The interaction of the substituted diene monomers isoprene, C 5 H 8 , and myrcene, C 10 H 16 , with a lanthanide was probed by examining their chemistry with (C 5 Me 5 ) 2 Sm. CH 2 dCHC-(Me)dCH 2 reacts with (C 5 Me 5 ) 2 Sm to form the bimetallic [(C 5 Me 5 ) 2 Sm] 2 [µ-η 2 :η 4 -CH 2 CHC-(Me)CH 2 ], 1. Both (C 5 Me 5 ) 2 Sm components in 1 exhibit trivalent metrical parameters. One Sm is oriented toward all four of the dienyl carbon atoms at distances of 2.604(9)-2.799(8) Å, and the other Sm interacts with only two carbons at distances of 2.544(9) and 2.674(9) Å. CH 2 dCHC(dCH 2 )CH 2 CH 2 CHdCMe 2 reacts with (C 5 Me 5 ) 2 Sm to form [(C 5 Me 5 ) 2 Sm] 2 [µ-η 2 : η 4 -CH 2 CHC(CH 2 )CH 2 CH 2 CHCMe 2 ], 2, which is similar in structure to 1. The double bond of the pendant olefinic substituent attached to the diene in myrcene does not coordinate. The structures of 1 and 2 are compared to the (C 5 Me 5 ) 2 Sm/PhCtCPh bimetallic reaction product [(C 5 Me 5 ) 2 Sm] 2 [µ-η 1 :η 1 -PhCCPh], 3, which also contains two (C 5 Me 5 ) 2 Sm units attached to an unsaturated hydrocarbon, but has equivalent trivalent (C 5 Me 5 ) 2 Sm units. Each Sm in 3 interacts with the reduced (PhCtCPh) 2primarily through a 2.52-2.54 Å Sm-C bond, although an ortho phenyl carbon is oriented toward each metal at a distance of >2.96 Å.