Reactions of (C5Me5)(2)Sm(THF)(2) with 1 equiv of K(ER) in THF gave in high yields the Sm(II) complexes [(C5Me5)Sm(THF)(m)(ER)(mu -C5Me5)K(THF)(n)](infinity) (m = 0 or 1; n = 1 or 2; ER = (OC6H2Bu2)-Bu-t-2,6-Me-4 (la), (OC6H3Pr2)-Pr-i-2,6 (Ib), (SC6H2Pr3)-Pr-i-2,4,6 (Ic), (NHC6H2Bu3)-Bu-t-2,4,6 (Id), or N(SiMe3)(2) (le)), in which the "(C5Me5)K(THF)(n)" unit acts as a neutral coordination ligand bonding to the Sm(II) center with the "C5Me5" part. The similar reaction of (C5Me5)(2)Yb(THF)(2) with 1 equiv of KN(SiMe3)(2) yielded the corresponding ro(II) complex [(C5Me5)Yb(N(SiMe3)(2)) (mu -C5Me3)K(THF)(2)](infinity) (If) in 90% yield. These complexes all adopt a similar polymeric structure via "intermolecular" interactions between the K atom and a C5Me5 ligand. The analogous reaction of (C5Me5)(2)Sm(THF)(2) with 1 equiv of KPHAr afforded [(C5Me5)Sm(THF)(mu -PHAr)K-(C5Me5)(THF)](infinity) (Ig, Ar = (C6H2Bu3)-Bu-t-2,4,6), in which the "C5Me5K" unit is bonded to the phosphide site with its K atom. The reaction of the silylene-linked bis(tetramethylcyclopentadienyl) samarium(n) complex Me2Si(C5Me4)(2)Sm(THF)(2) with 1 equiv of KOAr in THF yielded [Me2Si(C5Me4)(mu -C5Me4)K(THF)(n)Sm(OAr)](infinity) (Ar = (C6H2Bu2)-Bu-t-2,6-Me-4 (Ih, n = 2) or (C6H3Bu2)-Bu-t-2,6 (li, n = 1)), which can be viewed as a C5Me4/OAr-ligated Sm(II) species coordinated by the silylene-linked, neutral "C5Me4K'' ligand. The use of NaN(SiMe3)(2) in place of KN(SiMe3)(2) in the reactions with (C5Me5)(2)Ln(THF)(2) afforded the "C5Me5Na(THF)(3)"-coordinated, "monomeric" Ln(II) complexes (C5Me5)Ln(N(SiMe3)(2))(mu -C5Me5)Na(THF)(3) (Ln = Sm (1j) or YB (1k)). Reactions of the polymeric complexes 1a,e with 2 equiv of HMPA (per Sm) in THF yielded the corresponding HMPA-coordinated, monomeric Sm(II) complexes (C5Me5)Sm(ER)(HMPA)(2) (ER = (OC6H2Bu2)-Bu-t-2,6-Me-4 (2a), N(SiM3)(2) (2e)), This type of C5Me5/ER-ligated Sm(II) complexes, particularly 1a-c, showed unique reactivity toward styrene and ethylene, which can not only polymerize styrene and ethylene but also copolymerize them into block styrene-ethylene copolymers under the presence of both monomers. The less reducing Yb(II) complex If or the silylene-linked cyclopentadienyl Sm(II) complex Ih did not show an activity for the polymerization of ethylene under the same conditions, suggesting that the polymerization reaction in the present systems is initiated by dissociation of the neutral "C5Me5M" ligand (M = K or Na) from the Sm(II) center, followed one-electron transfer from the resultant C5Me5/ER-ligated Sm(II) species to an incoming monomer. As a leaving group, "C5Me5K" seemed more suitable than "C5Me5Na". Among the ER ligands, the thiolate ligand (SC6H2Pr3)-Pr-i-2,4,6 (Ic) showed the highest selectivity for the block copolymerization of styrene and ethylene, while the aryloxide (OC6H2Bu2)-Bu-t-2,6-Me-4 (la) and the silylamide N(SiMe3)(2) (le) gave the highest activity for the polymerization of ethylene and that of styrene, respectively. Possible mechanisms for the polymerization and copolymerization reactions are proposed.