Calculations of the ability of titanium ethylene complexes of the type, Ti:C2H4, to absorb molecular hydrogen have been performed using density functional theory. A maximum of 5H(2) molecules can be adsorbed on Ti:C2H4 thereby giving an uptake capacity of 11.72 wt%, in excellent agreement with previous experimental results reported by two of us (Phys. Rev. Lett., 100, 105505, 2008). Calculations of the vibrational frequencies in such complexes with both H-2 and D-2, Ti:C2H4(nH(2)) and Ti:C2H4(nD(2)), n = 1-5, have also been performed and the values obtained used to find the Equilibrium Isotope Effect (EIE). Measurements of the EIE are also reported and these are in excellent agreement with the EIE calculated for 5H(2) molecules adsorbed in the complex. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.