Matrix-isolation studies of the structures and reactions of small metal particles

Abstract

Small paramagnetic metal cluster, M_n(n < 13) of Group 1, 3, 11 and 13 elements have been prepared by the matrix-isolation technique and their geometric and electronic structures determined by EPR.

The trimeric clusters of the alkali metals have either static, obtuse-angled ²B₂ ground states or are dynamic pseudorotating Jahn–Teller species. Only static structures are found for the coinage metal trimers, the obtuse ²B₂ geometry being adopted in hydrocarbon matrices and the acute ²A₁ geometry in solid nitrogen; the unpaired electron is concentrated on the terminal nuclei in the obtuse form and on the central atom in the acute form. Al₃ and Ga₃ have quartet ⁴A₂ pseudorotating Jahn–Teller ground states.

The septamers of both the alkali and coinage metals have pentagonal bipyramidal structures in which the unpaired electron is largely located at the two apical atoms. There is a progressive loss of s-spin population as n increases with the population approaching that at the Fermi surface of the bulk metal even at the septamer stage.

The products of the reaction of Cu₃, Cu₅ and Al₃ with O₂, Ag₃ and Ag₇ with ethene and Ag₇ with CO have also been examined by EPR. For the copper clusters there is almost complete electron transfer to give tight ion pairs, Cu⁺_nO^–₂ with an equal residual spin population on each nucleus but with Al₃ a covalently bound peroxyl Al₃O₂ is formed. Ag₃ changes its structure from an obtuse to an acute triangle on reacting with ethene. Ag₇ reacts with CO and ethene with retention of its bipyramidal structure but with appreciable spin delocalisation on to the ligand.