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The active sites of heme proteins are closely surrounded by protein side chains in a manner which provides steric hindrance to entering ligands.* 1•2 For example, hemoglobin binds isonitriles with affinities decreasing in the order EtNC > t-PrNC > t-BuNC,3 a series also found in hindered model compounds.43 Additionally, the observations that the Fe-OO bond is bent from the heme perpendicular in both heme proteins and unhindered model compounds5 whereas such distortion occurs only in heme proteins for FeII *1-C06"9 groups seem to support the suggestion10 that the proteins reduce the ratio of CO binding to 02 binding through distal side steric effects.2•11•12 Using two new cyclophane hemes having different amounts of steric hindrance to ligand binding, we find that CO and 02 are subject to the same magnitudes of steric effects and are thus not differentiated by distal side steric effects.Comparisons of affinities of CO and 02 for simple unhindered hemes with those of heme proteins have led to conflicting conclusions concerning distal steric effects.11•13"16 Such comparisons are subject to uncertainties because dioxygen affinities are sensitive to solvent polarity,17 and both carbon monoxide and dioxygen affinities are affected by structural changes which could prefer-

The µ-oxo bridge is a common structural component of nonheme iron proteins, 1 but information about its reactivity is limited. 1,2 Here we demonstrate control of the rate of reductive µ-oxo bridge cleavage on the basis of obligatory ligation within one and only one of the two cyclophane-like 3 cavities trans to the oxo bridge in [Fe((DMG)BPh 2 ) 2 ] 2 O, 1 4 (Figure 1).Three ligated forms of 1 (eqs 1 and 2; L ) amines, imidazoles, pyridines, nitriles, etc.) have been characterized 4a and the X-ray structures for 1 and 1-(BuNH 2 ) 2 reported. 4b The