Several new mono- and dinuclear complexes of CoII and NiII with chelating guanidine ligands were synthesized. The molecular structures for most of the complexes in the crystalline state were derived from single-crystal X-ray diffraction, and some characteristic structural details are discussed. The molecular magnetism was further studied by superconducting quantum interference device (SQUID) measurements. In addition to the experiments, quantum chemical calculations were carried out. Finally, the paramagnetic NMR spectra for one of the complexes are discussed, which shows that 1JC,H correlation experiments are possible.