As an example, we illustrate one of the oldest covalent strategies used in organometallic catalysis that involves the direct attachment of a surface oxide to a transition metal ion. Most commonly, this is accomplished by treating a metal oxide material, such as silica, with a metal alkyl. Surface Si-OH groups react with the M-R group to release RH and form a new Si-O-M bond. This attachment inherently influences the metal center dramatically, because the catalytic center is directly bound to the surface. In addition, the Si-OH surface may be modified with an appropriate ligand (L) to generate a Si-O-L species for catalyst attachment. This approach is shown schematically in FIG. , in which surface Si-OH groups were modified with a phosphine for the anchoring of a Grubbs catalyst. Considering the breadth of methods for surface attachment ranging from covalent bonding to a silica surface or non-covalent interactions with modified surfaces, a practitioner may benefit from reflecting on the following questions in designing an anchored catalyst system. First, consider the application: Is the target reaction thermal, electrochemical, or photochemical? Thermal reactions require a support material that is thermally robust under the reaction conditions. Electrochemical reactions require a conductive support material and a linker that provides sufficient electronic coupling. Photochemical reactions generally require a transparent support material, and often materials with a high surface area so that a sufficient amount of photocatalyst can absorb light. Second, consider the reaction mechanism: Are there steps that involve two catalysts interacting with one another or ligands that dissociate? If multiple catalysts are required, the anchoring group should be sufficiently long and flexible to accommodate intermolecular interactions. If ligands dissociate, then the dissociating ligands should not be chosen for the attachment group so as to avoid catalyst leaching. Third, consider the reaction solvent: will the attachment method be robust under those conditions? Sequestration methods that rely on weak intermolecular forces, such as hydrophobic interactions, may be appropriate for reactions in water but not reactions that require nonpolar solvents. Finally, consider the synthetic strategy: how will the functional groups in the anchoring group influence the catalyst synthesis? Sometimes it is more effective to anchor an organic group with a key functionality, and then use a different reaction to anchor the metal unit. For example, a silyl ether containing an azide can be attached to a surface, and then an alkyne-containing metal complex can be connected to the azide in a Cu-catalyzed "click" reaction to form a robust linkage. 35