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A series of studies have been conducted by experimental and theoretical methods on the synthesis, structures, and reactions of CpRh boryl complexes that are likely intermediates in the rhodium-catalyzed regioselective, terminal functionalization of alkanes. The photochemical reaction of CpRh(eta(6)-C(6)Me(6)) with pinacolborane (HBpin) generates the bisboryl complex CpRh(H)(2)(Bpin)(2) (2), which reacts with neat HBpin to generate CpRh(H)(Bpin)(3) (3). X-ray diffraction, density functional theory (DFT) calculations, and NMR spectroscopy suggest a weak, but measurable, B-H bonding interaction. Both 2 and 3 dissociate HBpin and coordinate PEt(3) or P(p-Tol)(3) to generate the conventional rhodium(III) species CpRh(PEt(3))(H)(Bpin) (4) and CpRh[P(p-tol)(3)](Bpin)(2) (5). Compounds 2 and 3 also react with alkanes and arenes to form alkyl- and arylboronate esters at temperatures similar to or below those of the catalytic borylation of alkanes and arenes. Further, these compounds were observed directly in catalytic reactions. The enthalpies and free energies for generation of the 16-electron intermediate and for the C-H bond cleavage and B-C bond formation have been calculated with DFT. These results strongly suggest that the C-H bond cleavage process occurs by a metal-assisted sigma-bond metathesis mechanism to generate a borane complex that isomerizes if necessary to place the alkyl group cis to the boryl group. This complex with cis boryl and alkyl groups then undergoes B-C bond formation by a second sigma-bond metathesis to generate the final functionalized product.

The compounds, where Cp' = C(5)H(5), a series, and C(5)H(4)Me, b series, are generated via treatment of with HB(C(6)F(5))(2). When allowed to undergo irreversible methane loss in the presence of an excess of the sterically modest alkynes 2-butyne or phenylacetylene, the putative intermediates 1a and 1b are trapped as the tantala-3-boratacyclopentene compounds 2 and 3, respectively. In these complexes, the alkyne and borataalkene ligands have reductively coupled at the d(2) tantalum center. For the unsymmetrical alkyne, a kinetic product resulting from coupling in the opposite regiochemical sense is observed; the thermodynamic products 3-t incorporate the phenyl group in the alpha position of the tantalaboratacyclic ring. Two of these compounds (2b and 3b-t) were characterized crystallographically. For bulkier alkynes (diphenylacetylene, 1-phenyl-1-propyne, and 3-hexyne), intermediates with similar spectroscopic properties to the tantala-3-boratacyclopentenes were observed, but the ultimate products were the vinylalkylidene compounds 5-(R,R'). Compound 5-(Ph,Me) was characterized crystallographically, and it was found that the vinylalkylidene binds to the metal in an eta(1)-bonding mode, with the tantalum center receiving further ligation through a hydridoborate moiety. Mechanistic studies suggest that these products arise via retrocyclization of the tantala-3-boratacyclopentenes formed kinetically. These studies represent the first studies concerning the reactivity of a borataalkene ligand at a transition metal center and show that it can behave in an "olefin-like" manner, despite having a more flexible array of bonding modes available to it than an olefin.