges of plus one-sixth each which are located on the eighteen
hydrogen atoms that are on the periphery of this complex. This is, of course, a nice
situation because a distribution of charge of this sort corresponds to electrostatic
stability.
If we have a metallic sphere that is electrically charged, all of the charge is on
the surface of the sphere, even though it is a solid metallic sphere, the charge,
the elements of charge repel one another until they reach the surface. In fact, I
think that we may say that in aqueous solution, the hydrogen bonds that are formed
by these hydrogen atoms with surrounding water molecules neutralize these charges
to some extent and put the charges in still smaller increments still farther away
from the central part of this complex.
There’s another aspect of the structure of this complex that I want also to mention.
That is the utilization of the orbitals. Let me, let us consider the, let us consider
the orbitals that are available for cobalt. In the periodic table of the elements,
cobalt is seen with atomic number twenty-seven. Cobalt plus three, with three charges,
well, I’ve erased the plus three, cobalt plus three with three electrons removed from
it would have twenty-four electrons, that is, six more than the number for the argon
structure. If we consider the five 3d orbitals, we may place these six electrons
in three of the orbitals. Then, we have 4s and the three 4p orbitals. Here we have
left on the cobalt atom, six orbitals in the argon shell, krypton shell, krypton shell
is the shell with nine orbitals. Three are used by the six unshared electrons of
cobalt. Six orbitals are left. These orbitals are of such a nature that they are
nicely-suited to the formation of bonds, six bonds pointing toward the corners of
a regular octahedron. These six orbitals are orbitals of this sort. So that we have
a nice story, covering, accounting in a satisfactory way for the existence and stability
of the cobaltic hexammine complex ion.
In fact, the electro-neutrality principle, the, which is the striving of every atom
to achieve an electric charge that is close to zero, sometimes partial ionic character
of bonds keeps it from being exactly zero, but by increasing the ligancy, one, it
is often possible for the charge to be decreased closer to zero. This electro-static,
this electro-neutrality principle explains in a pretty satisfactory way why it is
that so many elements in the periodic table, especially in the transition region,
form ions in aqueous solution with charge plus two or plus three. Iron, cobalt, nickel,
copper, zinc, manganese, chromium, the principle ions, cations, of these metals are
those in which the charge on the ion is plus-two or plus-three. These metals all
have electro-negativity around in this region. The amount of partial covalent character
of the bonds is somewhere around one-third to one-half, which is just enough, with
octahedral coordination, to neutralize the charge of plus two or plus three on the
central ion and move the charge out toward the periphery of the hydrated ion in the
case of an ion in aqueous solution.
