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.
