Whenever the open metal site is in LS and the d 2 z orbital is empty, the geometrical optimization results in a configuration with H2 bound horizontally on top of the metal. This is the case for bpac and pz MOFs with M=Fe in S=0, and for the pz case for M=Mn in S=1/2. The corresponding large binding energies, i.e. above 1 eV, are consistent with the short metal-gas bond lengths and a Kubas type of binding (see results in Tables and). The Kubas interaction here involves the σ-donation from the H-H σbonding orbital into the empty d z 2 orbital of the metal, and the simultaneous π-back-donation from a filled metal d orbital into the σ * antibonding orbital of the molecule. It results in a lengthening of the H-H bond as found in this work. The computed H-H bond distance of the gas phase molecule is 0.762 Å. For the bpac and pz cases and Fe in LS, the adsorbed H2 molecules exhibit H-H=0.830 Å. For Mn, we compute a shorter values of H-H=0.823 Å in the bpac case, and H-H=0.818 Å in the pz case, and a larger H2-metal distance as compared with Fe (see Tables ?? and 3). The case of partially filled d z 2 in Ni (bpac) in HS is also consistent with a Kubas interaction: a slight weakening of the H-H bond is predicted with a H-H=0.773 Å and the average H-Ni distance 2.179 Å.