Reaction of the diazafluorenylidene-sub-stituted phosphaalkene 1 with [Cp*(2)Ln] [BPh4] (Ln = Dy, Tb, and Gd), followed by subsequent reduction with KC8, afforded complexes Cp*Dy-2(N,N'-1) (2), Cp*Tb-2(N,N'-1) (3), and Cp*Gd-2(N,N'-1) (4) in moderate yields, in which the phosphaalkene moiety is in the radical-anion state. Complexes 2 and 3 represent the first lanthanide metal complexes of a heavy main-group element-based radical. They have been characterized by single-crystal X-ray diffraction, UV/vis spectroscopy, and superconducting quantum interference device measurements. The magnetic studies reveal that the phosphorus-radical centers have a quite weak antiferromagnetic interaction with the lanthanide ions in 2-4. Furthermore, complex 2 shows slow magnetic relaxation behavior.