b2

[ryco]

Would a minor second be considered a leading tone?
ex: m2 > R Bb7(b5) to A

[Kam]

In certain contexts

One example:
ii-7 bii-7b5 Imaj7

instead of a standard ii-7 V7 Imaj7

This method is called a tri-tone substitution and can be used on almost any ii - V - I progression, but it does get kind of old if it's overused.