Zn4 clusters: Figure shows that, of the clusters incorporating four Zn 2+ cations, the lowest energy structures are dominated by cyclic structures, culminating in the fully coordinated [Zn 4 Im 12 ] -4 cycle, which constitutes the most stable structure of this class (-4.74 eV). Remarkably, the analogous tetrahedrally coordinated linear arrangement [Zn 4 Im 13 ] -5 , shown in Figure , right, is significantly less stable (-4.27 eV). Already cyclic clusters containing four Zn cations exhibit Zn-Im-Zn unstressed angles that allow the intermolecular forces stabilizing these more compact oligomers that are topologically similar to the extended ZIF structures. It can be noted that the NO - 3 counterions transiently stabilize the Zn 2+ sites, while imidazolate units progressively incorporate to the cluster, leading to three-fold, then four-fold coordination arrangements. An interesting situation arises in the [Zn 4 Im 8 ] 0 complex, where a cycle with two tetrahedral (ZnIm 4 ) sites, stabilized by two nitrate anions (-4.21 eV), is very similar in energy to the planar configuration in which all four Zn cations are three-fold coordinated to Im -(-4.20 eV). However, the difference is not significant enough to draw any particular conclusion here. Also noticeable is the observation of a [Zn 4 Im 9 ] -configuration in which an imidazolate cation bridges two Zn sites across the Zn4 ring.