As a result of the different binding strengths of different carboxylic acids, the low solubility of BDC in DMF/formic acid solutions, and the ratio of formic acid to BDC in these reactions being in excess of 40:1, it is not surprising that the BDC linkers may be outcompeted initially by formate (or hydroxide), and so coordinate to clusters later in the reaction. As hydroxide groups are more strongly coordinated than carboxylate groups, the joining of the single clusters to form the double cluster could therefore occur before the terminating ligands can exchange (in the linkercontaining reactions) for dicarboxylate linkers (which would either "trap" the single cluster species or slow down the cluster conversion). Alongside our discovery that the growth of the crystalline framework occurs simultaneously with, rather than after, the linking of clusters together, this further suggests that once the clusters link up into a framework, the dicarboxylate linkers are less labile, and that dissociation (required for rearrangement) is kinetically and entropically less favourable. This would therefore mean that, despite the lower temperature used here, the successful formation and linking-up of the double cluster determines the formation of the hcp framework over the fcu.