Metallocene borohydride complexes {(Me2Si(Cl3H8)(2))Nd(mu-BH4)[(mu-BH4)Li(THF)]}(2) (1) and (Me2Si(2,7-tBu(2)Cl(3)H(6))(2))Nd(BH4)(mu-BH4)Li(ether)(3) (2) were prepared by reaction of the dilithium salts of silylene-bridged bis(fluorenyl) ligands with the borohydride precursor Nd(BH4)(3)(THF)(3). The solid state structures of dimeric 1 and monomeric 2 ate complexes were established by X-ray diffraction studies. We showed that these complexes used in combination with (nBu)(nOct)Mg are highly efficient for cyclocopolymerization of ethylene with butadiene leading to a new class of elastomers. Catalyst 1/(nBu)(nOct)Mg provided elastomers with a polyethylene skeleton incorporating unsaturated groups (trans double bond and pendant vinyl units) and 1,2-cyclohexane rings. These rings are formed via an intramolecular cyclization which occurs with a high trans selectivity. The investigation of catalyst 2/(nBu)(nOct)Mg has revealed that the tertio-butyl substitution in positions 2 and 7 of fluorenyl ligands influenced the microstructure of copolymers since in addition to 1,2-cyclohexane rings, 1,4-cyclohexane rings were formed. This original microstructure was characterized using 2D NMR H-1-H-1 and H-1-C-13 with direct and long-range correlations. Mechanisms of stereoselective formation of trans-1,2-cyclohexane and trans-1,4-cyclohexane rings were fully investigated.