The adsorption of conjugated molecules on graphene is an attractive way of developing new two-dimensional materials with tailored properties. We investigate here the adsorption of metal-based porphyrins on graphene. The energetics of adsorption of different metalloporphyrins are explored and the electronic properties are analyzed, showing calculated band gaps in good agreement with previous reports. Then, we address the design of adsorbates to give better adsorption and band gaps. We show that adsorption depends on both the atomic radius and the d-orbital occupancy. The band gaps obtained correlate with the proximity of the 3d orbitals to the Fermi level.