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The intermolecular activation of the carbon-hydrogen bond in benzene has been investigated by the thermolysis of c;í-L4Os(H)R (L = P(CH3)3; R = CH3, 1; R = CH2C(CH3)3, 2; R = CH2Si(CH3)3, 3). Thermolysis of 2 at 70 °C in benzene results in activation of the arene C-H bonds to afford L4Os(H)(C6H5), 6, quantitatively. This reaction is shown to proceed, in the case of 2, via initial reversible L dissociation to form an unsaturated intermediate, L3Os(H)Np, 9, which is configurationally stable during its lifetime and which rapidly and reversibly activates the C-H bonds of its own neopentyl ligand. Unsaturated intermediate 9 reacts with benzene by oxidative addition of Ar-H to form an Os(IV) intermediate, L3Os(H)2(Np)(Ar), which then undergoes reductive elimination of NpH. The activation step exhibits a kinetic isotope effect, *c6h6/*C6D6! of 3.32 ± 0.30. The thermolyses of 3 in benzene were shown to proceed by the same mechanism of initial dissociation of L, followed by oxidative addition of benzene, reductive elimination of SiMe4, and uptake of L. The unsaturated intermediate

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Nuclear relaxation due to chemical shift anisotropy provides an efficient mechanism for modulating apparent J Tl - H and J Tl - C coupling constants in tris(pyrazolyl)hydroborato complexes, Tl[TpRR‘]. Specifically, thallium relaxation via chemical shift anisotropy results in apparent coupling constants to thallium being dramatically reduced at (i) higher applied magnetic field strengths and (ii) lower temperatures. As a result of this phenomenon, the absence of observable J Tl - H and J Tl - C coupling constants, per se, should not be taken as definitive evidence that either (i) the structure is static with a coupling constant of zero or (ii) dissociation of Tl is rapid on the NMR time scale, thereby resulting in an observed loss of coupling.

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