Factors affecting orbital overlap, boron halides

There are three main factors which affect orbital overlap, the first two have been described earlier.

1. Symmetry

2. Proximity of energy levels

3. Size of the orbitals

This third factor helps explain why boron trifluoride is a weaker lewis acid than boron trichloride:

From simple electronegativity arguments we would expect BF₃ to be a stronger acid, since electrons would be pulled away from the boron atom hence encouraging it to accept an electron pair. 

The reason BF₃ is weaker is that the halogen atoms contain electrons in p orbitals, which in boron trihalides have a small overlap with the empty p orbital of boron. This effect is weaker in heavier halogens because of the size difference:

The p-orbital overlap can also represented as resonance structures:

This overlap/resonance/extra stability cannot happen in an adduct, since in an adduct the orbital is already occupied by a lone pair.