The vibrational properties of interstitial silane (SiH4)i and silyl (SiH3)i molecules in crystalline silicon are calculated using a first-principle, cluster-based, spin-polarized local-density method. The Si-H stretch modes are found to be redshifted by ∼300 cm-1 from those of the isolated molecule, which lie around 2200 cm-1. These results refute recent suggestions that modes observed around 2200 cm-1, and previously assigned to hydrogenated vacancy defects, are due to these interstitial molecules.