Changes in the polarisation of the same subunit within different pentalenides can also be explained by the NBO analyses. For example, the observed NMR shifts of the wingtip protons in the unsubstituted parts of 9 and 10 were shifted slightly downfield from those in 1. This is due to the fused nature of the two (hypothetical) subunits, since some charge density was transferred from the unsubstituted Cp 1 into the arylated Cp 2 where it may be delocalised into the electron-withdrawing substituents. The NBO charges also reflected the electronic influence of the alkyl substituent in 12 well, as the C 1 carbon was markedly less negatively charged than the C 3 and C 4 carbon atoms due the +I effect of the methyl group (Figure ). As expected, the methyl substituent itself did not accept a significant amount of charge from the pentalenide core, but its presence in 1-position lead to a slightly higher charge density in the 3-phenyl group on the same ring compared to the two phenyls on the other subunit. Due to the electron-donating influence of the methyl group Cp 1 showed a higher charge density than Cp 2 , consistent with its wingtip proton resonating more highfield (6.19 ppm) from that of the diphenyl substituted subunit (6.67 ppm). Due to the interaction of the fused five-membered rings this polarisation caused the H 5 chemical shift to move upfield compared to that of 2.