It's only inm- words in Spanish. I did a little browse through my diccionario usual and found e.g., ilegal, imposible, and imbecilidad. It might could be just the Spanish disinclination to use double letters except where they represent a different sound as in ll, rr or cc where the second c is followed by an i or e but that doesn't explain why not, e.g., imortal. And we also have such words as innegable.

Go figure.