The pendant-arm macrocycles 12-methyl-1,4,7,10-tetraazacyclotridecan-12-amine (L¹³), 6-methyl-1,4,8,11-tetraazacyclotetradecan-6-amine (L¹⁴) and 10-methyl-1,4,8,12-tetraazacyclopentadecan-10-amine (L¹⁵) react readily in water with cobalt(II) ion and air, followed by aqueous hydrochloric acid addition and equilibration with activated charcoal, to yield almost exclusively cis-[Co(Lⁿ)Cl]²⁺ complexes (n= 13, 14 or 15). A trans isomer is detected in addition to the cis only in the case of L¹⁴, and then to only ca. 2% of total complexes. With 3-methyl-1,5,9,13-tetraazacyclohexadecan-3-amine (L¹⁶) a trans isomer is the sole product, however. The three cis complexes were crystallized readily as perchlorate or tetrachlorocobaltate salts: [Co(L¹³)Cl][CoCl₄]·0.25MeOH, triclinic, space group P, a= 15.682(8), b= 14.864(2), c= 8.938(4)Å, α= 96.32(4), β= 97.34(6), γ= 102.59(2)°; [Co(L¹⁴)Cl][ClO₄]₂, monoclinic, space group P2₁/c, a= 9.856(4), b= 17.36(2), c= 12.08(1)Å, β= 98.95(5)°; and [Co(L¹⁵)Cl][ClO₄]₂·H₂O, orthorhombic, space group Pn2₁a, a= 16.708(4), b= 13.343(3), c= 9.886(1)Å. Single crystal X-ray structure determinations were refined to residuals of 0.050, 0.051 and 0.037 for 4848, 3035 and 1549 ‘observed’ reflections respectively. In all cases the pendant primary amine and two adjacent secondary amines necessarily occupy an octahedral face, with the chloro ligand cis to the primary amine, and secondary amines adopt RRSS stereochemistries. There is an increase in average Co–N distances with macrocycle size (1.94₈, 1.95₄, 1.98₁Å for L¹³, L¹⁴ and L¹⁵ respectively), although the Co–N (pendant primary amine) distance is relatively invariant [range 1.955(4)–1.962(5)Å]. The Co–Cl distance, however, is minimized for L¹⁴[2.245(2), 2.236(2), 2.273(2)Å for L¹³, L¹⁴ and L¹⁵ respectively]. This latter trend is reflected in the comparative rate constants for stereoretentive base hydrolysis (k_OH 4300, 76, 6700 dm³ mol^–1 s^–1 respectively), suggesting a ground-state influence on the reaction rate.