1 Zn(II)P (Figure ) belongs to the 0 U group, in which electron repulsion is sufficiently large to both induce a distortion and to open a band gap. This corresponds to a closed-shell electronic structure in cyclobutadiene at its equilibrium geometry (Figure ). The relatively high U/t ratio in 0 U is reflected in large effective masses (corresponding to poorer electron mobility; Table ), compared to similar porphyrin-based systems. 42 Table . Properties of selected gapless metalloporphenes. In the lowest triplet state of cyclobutadiene, both rotationally related orbitals A and B are singly occupied with same-spin electrons, resulting in a Baird aromatic D4h geometry (Figure bottom). If U is sufficiently large, the triplet can even become the ground state, as found in some indenofluorenees. This is analogous to the electronic structure of 0 UU metalloporphenes (Figure ), which must be spin-polarized. As the frontier bands in 0 U are spatially almost identical, these metalloporphenes have similar electron and hole mobilities. Due to their Baird aromatic character, they tend to have significantly larger band gaps and smaller BLA than 0 U metalloporphenes (Table ). Groups -1 and 1. When the transition metal inserted into porphene has a vacant low-lying d orbital, an electron can be transferred from the π-to the d-system, resulting in a group -1 metalloporphene (Figure ). One such example is cobalt porphene in its quartet state (Figure ). If we assume cobalt is divalent, it will have a singly occupied low-energy dxy orbital which can accept a π-electron from porphene, thus making 4 Co(I) + P -. Conversely, when the transition metal has an occupied highenergy d-orbital, d → π single-electron transfer can occur, forming a group 1 metalloporphene. An obvious example is scandium porphene: in the absence of axial ligands, scandium will oxidize to Sc(III) by giving its electron to the π-system to form 2 Sc(III) 3+ P -(Figure ).