A personal reminiscence about theories used and misused in structural chemistry

In the last half-century, chemists acquired enough information about the structures and force fields of molecules to formulate what appeared to be successful theories (better called models) of molecular properties. For various reasons, chemists relied particularly heavily on studies of small hydrogen-containing compounds to shape their thinking. As discussed in the following text, this led to some inappropriate generalizations. In my first studies of molecular structure by gas-phase electron diffraction (GED), I was fortunate to find contradictions to the theories proposed at the time. What follows are reminiscences of how that came about and what happened subsequently. The main lesson to be earned, at least for molecules with central atoms that are not extremely large, is that the principal forces governing molecular structure are neither hybridization nor even the popular valence shell electron pair repulsions, explanations still taught routinely in chemistry courses but, instead, are the forces between geminal atoms resulting in the close-packing of ligands.