Diverse reactivity patterns of mixed tetramethyl/methylidene rare-earth complexes bearing bulky benzamidinate coligands L₃Ln₃(μ₂-Me)₃(μ₃-Me)(μ₃-CH₂) [L = [PhC(NC₆H₃ⁱPr₂-2,6)₂]⁻; Ln = Y(1a), Lu(1b)] with PhCN, alkynes, and CS₂ have been established. Reaction of complexes 1 with PhCN gave the μ₃-CH₂ addition complexes (NCN^dipp)₃Lu₃(μ₂-Me)₃(μ₃-Me)[μ–η¹:η¹:η³-CH₂C(Ph)N] [Ln = Y(2a), Lu(2b)]. Treatment of complexes 1 with phenylacetylene afforded unexpected alkenyl dianion complexes L₃Ln₃(μ₂-Me)₃(μ₃-Me)(μ–η¹:η³-PhCCMe) [Ln = Y(3a), Lu(3b)] through the insertion of rare earth methylidene into a C–H bond in a reductive fashion. However, reaction of complexes 1 and HCCSiMe₃ gave μ₃-Me protonolysis complexes L₃Ln₃(μ₂-Me)₃(μ₃-CCSiMe₃)(μ₃-CH₂) [Ln = Y (4a), Lu (4b)] in excellent yields. Treatment of complexes 1 with CS₂ led to the formation of the methyl activation complexes L₃Ln₃(μ₂-Me)₂(μ₃-CH₂)(μ₃–η¹:η²:η²-S₂CCH₂) [Ln = Y(5a), Lu(5b)]. All the new complexes were fully characterized.