The isomeric structures for adsorbed hydrocarbons found by global optimization show the consequences of these electronic structure trends. If we compare the first and last isomers of Pt 4 C 2 in Figure (a) with the isomers of Pt 4 GeC 2 in Figure (b), we see structural sim-ilarities, even though these clusters have different reactivities. The binding sites for ethane and ethylene on Pt 4 C 2 and Pt 4 GeC 2 are similar (Figure , S5), consistent with the hypothesis that Ge influences reactivity electronically. There is also a significant Ge effect on the energetics of the isomers with separated vs bonded C atoms. In Pt 4 C 2 the C-C bonded isomer lies +0.17 eV above the C-C separated isomer, while in Pt 4 GeC 2 the corresponding gap is reduced to +0.06 eV due to C-C bond strengthening by electron transfer from Ge. Furthermore, the equivalent barrier for breaking the C-C bond in Pt 4 C 2 (1.16 eV) is lower than that for Pt 4 GeC 2 (1.33 eV). See Table , and Figure for details.