Vibrational studies of interstitial hydrogen in metal carbonyl clusters

Abstract

A low-temperature vibrational study of the interstitial hydride clusters [Co₆(CO)₁₅H]^–, [Ni₁₂(CO)₂₁H]^3–, and [Ni₁₂(CO)₂₁H₂]^2– has enabled the ν(M–H) modes to be assigned. The salts of [Co₆(CO)₁₅H]^– show the presence of two ν(Co–H) vibration modes at 1 086 and 949 cm^–1, which may be attributed either to different restoring forces, or to the presence of two [Co₆(CO)₁₅H]^– isomers differing in the co-ordination of the hydrogen atom, e.g.µ₆-interstitial versusµ- or µ₃-external co-ordination. This study has also provided evidence for the occurrence in the solid state of an unexpectedly ready deprotonation of the cluster [Co₆(CO)₁₅H]^–, presumably due to interaction with traces of CO₃^2– present in the alkali halide support. The clusters [Ni₁₂(CO)₂₁H_4–n]^n–(n= 2 or 3), in contrast, show only a broad absorption in the region 650–700 cm^–1 which has been assigned to different ν(Ni–H) modes according to the different position of the interstitial H with respect to the Ni₆ cage. The low value of their frequency partially reflects the larger dimensions of the Ni₆ cavity of [Ni₁₂(CO)₂₁H_4–n]^n–, with respect to the Co₆, cavity of [Co₆(CO)₁₅H]^–, but probably indicates an increasing similarity of higher nuclearity hydride clusters to bulk metal hydrides.