We report here our high-resolution neutron powder diffraction and neutron vibrational spectroscopy study of CaSiD1+x along with first-principles calculations, which reveal the deuterium structural arrangements and bonding in this novel alloy hydride. Both the structural and spectroscopic results show that, for x > 0, D atoms start occupying a Ca3Si interstitial site. The corresponding Si-D bond length is determined to be 1.82 Å, fully 0.24 Å larger than predicted by theory. Thus, our neutron measurements are at odds with the strongly covalent Si-H bonding in CaSiH1+x that such calculations suggest, a result which may have implications for a number of ongoing studies of metal-hydrogen systems destabilized by Si alloying.