Experiments are described that provide indirect evidence for the involvement of alkane sigma-complexes in oxidative addition/reductive elimination reactions of Tp'Rh(L)(R)H complexes (Tp' = tris-3,5-dimethylpyrazolylborate, L = CNCH(2)CMe(3)). Reductive elimination rates in benzene-d(6) were determined for loss of alkane from Tp'Rh(L)(R)H, where R = methyl, ethyl, propyl, butyl, pentyl, and hexyl, to generate RH and Tp'Rh(L)(C(6)D(5))D. The isopropyl hydride complex Tp'Rh(L)(CHMe(2))H was found to rearrange to the n-propyl hydride complex Tp'Rh(L)(CH(2)CH(2)CH(3))H in an intramolecular reaction. The sec-butyl complex behaves similarly. These same reactions were studied by preparing the corresponding metal deuteride complexes, Tp'Rh(L)(R)D, and the scrambling of the deuterium label into the alpha- and omega-positions of the alkyl group monitored by (2)H NMR spectroscopy. Inverse isotope effects observed in reductive elimination are shown to be the result of an inverse equilibrium isotope effect between the alkyl hydride(deuteride) complex and the sigma-alkane complex. A kinetic model has been proposed using alkane complexes as intermediates and the selectivities available to these alkane complexes have been determined by kinetic modeling of the deuterium scrambling reactions.

While X-ray crystallography provides reliable solid-state information, the assignment of hapticity in [HB(3,5-dimethylpyrazolyl)3]ML n (M = Rh, Ir, Pt) complexes in solution is complicated by the dynamic nature of the tris(pyrazolyl)borate ligand. 1H and 13C NMR spectroscopy is ambiguous as to the nature of the coordination environment around the metal center. 11B NMR data are shown to correlate with the hapticity of the tris(pyrazolyl)borate ligand, making it a useful tool for the rapid determination of solution-state structure of tris(pyrazolyl)borate metal complexes.

Tp*ReO(H)Cl, TpRe(O)H(Cl), and Tp*ReO(H)2 have been prepared by alkoxide-for-hydride metathesis using BH3·THF. The triflate derivative Tp*Re(O)H(OTf) inserts olefins and is oxidized by oxygen-atom transfer reagents to give HOTf and Tp*ReO3, likely by a pathway involving hydride migration to an oxo ligand (Tp* = hydrotris(3,5-dimethyl-1-pyrazolyl)borate; triflate = OTf = OSO2CF3).

Generation of the 16-electron fragment {[HB (3,5-dimethylpyrazolyl) 3 ]Rh(CNCH 2 CMe 3 )} (Tp′RhL) in the presence of cyclopropane results in C-H activation of the hydrocarbon. The cyclopropyl hydride complex rearranges in benzene solvent to the metallacyclobutane complex Tp′Rh(CNCH 2 CMe 3 )(CH 2 CH 2 CH 2 ). Thermolysis of the rhodacyclobutane complex produces an η 2 -propylene complex. The related complex Tp′Rh(CN-2,6-xylyl)(C 2 H 4 ) has been structurally characterized and displays η 3 -Tp′ coordination, both in the solid state and in solution. Thermolysis of the rhodacyclobutane complex in the presence of neopentyl isocyanide leads to insertion of isocyanide into both Rh-C bonds of the metallacycle. Cyclobutane undergoes C-H but not C-C bond cleavage.

Rhenium-oxo complexes with the hydridotris(3,5-dimethyl-1-pyrazolyl)borate (Tp*) ligand are described. Halide complexes Tp*Re(O)X(Cl) (X ) Cl, I) are prepared by treatment of Tp*Re(O)(OH)Cl with HX. Reaction of Tp*Re(O)Cl 2 (3) with 1 or 2 equiv of LiPh/ZnCl 2 or Et 2 Zn produces the corresponding oxo-aryl and oxo-alkyl complexes Tp*Re(O)(Ph)Cl, Tp*Re(O)(Ph) 2 , and Tp*Re(O)(Et)Cl. Alkoxide complexes Tp*Re(O)(OR)Cl and Tp*Re(O)-(OR) 2 are prepared from 3 and ROH, PhOH, or catechol. Triflate complexes Tp*Re(O)X-(OTf) (X ) halide, H, Et, Ph, OEt, OPh) have been prepared by halide metathesis with AgOTf or by alkoxide metathesis with Me 3 SiOTf. Hydride complexes Tp*Re(O)(H)X (X ) Cl, H) are generated from the corresponding alkoxide complexes with BH 3 ‚THF. Oxidation of Tp*Re-(O)Ph(OTf) with Me 2 SO gives a phenoxide complex, and oxidation of Tp*Re(O)Et(OTf) with pyridine N-oxide gives acetaldehyde. Both reactions are similar to oxidations of related Tp compounds. Tp*Re(O)H( OTf) is oxidized by either reagent to Tp*Re(O) 3 , with liberation of H + ; such a hydride complex has not been accessible in the Tp system. The Tp* ligand imparts added stability to the rhenium(V) derivatives, making preparations easier. Steric constraints of the Tp* ligand are illustrated by the lack of phenyl and pyridine ligand rotation on the NMR time scale. The stability and crowding of the Tp* compounds inhibits reactions with oxygen atom donors so that heating is often required, and the resulting oxidations are more complex than for analogous Tp compounds.