Identifying the form and role of the chemical species that traverse the stages of crystallization is critical to understand the formation process of solids. Such information is crucially lacking concerning the inorganic species involved in the formation of coordination polymer materials. Herein, we report the use of the combination of in situ atomic force microscopy and mass spectrometry to identify preformed complex cadmium ethylimidazole-containing solution species in the growth solution of the cadmium 2-ethylimidazolate metal-organic framework, CdIF-4, and show that they are critical in the surface nucleation of crystal growth of this material. Indeed, surface nucleation appears only to be instigated by these complex inorganic species and not by the more common simple addition of organic ligand molecules only. Such greater understanding of the solution species and their role in crystallization will guide future synthesis of designed functional coordination polymer materials.