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"Valence and Molecular Structure," Lecture 3.

"Valence and Molecular Structure," Lecture 3. 1957.
Produced for the Institutes Program of the National Science Foundation. Robert and Jane Chapin, producers.

Lecture 3, Part 3. (7:15)

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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.


Associated: Linus Pauling, Robert Chapin, Jane Chapin, National Science Foundation
Clip ID: 1957v.2-03

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Creator: National Science Foundation
Associated: Linus Pauling, Robert Chapin, Jane Chapin

Date: 1957
Genre: video
ID: 1957v.2
Copyright: More Information

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