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The journal of physical chemistry letters 13(36), 8447 - 8454 (2022). doi:10.1021/acs.jpclett.2c02157, Through a series of high pressure diamond anvil experiments, we report the synthesis of alkaline earth (Ca, Sr, Ba) tetrahydrides, and investigate their properties through Raman spectroscopy, X-ray diffraction, and density functional theory calculations. The tetrahydrides incorporate both atomic and quasi-molecular hydrogen, and we find that the frequency of the intramolecular stretching mode of the $H^{𝛿−}_2$ units downshifts from Ca to Sr and to Ba upon compression. The experimental results indicate that the larger the host cation, the longer the $H^{𝛿−}_2$ bond. Analysis of the electron localization function (ELF) demonstrates that the lengthening of the H–H bond is caused by the charge transfer from the metal to $H^{𝛿−}_2$ and by the steric effect of the metal host on the $H–H$ bond. This effect is most prominent for BaH$_4$, where the precompression of $H^{𝛿−}_2$ units at 50 GPa results in bond lengths comparable to that of pure $H_2$ above 275 GPa., Published by ACS, Washington, DC