J . Phys. Chem. 1984, 88, 4037-4041
4037
Substituent Effects in the Surface Coordination Chemistry of Nitriles R. M. Wexler*+and E. L. Muettertiest Materials and Molecular Research Division, Lawrence Berkeley Laboratory, and Department of Chemistry, University of California. Berkeley, California 94720 (Received: February 16, 1984)
A detailed kinetic analysis is presented for the thermal desorption of alkyl and perfluoroalkyl cyanides (nitriles) from Ni(ll1). The activation energies for the desorption of the perfluoroalkyl derivatives were about 8 kcal/mol greater than for the alkyl analogues. These data suggest that the perfluoronitriles, the better acceptor molecules, are more strongly bound to the d-electron-rich nickel surface. Only CH,CN and CF,CN exhibited coverage dependence in the magnitude of the desorption activation energies. Nitriles with functional groups such as chloro, acetyl, vinyl, and aryl showed only a minor degree of molecular desorption from Ni( 11 1); thermal decomposition was the dominant process in these cases.
Introduction From the work of Shustorovich,' there is reason to believe that variations in the donor-acceptor properties of a ligand coordinated to a metal surface may be an important factor in determining the strength of the interaction between the ligand and surface. In order to probe this relationship experimentally, a series of closely related molecules must be studied so that the electronic and steric properties of the ligand can be varied systematically. Since most molecules studied on metal surfaces have been quite simple, it has not been possible to investigate the effect of small perturbations of electronic or steric properties on the binding of ligands without completely changing the nature of the ligand and its mode of coordination. As we looked for a series of related molecules for investigation of systematic variations on the desorption of ligands from surfaces, the nitrile ligand appeared to be eminently suited. Previous studies indicated that acetonitrile underwent minimal decomposition and no detectable reversible bond breaking on Ni( 111) upon heating2z3 Thus, investigation of substituent effects on the desorption of nitriles (R-CN) seemed possible. This would allow systematic variation of the steric bulk and inductive effects of the R group provided there was no direct and significant interaction of substituent atoms with the surface. Also, the a system of the cyano group might allow investigation of donor and acceptor resonance effekts on the desorption energy of the nitrile ligand. Unfortunately, as described below, many functionally substituted nitriles coordinated to Ni( 111) decomposed to a large extent upon heating, thus making analysis of the substituent effects on the desorption barrier ~nfeasible.~However, alkyl and fluoroalkyl cyanides were highly resistant to decomposition and a detailed analysis of their desorption kinetics was completed. It is important to note that there can be substantial variation in stmeochemical features of the nitrile-surface bonding. For a flat metal surface, the C-N nitrile bond vector may be normal or parallel to the surface plane or intermediate between those two idealized models. If normal or more or less normal, the nitrogen atom may be sitting atop a single metal atom or bridging between two or more metal atoms. If parallel to the surface plane, the C-N bond may have a wide array of registries with respect to the surface unit cell. Also, if the C-N bond is not normal to the surface plane, substituent atoms in the R group may closely approach surface met61 atoms (in the extreme case forming a chemical bond). Such a configuration can substantively affect the thermal reactivity of the chemisorbed nitrile. The reactivity of the functionalized nitriles is considered in this relationship to ground-state geometry.
chemical etching of the crystals was omitted since this often led to formation of an oxide film. The ultrahigh-vacuum system used in these studies has been described previously. Ni( 11 1) surfaces torr of Ar, were cleaned by argon ion bombardment (5 X 600 eV, 10 kA/cm2, 25 "C, 5-10 min) to remove sulfur. Carbon was removed by heating to 500 "C in 2 X lo-' torr of oxygen for about 1 min. Annealing to 600 "C for 1 min removed residual oxygen and nitrogen. Working base pressures were always less torr and in most cases
