While open metal sites in MOFs capable of engaging in strong orbital interactions with H2 are rare, the trigonal pyramidal Cu I sites in Cu I -MFU-4l (Cu2.2Zn2.8(Cl/O2CH)1.8(btdd)3) have been reported to exhibit an excessively strong H2 adsorption enthalpy of -32 kJ/mol. The electron rich d configuration of Cu I with a C3v trigonal pyramidal geometry results in high-lying occupied dyz and dxz orbitals suitable for π-backbonding with H2. We note that Cu I -MFU-4l is the only example of a material that exhibits strong H2 binding up to near stoichiometric H2 loading per Cu, although other Cu I -based frameworks have been reported with different Cu I coordination geometries. The significance in the geometry for π-backbonding by trigonal Cu I sites can be rationalized by the energy level of the π-symmetric dyz and dxz orbitals with ligand field theory.