Copper complexes of the phenolic oxime family of ligands (3-X-salicylaldoximes) are used extensively as metal solvent extractants. Incorporation of electronegative substituents in the 3-position, ortho to the phenol group, can be used to buttress the interligand H-bonding, leading to an enhancement in extractant strength. However, investigation of the relevant H-bonding in these complexes can be exceedingly difficult. Here, we have combined EPR, END OR, DFT, and X-ray crystallography to study this effect. Analysis of the H-1 ENDOR data revealed a variation in the Cu center dot center dot center dot H-16 (oxime proton) distance from 2.92 angstrom for the unsubstituted complex [Cu(L-2)(2)] to 3.65 angstrom for the X = CH2N(C6H13)(2) substituted complex [Cu(L-3)(2)]. DFT calculations showed that this variation is caused by changes to the length and strength of the H-bond between the oximic hydrogen and the phenolate oxygen. Noticeable changes to the Cu center dot center dot center dot H-15 (azomethine proton) distances and the Cu center dot center dot center dot N bonding parameters were also observed in the two complexes, as revealed through the (N)A and (N)Q END OR data. Distortions in the structure of the complex and variations in the oximic proton to phenolate oxygen H-bond strength caused by the substituent (X) were confirmed by DFT and X-ray crystallography. DFT directly evidenced the importance of the interaction between H-16 and the amine nitrogen of CH2N(C6H13)(2) in the buttressed complex and indicated that the high strength of this interaction may not necessarily lead to an enhancement of copper extraction, as it can impose an unfavorable geometry in the inner coordination sphere of the complex. Therefore, END OR, DFT, and X-ray structural data all indicate that the aminomethyl substituent (X) ortho to the phenolic oxygen atom provides a particularly strong buttressing of interligand H-bonding in these copper complexes and that these outer sphere interactions can significantly influence structure and stability.