The formation, reactions and structures of binary cobalt phosphide clusters [CoxPy]− in the gas phase

Abstract

Laser ablation of CoP generated 152 binary anionic cobalt phosphide clusters [Co_xP_y]⁻, ranging up to [Co₂₅P₁₆]⁻. This is a more extensive set of [Co_xP_y]⁻ clusters than obtained in previous laser ablation of a mixture of Co metal and red phosphorus: the composition maps for the two experiments overlap, but with generally lesser y:x ratios for laser ablation of CoP. The reactivities, reactions, and collisional dissociation of selected ions have been investigated by Fourier transform ion cyclotron resonance mass spectrometry. The majority of the [Co_xP_y]⁻ ions react with H₂S by addition of (up to three) S atoms, substitution of P₂ by S, and with elimination of HP₂⁻. Reaction with NO₂ or N₂O causes addition of one O atom. Collisional activation causes dissociation of P₂, P₄, CoP₂ or CoP₄. The ions [CoP₈]⁻, [Co₄P₈]⁻, [Co₅P₉]⁻ and [Co₆P₁₀]⁻ are unreactive, and for these and the reactive [Co₄P₄]⁻ ion, density functional investigations of 30 postulated structures have been performed. The probable structures, calculated electron affinities, and electronic structures are reported. The structural principles evident so far are that P atoms, P₂ groups, and P₃ groups bridge the faces, and to a lesser extent the edges, of Co_x polyhedra. The lack of reactivity for relatively P-rich clusters correlates with the absence of co-ordinatively unsaturated metal sites on the clusters: it is postulated that the more reactive species undergo addition at under-co-ordinated Co atoms. The calculated electronic structures generally have close-lying electronic states with unpaired electrons, which explains the overall high reactivity of the [Co_xP_y]⁻ clusters. The structural differences between these [Co_xP_y]⁻ clusters and characterised molecules Co_xP_yL_z with ancillary ligands are discussed, as are possible applications of the new [Co_xP_y]⁻ clusters in synthesis of novel materials.