Surface-cluster analogy, bonding

This molecular picture of the surface states of chemisorbed molecules was consolidated by the analogy of the bonding of such molecules on molecular metal cluster and on small metal particles [11-13]. 

Scheme 2. Schematic relationships between C2 hydrocarbon adsorbates and the surface species that could be derived from them. Possible relationships are indicated between surface species that involve not more than one addition/subtraction of H and/or M atoms. Strong bonding of these species to metal atoms could give rise to structures approximating to metallocyclopropanes or metallocyclopropenes, respectively. These structures with C=C or C = C groupings frequently occur on surfaces with additional n bonds to further metal atoms, M in other cases, two additional CM bonds may replace a CC double bond. JOn surfaces the metal atoms are usually bonded to each other so that the analogous molecular cluster compounds would be metallocyclopropanes or metallocyclobutanes, etc. 1 See footnote to Scheme I. The dashed rectangles indicate surface species that involve no CH bond breaking on adsorption of the parent hydrocarbon. These are most likely to be present under low-temperature adsorption conditions.

The analogy with both surface-bound and cluster faces bridging i,3-CO has its counterpart in the q3-CNR ligation. This triply bridging bonding mode was crystallographically evidenced in some homometallic and heterometallic... 

The cluster approach is, like the extended-surface approach, characterized by many different calculational schemes. A recent review stresses electronic aspects of bonding. In this review we have chosen to concentrate on geometric aspects. We shall discuss a number of major techniques in order of increasing computational complexity the extended H uckel theory, self-consistent Xa scattered wave calculations, and self-consistent ab initio Hartree-Fock and valence bond methods. In that order these techniques allow increasing accuracy. However, the cluster size must decrease simultaneously due to calculational complexity, ultimately reducing the degree of analogy with surfaces. 

The study of polynuclear cobalt-alkyne clusters has provided insight into the coordination, protection, and activation of the carbon-carbon triple bond. It is possible to draw analogies between alkynes coordinated to molecular species and those to surfaces. 

---