The preparation of a series of homoleptic, four-coordinate lanthanide(II) complexes, [Ln(Priso)2] (Ln = Sm, Eu or Yb) incorporating the bulky guanidinate ligand Prisoa?? ([(ArN)2CNPri2]a??, Ar = 2,6-diisopropylphenyl) is described. X-ray crystallography shows the complexes to be isostructural and to exhibit coordination geometries midway between tetrahedral and planar. Comparisons between the geometries of the complexes and those of the bulkier systems, [Ln(Giso)2] (Gisoa?? = [(ArN)2CNCy2]a??, Cy = cyclohexyl) are discussed. Attempts to prepare the less hindered amidinate complexes, [Ln(Piso)2] (Pisoa?? = [(ArN)2CBut]a??), were not successful, but did give rise to the heteroleptic complex, [(1-N,6-Piso)Sm(THF)(-I)2Sm(1-N,6-Piso)]. Whereas the amidinate ligands in this complex chelate the samarium centre in an 1-N,6-Ar-fashion, in the closely related complex, [ (2-N,N-Priso)Yb(THF)(-I) 2], the ytterbium atoms are 2-N,N-chelated by the guanidinate ligand. The facility of the planar four-coordinate complex, [Sm(Giso)2], to act as a one-electron reducing agent towards a variety of unsaturated substrates has also been explored. The complex has been shown to selectively reductively couple CS2, via a Ca??S bond formation, to give the samarium(III) dimer, [(Giso)2Sm(-2-:2-S2CSCS)Sm(Giso)2].