"MOF-74 is a well-studied framework with a high density of coordinatively unsaturated metal centers (also called open metal sites) in metal-oxide pyramid
clusters"
Does this mean that the cation in question has unfilled orbitals?
Yes! You could see this as an unsaturated complex in general ligand field theory, thus making the MOF active in some regard toward various ligands. This may be useful for various applications, such as gas separation/sorption, catalysis etc.