Chemical Shift Anisotropy as a Mechanism for Modulating Apparent J Tl - H and J Tl - C Coupling Constants in Tris(pyrazolyl)hydroborato Thallium Complexes

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.