I would think 'covalent' bonding and thus creating a 'new' molecule (thus the term molecular spectroscopy) that has its own unique spectrum.  However, most salts appear (exception is NaCl, CsCL...) in FTIR but I think the spectrum is due to each specific ion.

It's far from simple to answer this. In general, a change in the chemical environment in a molecule will change the vibrational modes of its bonds and hence changes in its IR spectrum. Thus for example you can identify hydrogen bonding when it happens.

You have to do some serious reading about what IR can do and what the spectra tell you.