The thermal reactions of the per-deuterated yttrium phenyl cation with carbon dioxide have been investigated experimentally by using electrospray-ionization mass spectrometry and computationally by means of density functional theory. The process proceeds by migratory insertion under C-C bond formation followed, to some extent, by decarbonylation. Labeling experiments with (CO2)-O-18 confirm the origin of the oxygen incorporated in both productions. With regard to the mechanisms, the calculations reveal that the benzoate salt of yttrium corresponds to the most stable isomer. While extrusion of CO starting from this ion; or from two isomeric encounter complexes, is hampered by kinetic barriers, decarbonylation is accessible under thermal conditions from other intermediates which are generated after insertion of CO2 into the Y-C bond of [YC6D5](+). (C) 2015 Elsevier Inc. All rights reserved.