The syntheses and characterisation of the new macrocyclic hexaamine trans-(5(S),7(S),12(R),14(R)-tetramethyl)-1,4,8,11-tetraazacyclotetradecane-6,13-diamine (L(6)) and its Co(III) complex are reported. The X-ray crystal structural analyses of [CoL(6)]Cl(2)(ClO(4)) [monoclinic, space group C2/c, a = 16.468(3) Å, b = 9.7156(7) Å, c = 15.070(3) Å, beta = 119.431(8) degrees, Z = 4] and the closely related cis-diamino-substituted macrocyclic complex [CoL(2)](ClO(4))(3). 2H(2)O (L(2) = cis-6,13-dimethyl-1,4,8,11-tetraazacyclotetradecane-6,13-diamine) [orthorhombic, space group Pna2(1), a = 16.8220(8) Å, b = 10.416(2) Å, c = 14.219(3) Å, Z = 4] reveal significant variations in the observed Co-N bond lengths and coordination geometries, which may be attributed to the trans or cis disposition of the pendent primary amines. The Co(III/II) self-exchange electron transfer rate constants for these and other closely related hexaamines have been determined, and variations of some 2 orders of magnitude are found between pairs of trans and cis isomeric Co(III) complexes.