Intrigued by the similarities in catalyst performance across ROCOPs, we turned to DFT calculations (ESI Section S9 and S10) to characterize the mechanism of ROCOP at the ωB97XD,THF/BS2//ωB97XD,THF/BS1 level of theory. LCr Rb was chosen as it shows no initiation time, and a monomeric form (in contrast to the dimers observed by XRD) was surveyed computationally as monomers have been shown to be the active species in solution for related systems. Ethylene oxide was modelled as the epoxide substrate in place of the larger CHO utilized experimentally. Moreover, only one anionic coligand performing the catalysis, with the second kept as acetate serving as a spectator coligand, was modelled, where it's speculated that the second carboxylate equivalent dissociates to create a vacant coordination site at the Cr centre, as has been hypothesized in previous computational studies of heterobimetallic catalytic ROCOP. A simplified mechanism in which one alternating monomer enchainment occurs was considered as these steps must occur prior to other side reactions and allow for some general conclusions to be drawn regarding changes to the ROCOP mechanism, and thus the kinetics when alternating the feedstock heteroallene. Starting from the model ethoxy intermediate A (stemming from epoxide ring opening), heteroallene insertion to form the carbonate C or dithiocarbonate DTC, which then undergoes epoxide insertion to regenerate an extended alkoxide A , was computationally characterized, the results of which are presented in Figure . As can be seen in Figure , CE2 activation occurs in both cases at Rb with the alkoxide ligand oriented at Cr. Heteroallene insertion via TS1 is more kinetically facile for CO2 (+9.8 kcal mol -1 ) than for CS2 (+19.1 kcal mol -1 ) reflecting the decreased electrophilicity of the heavier homologue. This forms dithiocarbonate complex DTC (+6.8 kcal mol -1 ) or carbonate complex C (+9.1 kcal mol -1 ). The (dithio)carbonate moiety then reorientates in a series of rearrangements steps to exclusively coordinate to Rb in DTC'' (+5.7 kcal mol -1 ) and C'' (+19.7 kcal mol - 1 ) and concomitantly reveals a vacant site at the Cr centre to allow for subsequent epoxide coordination. Ring-opening via TS2 then transforms the heterocarbonate coligand into an alkoxide A , which in both the case of CS2 and CO2 ROCOP is an overall exergonic process (where for CO2, ΔG = -5.3 kcal mol -1 and for CS2, ΔG = -16.4 kcal mol -1 ). Based on the energetic spans between TS2 and (DT)C' for the CO2 (+22.0 kcal mol -1 ) and CS2 (+28.8 kcal mol -1 ), epoxide ring opening is characterized as the ratedetermining step in the case of both heteroallenes, and is more kinetically facile for CS2, which reflects the increase in nucleophilicity when moving down the periodic table and is well aligned with the lower reaction temperature that is required for CS2 than for CO2 ROCOP. Taken together, the computational study reveals that all favourable intermediates and transition states for the main-propagation events are similar in structure for the two different heteroallenes, where alkoxide coordination at Cr, heteroallene activation at Rb as well as heterocarbonate coordination at Rb and epoxide activation at Cr prior to ring-opening feature closely related geometries. The distinct roles of the cooperative partners are hence preserved from O to S which we propose is the reason behind the maintenance of the activity order for LCr AM . As epoxide ring-opening is the ratedetermining step for E = O and S in which the nucleophilic heterocarbonate is oriented by the alkalimetal, we infer that the decrease in Lewis-acidity when moving to heavier alkali metals leads to looser chain-ends which facilitates this insertion event and tentatively explains the increase in activity when moving from Li to Cs. However, the subsequent reaction pathways leading to the complex product distribution for CS2 ROCOP remain to be explored.