It is pertinent to note that, under typical synthesis conditions, there is an excess of imidazolate anions in solution with respect to the metal cations. Hence, cluster growth by addition of Im -anions is kinetically more likely than by addition of cations. One further reason supporting that Im -binding to the growing clusters may be faster than Zn 2+ binding is related to the two-fold charge difference, which should lead to lower electrostatic energy barriers for imidazolate addition. Moreover, while Im -anions stabilize with two-fold coordination, Zn 2+ cations require a higher number of coordinating ligands to be stabilized. For these reasons, our calculations suggest that there will a faster growth by addition of imidazolate or nitrate anions (in the downward direction of the chart in Figure than by addition of zinc cations (moving left to right in Figure ).