where 𝑞 𝑘 (𝑁) describes the atomic charges at the respective atomic center 𝑘 of the original system and 𝑞 𝑘 (𝑁 + 1) those of the system with one additional electron. After normalization, this provides a percentage value of the additional electron density at any given atom. For clarity, we have combined the resulting atomic percentages into three molecular sub-units, i.e. the 2,2'-bipyridine ligands (bpy), the platinum-chloride fragment {PtCl2} and the POM metaloxo framework {MnMo6O24}, see Table and SI, Section 8 for computational details. Table . Percentage distribution of an additional electron taken up by POM-PtCl, POM-Ref and [Pt(bpy)Cl2] as calculated using normalized nucleophilic Fukui functions and Hirshfeld population analysis. As shown in Table , analysis of the nucleophilic Fukui function for the one-electron reduced POM-PtCl indicates that additional electron density will be distributed on the Pt-HER catalyst, either on the {PtCl2} fragment (18.8 %) or the bpy ligand (75.5 %), while only a low percentage of the additional electron is distributed on the POM framework (5.7 %). For the reference systems, the expected distributions are observed: in POM-Ref, 88.6 % of the electron density is located on the POM framework, while in [Pt(bpy)Cl2], 79.7 % are located on the bpy and 20.3 % on the {PtCl2} frag-ment (Figure and SI, Table ). This is in line with experimental electrochemical and spectro-electrochemical data, which indicate that an electron transfer to the one-electronreduced POM-PtX, i.e., a second reduction of POM-PtX occurs on the {PtX(bpy)} moiety (SI, Figures ).