Reaction of [(RhC5Me5)2Cl4] with methyl-lithium gives cis-[(RhC5Me5)2(µ-CH2)2Me2](2) which slowly isomerises to trans-[(RhC5Me5)2(µ-CH2)2Me2](3); pyrolysis of (2) gives propene, methane, and some ethylene.
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Isobe et al. (1981) studied this question.