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 CoIII 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 CoIII 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.