Crucially, these results demonstrate the presence of qualitatively different electronic behavior in the Cs 2 SnX 6 and Cs 2 TiX 6 families, where despite retaining the same cation valence, the change in frontier orbital character upon substitution of Sn 4+ with Ti 4+ dramatically alters the electronic structure and optical absorption. From the band structures in Fig. and values in Table , it is clear that the electron effective masses (m e ) dictate the exciton behavior in this family, with the weak dispersion and strong real-space localization of the flat d orbital conduction bands in Cs 2 TiX 6 , aided by the zero-dimensional crystal structure, yielding strong electron-hole interactions. This strong excitonic renormalization of the optical absorption in Cs 2 TiX 6 explains the origin of longstanding discrepancies between experiment and theoretical models of their electronic structure. Moreover, these findings serve as a warning of the changes that can occur when employing ionic substitution as a materials design approach, when such strategies involve changes in valence orbital character.