The rare-earth metal complexes bearing NSN-bidentate beta-diimidosulfonate ligands (RNSNdipp)Ln-(CH2SiMe3)(2)(THF)(n) (R = Ph, Ln = Lu (1a), n = 1, Y (1b), n = 2, Sc (1c), n = 1; R = PhNMe2, Ln = Lu (1d), n = 1) were synthesized by treatment of the ion-pairs [Ln(CH2SiMe3)(2)(THF)(x)][BPh4] with equimolar amount of the ligand lithium salts (RNSNdipp)Li(THF)(2) (NSNdipp = S((NC6H4Pr2)-Pr-i-2,6)(2)). Addition reaction between lutetium tris(alkyl)s, Ln(Z)(3)(THF)(n) and NSNdipp gave the corresponding dialkyl complexes (ZNSN(dipp))Lu(Z)(2)(THF)(n) (Z = CH2SiMe3, n = 1 (le); Z = o-CH2C6H4NMe2, n = 0 (1f)). Deprotonation of beta-imidophosphonamido ligands H-NPNdipp and H-NPNdipp (NPNdipp = Ph2P(NC(6)H(3)(i)Pr2-2,6)(2), NPNEt = PPh2((NC6H3Pr2)-Pr-i-2,6)(NC6H4-Et-2)) with Lu(CH2SiMe3)(3)(THF)2 yielded the corresponding dialkyl complexes (NPNdipp)Lu(CH2SiMe3)(2)(THF) (2) and (NPNEt)Lu(CH2SiMe3)(2)(THF) (3). All the complexes had been structurally well-defined, and 1a, 1b, 1e, 2, and 3 were further characterized by X-ray diffraction analysis where the almost planar NSN rare-earth metal unit is C-s (or pseudo C-s) symmetry with the two alkyl groups arranging on both sides and a coordinated THF against it. Upon activation with [PhMe2NH][B(C6F5)(4)] and (AlBu3)-Bu-i, all these complexes exhibited high 3,4-regioselectivity (ranging from 91% to >99%) for the polymerization of isoprene. Moreover, the excellent isospecific selectivity up to mmmm > 99% have been achieved with complexes 1 depending on the electronics of the sulfur substituents to give crystalline polyisoprene with the highest T-m (170 degrees C) reported to date. The NPN-bidentate beta-imidophosphonamide ligated rare-earth metal complexes provide both high syndio- and iso- 3,4-selectivities (3,4 > 99%, rr = 66%, mmmm = 96%) depending on the frameworks, steric environment and geometry of the ligands. The regio- and stereo- selective mechanisms proceeded in these systems were explicated by DFT simulation.