For a few years it looked as if chemists would be forced to choose either Pauling's
view or Mulliken's. But at root the two approaches were not as different as they seemed.
Both were based on Schrödinger's wave equation, and Slater and others found in the
mid-1930's that if the mathematics was carried through far enough, the two approaches
ended up providing the same results. It was rather like the choice physicists had
to make between Heisenberg's matrix approach to quantum mechanics and Schrödinger's
wave equation: Although seemingly very different, both were paths to the same destination.
The choice of the paths depended on which was easier to use and which worked better
in a given situation.
Pauling, of course, thought his was the better approach to understanding the chemical
bond. He understood that the molecular-orbital approach was useful - he had employed
it in some cases while searching for a breakthrough on the chemical bond - but he
largely dropped it when he found how to make his own variations on the Heitler-London
theme work in 1931. Once Slater showed the essential equivalence of his and Mulliken's
methods, Pauling saw no need to refer to the molecular-orbital approach. His ideas
worked out of what chemists already believed about the chemical bond; Mulliken's were
by comparison anti-intuitive and, Pauling thought, confusing to students.
And Pauling's notion of the chemical bond took off, while Mulliken's languished in
relative obscurity. There were several reasons, prominent among them the fact that
Pauling was an eloquent teacher and a persuasive writer who knew how to communicate
in language chemists could understand. When Pauling spoke, the valence-bond approach
seemed like revealed wisdom. When Mulliken talked, people went to sleep. He was a
terrible teacher, ill at ease in front of crowds, his voice almost inaudible. He refused
to pander to his chemistry students, and his lectures were notoriously digressive,
heavy with mathematics, and hard to follow. He was not much better in print. As the
years went by, Mulliken and a small band of followers would continue to improve their
molecular-orbital approach, refining the equations and using it successfully to attack
a number of problems. Twenty years later, a new generation of chemists would come
to prefer it over Pauling's approach. But in the 1930's, Mulliken's ideas would be
lost in a blizzard of results blowing out of Pasadena.