⁵⁹Co and ¹³C Nuclear Spin Relaxation Studies in Solutions of Symmetric, Bidentate Cobalt(III) Complexes. On the Mechanism of ⁵⁹Co Spin Relaxation. Crystal Structure Determination of Tris(tropolonato)cobalt(III)

Abstract

Valiev's suggestion of excitation of the vibrational modes of the octahedron is considered as a spin relaxation mechanism for ⁵⁹Co spins in some symmetric, bidentate Co^III complexes. His theory is extended to the spin 7/2 system; the efficiency of the mechanism is very sensitive to the molecular size with larger complexes having much shorter T₁ values. Experimental evidence in support of the mechanism is provided by the contradictory values of quadrupole coupling constants calculated experimentally by assuming rotational reorientation to be the dominant time process and by measurements on a variety of Co^III complexes. Rotational correlation times were measured from ¹³C T₁ values of protonated carbons in the complex. An X-ray structure determination of tris(tropolonato)-cobalt(III) is reported in support of the suggested relaxation mechanism.

The differences between T₁ and T₂ in some CoN₆ complexes are shown to arise from unresolved ⁵⁹Co-¹⁴N scalar coupling.