Structural studies of metalloporphyrins. VI—¹³C relaxation time, T₁, measurements and the nature of the axial N—metal bond in some macrocyclic complexes

Rotation barriers around the metal—pyridine bond of various macrocyclic complexes of cobalt(III), rhodium(III), zinc(II) and magnesium(II) have been studied using the 13 C longitudinal relaxation times, T 1 , of the carbon atoms of pyridine and the macrocycle. The data have been rationalized on the basis of steric and electronic factors. The influence of π back‐donation on the rates of rotation around the axial CoN and RhN bonds is discussed. The agreement found between the conclusions drawn from this study and X‐ray data reported for complexes of identical or closely related structure demonstrates the validity of this method.