CpRh fragment

How many different neutral square-pyramidal cluster compounds can you synthesize on paper using the isolobal principle, the cluster electron-counting rules, and SiH and CpRh fragments only. 

Two E2 units may connect two metal centers (123 Table XXXI). These compounds resemble trans-E4M2 octahedra with two E-E bonds broken (or weakened) in the final product. To date the observed molecules are all made of metal fragments that have either 13 or 14 valence electrons. Unfortunately, the structure of Cp Fe 2(P2)2 was disordered, making its detailed comparison with the other examples difficult. The Nb compound has also not been structurally characterized, but the Cp Nb(CO)2 fragment is isoelectronic to the CpCo and CpRh units so that it would be expected to adopt a similar arrangement. 

Over the past 15 years, many theoretical treatments of C-H activation have appeared. Early work by Hoffmann addressed qualitative orbital approaches to C-H activation by CpML fragments [52]. More quantitative approaches have appeared recently for the addition of methane to the [CpRh(CO)] fragment [53— 56]. These more recent calculations provide support for the presence of methane a-complexes along the reaction coordinate for methane oxidative addition, and confirm the weak nature of the interaction between the metal center and the C-H sigma bond ( 20 kj mol-1). A more detailed comparison of these results is beyond the scope of this chapter. 

Numerous theoretical studies have addressed the problem of alkane transition metal complexes.- " In the first theoretical investigation for [Cr(CO)s] and [CpRh(CO)] fragments, Saillard and HoffmaiiiV proposed an q complex, although an r -C,H tilted coordination mode was also possible. For the structure of the intermediate [CpRh(CO)(CH4)] complex. Song and Hall predicted an r 2-C,// interaction, which is very similar to the structure of Cp qih(C,H ) [BF4] characterized by XRD. Most of the subsequent studies showed that the... 

Rh(CO)4, and CpRh(CO) fragments by extended Huckel methods.91 The HOMO... 

Other calculations93 97 focus mainly on adducts of CH4 and other small molecules with CpML and unsaturated Pd/Pt fragments because of their extensive use in experimental studies of alkane activation (see below). The binding and activation is similar to Eq. (12.17), and t 2-C,H coordination of CH4 is shown by Hall for CpRh(CO)(CH4) and [ 1 ( )( 3)( 4)]+ and by Smith for CpM(NO)(CH2)(CH4) (M = Mo,W).93,97 The energy profiles are in agreement with... 

The reactions involving the oxidative addition of dihydrogen and methane to the fragment Rh(CO)4 or the corresponding isolobal complex CpRh(CO), or to the metallic surfaces of nickel and titanium have also been analyzed [60],... 

The first notable theoretical investigation of alkane binding was performed by Saillard and Hoffmann, who compared H2 and CH interaction with Cr(CO)j, Rh(CO)4, and CpRh(CO) fragments by extended Huckel methods. The HOMO... 

A series of dirhodium chalcogenide-bridged complexes with ethenyl 356, 357, and 358, and enone 359 ligands was described and coordinated to CpRh dimetallic fragments. 

The use of metals to activate the alkane C-H bond cleavage has been known for some time. First observations were made upon irradiation of the rhodium r -C5H5Rh(C02)2 complex. It has been well established that C-H bond activation involves the initial fragment CpRh(CO) (Cp = Tj -CjHj) generated upon irradiation of the corresponding dicarbonyl complex. At room temperature, in cyclohexane, the CpRh(CO)(cyclohexane) solvate is formed, which undergoes conversion to CpRhCO(cyclohexyl)(H) on the nanosecond to microsecond time scale. 

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