| Summary: | <p>We report a combined experimental and computational study of carbon dioxide activation at gas-phase Ho<small><sup>+</sup></small> and HoO<small><sup>+</sup></small> centres. Infrared action spectra of Ho(CO<small><sub>2</sub></small>)<small><sub><em>n</em></sub></small><small><sup>+</sup></small> and [HoO(CO<small><sub>2</sub></small>)<small><sub><em>n</em></sub></small>]<small><sup>+</sup></small> ion–molecule complexes have been recorded in the spectral region 1700–2400 cm<small><sup>−1</sup></small> and assigned by comparison with simulated spectra of energetically low-lying structures determined by density functional theory. Little by way of activation is observed in Ho(CO<small><sub>2</sub></small>)<small><sub><em>n</em></sub></small><small><sup>+</sup></small> complexes with CO<small><sub>2</sub></small> binding end-on to the Ho<small><sup>+</sup></small> ion. By contrast, all [HoO(CO<small><sub>2</sub></small>)<small><sub><em>n</em></sub></small>]<small><sup>+</sup></small> complexes <em>n</em> ≥ 3 show unambiguous evidence for formation of a carbonate radical anion moiety, (see article for equation). The signature of this structure, a new vibrational band observed around 1840 cm<small><sup>−1</sup></small> for <em>n</em> = 3, continues to red-shift monotonically with each successive CO<small><sub>2</sub></small> ligand binding with net charge transfer from the ligand rather than the metal centre.</p>
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