Finding the ‘Goldilocks Zone’: cationic size and tilting of carbodiimide and cyanamide anions

Abstract Solid-state carbodiimides and cyanamides are a group of compounds that generally shows motifs of layered structures, with an alternating sequence of metal cations and NCN2− anions. A study of the relationship between the ionic sizes in the cation layer and the crystal structures found is presented using the geometric characteristics of the NCN2− coordination environment. The results of this analysis reveal a ‘Goldilocks zone’ of cationic sizes, from Al3+ to Tl+, that are capable of forming stable “layered” metal carbodiimide structures. Furthermore, by employing a vectorial approach a correlation between the size difference of the cations and the degree of tilting of the NCN2− framework has been found, capable of predicting the likelihood of new phases crystallizing in such structures.