Solution data at 20 °C are presented for the binding oft-butylisocyanide and tosylmethylisocyanide (TMIC) to the five-coordinate hemes Fe(porp)(Cy2Im), where Cy2Im = 1,5-dicyclohexylimidazole and porp = the dianion of some durene-capped porphyrins with variable length linking methylene straps on either side of the durene moiety (4/4, 5/5, or 7/7 methylenes). ThetBuNC and TMIC ligands are used to probe the steric nature of the distal binding site under the cap, in particular the respective "central" and "peripheral" effects as defined by Trayloretal. The binding constants range from 1.1 × 109 M−1for the 7/7—TMIC system to immeasurably small for the 4/4—tBuNC system, this range spanning data reported in the literature for related anthracene- and adamantane-capped systems.