A detailed comparison of structural parameters obtained via microwave rotational spectroscopy in a systematic study of protic acid-haloethylene heterodimers is used to investigate the forces contributing to intermolecular interactions. Conclusions reached using structural data and chemical intuition are supplemented with information obtained from quantum-chemistry calculations to refine the understanding of the various contributions to complex formation. The observed structures, representative of the global minimum on the potential energy surface, are found to reflect a balance between optimal electrostatics and steric requirements, or in other words, how well the two interacting molecules fit together. Structural trends are rationalized in terms of familiar chemical concepts of the electrophilicity or nucleophilicity of interaction sites as modulated by electron-withdrawing and electron-donating groups along with the geometric requirements for optimal interactions between the two molecules.