Valence electrons, primary steric effects

Primary steric effects are due to repulsions between electrons in valence orbitals on adjacent atoms which are not bonded to each other. They are believed to result from the... 

Primary steric effects are due to repulsions between electrons in valence orbitals on atoms which are not bonded to each other. They are believed to result from the interpenetration of occupied orbitals on one atom by electrons on the other resulting in a violation of the Pauli exclusion principle. All steric interactions raise the energy of the system in which they occur. In terms of their effect on chemical reactivity, they may either decrease or increase a rate or equilibrium constant depending on whether steric interactions are greater in the reactant or in the product (equilibria) or transition state (rate). 

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