The presence of C2 does, however, alter the electronic structure of the cluster, apparently stabilizing the system, as it collapses the Pt4Ge isomer distribution to just the one structure in Fig. -a thermodynamic sink. The C2 unit acts as an oxidant, developing a net negative charge of -0.42 e, while the Ge atom has a +0.80 e positive charge, i.e. it loses further electron density compared to Ge in Pt4Ge. The overall charge on the Pt4 moiety remains negative, decreased slightly from the values typical for Pt4Ge isomers, but is still higher than that for Pt4/alumina (Figs. and). The interaction of C2 with Pt4Ge is strikingly different from its interaction with Pt4, as shown in Fig. . To investigate the electronic structure differences between the Pt4C2 and Pt4GeC2 clusters in more detail we calculated the molecular orbitals (MOs) of unsupported anionic gas-phase models in which the coordinates of all atoms were held fixed at values determined for the supported clusters (Fig. and). These minimal models are validated by their Bader charge distributions, which show the same trends as their supported counterparts . The MO diagrams (Fig. ) show that the Ge atom in Pt4GeC2 is in the +2 oxidation state; the first three unoccupied MOs are all Ge-Pt antibonding combinations with predominant Ge 4p character, while the Ge 4s-based MO is found far below the HOMO in energy. Of further interest is the LUMO of Pt4C2, which corresponds to the HOMO in Pt4GeC2 after accepting an electron pair from Ge. This orbital has clear C-C π bonding character, so the effect of adding Ge to Pt4C2 is to strengthen the C-C bonding in the partially coked cluster. While Pt4C2 and Pt4GeC2 are structurally similar, have similar total amounts of electron transfer from the support, and feature the partially negatively charged C2 unit, they differ in the source of electron transfer to C2: In Pt4GeC2, C2 receives electrons largely from Ge, leaving Pt still quite anionic. In Pt4C2, C2 draws electrons from Pt, leaving it closer to the charge neutral state. Thus, it appears that in Pt4Ge/alumina, Ge and C2 are in a synergistic electronic relationship that preserves the net negative charge on the Pt4 moiety. Hence, Pt sites are neither blocked nor significantly changed electronically in Pt4GeC2/alumina as compared to Pt4Ge, and therefore, it is not surprising that Pt4GeC2 retains the C2D4 binding properties seen in the TPD.