Alkane binding

What purpose does the alkane binding to the Pt(II) center serve For the electrophilic pathway (Scheme 5, b), this is immediately apparent, a-Alkane complexes should be considerably more acidic than free alkanes, such that deprotonation may become a viable C-H activation pathway. While the acidic character of alkane complexes has not been directly observed, it can be inferred from the measured acidity of analogous agos-tic complexes (36) and from the acidity of the a-complexes of dihydrogen (37), both of which can be regarded models for alkane complexes (see Section III.E). 

TABLE 11.2 Alkane Binding Energies (kcal/mol) for W(CO)s(alkane) Complexes... 

The thermodynamics of bonding are discussed below for a complexes that are stable at 25°C, complexes that are thermally unstable but observable by IR, and also bare metal species that can only be observed by mass spectroscopy. The data are quite useful for understanding the electronic factors that stabilize the various types of ligand coordination and may predict, e.g., which metal fragments are best for alkane binding. 

Figure 12.6. Possible coordination modes for methane and related alkane binding.

Solution studies of transient alkane binding at room temperature were first performed in 1973 by flash photolysis of Cr(CO)6 in cyclohexane, which gave a UV/vis band at 503 cm-1 similar to that seen in CH4 matrices71 72 ... 

Because alkanes bind primarily by donation to M, one approach to forming a stronger bond might be to increase the elcctrophilicity of M as much as possible,... 

A and 2.62-2.65 A for the proximal and distal C-H, respectively. Binding energies are 10.5-16.7 kcal/mol, generally increasing with size, and are somewhat higher than those calculated for alkane binding to W(CO)5 discussed below. 

The first notable theoretical investigation of alkane binding was performed by Saillard and Hoffmann, who compared H2 and CH4 interaction with Cr(CO)5,... 

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