New reactive, divalent lanthanoid formamidinates [Yb(Form)2(thf)2] (Form=[RNCHNR]; R=o-MeC6H4 (o-TolForm; 1), 2,6-Me2C6H3 (XylForm; 2), 2,4,6-Me3C6H2 (MesForm; 3), 2,6-Et2C6H3 (EtForm; 4), o-PhC6H4 (o-PhPhForm; 5), 2,6-iPr2C6H3 (DippForm; 6), o-HC6F4 (TFForm; 7)) and [Eu(DippForm)2(thf)2] (8) have been prepared by redox transmetallation/protolysis reactions between an excess of a lanthanoid metal, Hg(C6F5)2 and the corresponding formamidine (HForm). X-ray crystal structures of 2–6 and 8 show them to be monomeric with six-coordinate lanthanoid atoms, chelating N,N′-Form ligands and cis-thf donors. However, [Yb(TFForm)2(thf)2] (7) crystallizes from THF as [Yb(TFForm)2(thf)3] (7 a), in which ytterbium is seven coordinate and the thf ligands are “pseudo-meridional”. Representative complexes undergo CX (X=F, Cl, Br) activation reactions with perfluorodecalin, hexachloroethane or 1,2-dichloroethane, and 1-bromo-2,3,4,5-tetrafluorobenzene, giving [Yb(EtForm)2F]2 (9), [Yb(o-PhPhForm)2F]2 (10), [Yb(o-PhPhForm)2Cl(thf)2] (11), [Yb(DippForm)2Cl(thf)] (12) and [Yb(DippForm)2Br(thf)] (16). X-ray crystallography has shown 9 to be a six-coordinate, fluoride-bridged dimer, 12 and 16 to be six-coordinate monomers with the halide and thf ligands cis to each other, and 11 to have a seven-coordinate Yb atom with “pseudo-meridional” unidentate ligands and thf donors cis to each other. The analogous terbium compound [Tb(DippForm)2Cl(thf)2] (13), prepared by metathesis, has a similar structure to 11. CBr activation also accompanies the redox transmetallation/protolysis reactions between La, Nd or Yb metals, Hg(2-BrC6F4)2, and HDippForm, yielding [Ln(DippForm)2Br(thf)] complexes (Ln=La (14), Nd (15), Yb (16)).