Use of 0.0 mm Pathlength Gas Cell to Eliminate CO( g ) Absorbance from High-CO-Pressure Solid-State IR Spectra of Nonclassical Metal Carbonyls

A transmission-IR gas cell was constructed from 18%-Ni maraging steel so that two parallel 10 mm diameter × 3 mm thick sapphire windows were held approximately 0.1 mm apart. The cell was engineered to withstand static gas pressures up to 150 atm. When 100 atm CO was placed in the cell, the % transmittance was essentially zero between 2200 and 2080 cm⁻¹ because of the intense v(CO) band centered at 2143 cm⁻¹ (this is an important region of interest for the growing subset of metal carbonyl complexes known as nonclassical metal carbonyls). When the cell was assembled with Nujol or Fluorolube mulls of several microcrystalline copper(I) and silver(I) salts between the windows and CO gas was added to the cell, the gas dissolved in the mulling agents and reacted with the metal salts, but no gaseous CO was observed in the spectrum. New spectral bands were observed that are attributed to unusual metal carbonyl species that are stable only under high CO pressure. A relatively weak band due to free CO dissolved in the mulling agents was detected at 2138–2135 cm⁻¹. The new cell was used to demonstrate that the compound Ag(OTeF₅) takes up only one equivalent of CO at 100 atm and that the compound Cu(CF₃SO₃) takes up two equivalents of CO at 136 atm.