Coordination chemistry of metal surfaces: Chemisorption of PF3

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Abstract
The electronic and thermal properties of PF3 bonded to surfaces of Cr, Fe, Ni, Cu, Ru, Pd, Ir, and Pt were investigated by means of ultraviolet photoelectron spectroscopy (UPS), electron energy loss sectroscopy, LEED, and thermal desorption. Bond formation can be described in terms of coupling of the highest occupied ligand orbital (σ-donor 8a1) to metallic s states and of ’’back donation’’ of metallic d electrons into the lowest empty ligand orbital (π-acceptor 7e). The observed lowering of the ionization potential of the 8a1 level exhibits close similarities with the properties of the corresponding zero-valent mononuclear complex compounds, indicating that in this case the chemisorption bond has to be considered as an essentially local phenomenon. The agreement is best with the fcc metals Ni, Pd, Ir, and Pt, whereas larger variations (as well as a dependence on the crystallographic orientation of the surface) were found with the bcc metals Cr and Fe. Ordered 2×2 overlayers observed with Ni(111) and Pd(111) surfaces are interpreted in terms of a simple structure model based on the van der Waals diameter of PF3 and on linear bond formation. Thermal desorption takes place at temperatures which are very close to those observed for CO, indicating very similar bond strengths for both ligands. Thermal decomposition of PF3 (which obviously needs several metal atoms and is therefore not observed with mononuclear complexes) was found to dominate with Cr and Fe surfaces. Although the σ-donor coupling is rather pronounced with the system PF3Cu(110) in this case thermal desorption already takes place around 200 K. This low thermal stability is presumably the reason why so far no mononuclear Cu–PF3 complex could be prepared.