Size and charge effects on the binding of CO to late transition metal clusters

Abstract

We report on the size and charge dependence of the C-O stretching frequency, ͑CO͒, in complexes of CO with gas phase anionic, neutral, and cationic cobalt clusters ͑Co n CO −/0/+ ͒, anionic, neutral, and cationic rhodium clusters ͑Rh n CO −/0/+ ͒, and cationic nickel clusters ͑Ni n CO + ͒ for n up to 37. We develop models, based on the established vibrational spectroscopy of organometallic carbonyl compounds, to understand how cluster size and charge relate to ͑CO͒ in these complexes. The dominating factor is the available electron density for backdonation from the metal to the CO * orbital. Electrostatic effects play a significant but minor role. For the charged clusters, the size trends are related to the dilution of the charge density at the binding site on the cluster as n increases. At large n, ͑CO͒ approaches asymptotes that are not the same as found for ͑CO͒ on the single crystal metal surfaces, reflecting differences between binding sites on medium sized clusters and the more highly coordinated metal surface sites.

Controlling the Bonding of CO on Cobalt Clusters by Coadsorption of H2

“…No absorption bands were detected in the 1600 -1950 cm -1 region, indicating that CO is exclusively atop (μ 1 ) bound to hydrogen covered Co n + clusters. This has been found earlier for CO adsorption on bare Co n + clusters [9] . H 2 adsorbs dissociatively on cationic Co clusters [10] .…”

“…The cluster size plays a role due to delocalization of charge over the cluster surface. The validity of this assumption has been shown before [9] [19] .…”

“…We can use the recently measured charge dependence of ν(CO) in cobalt cluster monocarbonyl complexes [9] to estimate the amount of charge transferred into the Co-H bonds. More generally we can extend the quantitative model [9] for the charge dependence of ν(CO) in late transition metal carbonyl clusters to present a global picture of charge transfer in Co n (CO)H m + complexes.…”

“…More generally we can extend the quantitative model [9] for the charge dependence of ν(CO) in late transition metal carbonyl clusters to present a global picture of charge transfer in Co n (CO)H m + complexes. The model incorporates the electrostatic interaction between the charge of the cluster and the CO dipole and the effect of donation of electron density from the metal to the 2π* orbital of CO.…”

H2 Adsorption on 3d Transition Metal Clusters: A Combined Infrared Spectroscopy and Density Functional Study

“…It has indeed been shown that the cluster charge is delocalized over the surface of the cluster. 57,58 However, a combination of a theoretical and more detailed spectroscopic study is needed to identify the reason for the absence of a D-D stretch absorption band for the nickel hydride complexes with more than 10 nickel atoms. 6.…”