The first homoleptic lanthanide(II)-guanidinate complexes have been prepared and shown to have differing coordination geometries that depends on the size of the lanthanide metal. Synthetic route to very bulky guanidinate ligands was developed and utilized in the stabilization of novel low oxidation state main group complexes. The bulky ligands could also stabilize low coordinate lanthanide(II) complexes. The stabilizing properties of Giso- appears to be similar to the bulky β-diketiminates that is utilized in the formation of tetrahedral Yb(II) complexes. A property of the yellow 4 was observed to reduce pressure at 25 °C in the solid state for several minutes. Recrystallization of the solid from toluene afforded red crystals. NMR spectra of the related complexes are symmetrical and unchanging at temperature down to -90°C. The molecular structure of heteroleptic 4 reveals it to form dimeric units through effectively symmetrically bridging iodide ligands.