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We report the syntheses and magnetic property characterizations of four mononuclear cobalt( ii ) complex salts featuring a tripodal iminopyridine ligand with external anion receptor groups, [CoL 5-ONHtBu ]X 2 (X = Cl ( 1 ), Br ( 2 ), I ( 3 ) and ClO 4 ( 4 )). While all four salts exhibit anion binding through pendant amide moieties, only in the case of 1 is field-induced slow relaxation of magnetisation observed, whereas in the other salts this phenomenon is absent at the limits of our instrumentation. The effect of chloride inducing a seventh Co–N interaction and concomitant structural distortion is hypothesized as the origin of the observed dynamic magnetic properties observed in 1 . Ab initio computational studies carried out on a 7-coordinate Co( ii ) model species survey the complex interplay of coordination number and trigonal twisting on the sign and magnitude of the axial anisotropy parameter ( D ), and identify structural features whose distortions can trigger large switches in the sign and magnitude of magnetic anisotropy.