Arising from the unique properties of rare-earth metal ions, such as strong Lewis acidity, variable ionic radius, stable +3 oxidation state, C-H activation reactions promoted by rare-earth metals display distinct reaction pathways, substrate scopes as well as chem-, regio-or stereoselectivities with late-transition metal catalysis. Taking use of strong heteroatom-affinity of rare-earth metal ions and high reactivity of RE-C bond towards unsaturated carbon-carbon bonds, rare-earth organometallic complex enables to transform heteroatom-containing substrates and alkene or alkynes into value-added molecules in a highly efficient and atom-economical manner. Except heteroatoms, the interplays of rare-earth metal ions with -bonds of alkenes and arenes have been disclosed as well. Such interactions also take play an important role at the alkene insertion step in the transformation of heteroatomcontaining substrates with alkenes as well as olefin polymerization. In addition, rare-earth metal allyl species has been well-characterized 8 and postulated as the key intermediate in the stereoselective (co)polymerization of dienes and styrenics. 9 Nevertheless, the use of RE... interaction for C-H activation or the application of rareearth metal allyl species in catalytic C-H activation were elusive. 10 Inspired by the above achievements and as part of our continuous efforts in rare-earth metal catalysis, 11 we sought to verify whether imidazolin-2-iminato rare-earth metal alkyl complexes developed by Tamm and our group 11b-d can be used in the allylic C(sp 3 )-H functionalization reactions of alkenes. As depicted in Scheme 1c, the interaction between rare-earth metal ions and -bonds of alkene may increase the acidity of allylic proton, 2b enabling to form a nucleophilic allylic rare-earth metal species through basic ligand-involved deprotonation. The insertion of allylic rare-earth metal intermediate with an electrophile, eg. alkene, led to the formation of rare-earth metal alkyl species. The protonation of rare-earth metal alkyl species with another molecular alkene substrate liberates the alkene-coupling product and regenerates the catalyst. There are several challenges associated with the above transformation: (1) the selective activation of allylic position through relatively weak REalkene interaction 8a-b ; (2) the influence of internal alkene configuration and highly fluxional behaviour 14 of allylic rare-earth metal intermediate on the reactivity and stereoselectivity; and (3) the control of cross-coupling of two different alkenes instead of homo-coupling of alkenes. Herein, we wish to disclose our endeavour along this line. Cationic imidazolin-2-iminato (NHI) scandium(III) alkyl complexes were identified to be efficient promoters for the allylic alkylation of alkenes with styrenes as well as dimerization of internal and terminal alkenes. The corresponding coupling products were generated in an atom-economical and efficient manner with moderate to good yields and high E/Z ratio (64 examples, 16-99% yields, 1:3.4->19:1 E/Z ratio). Remarkably, the NHI ancillary ligand, basic ligand and Lewis base additive significantly affected the reactivity and E/Z selectivity of alkene coupling reactions. In addition, toluene derivatives were suitable substrates. Based on the experimental studies and theoretical calculations, a possible catalytic cycle involving the assistance of Lewis base was provided to understand the reaction mechanism and E/Z selectivity.