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A series of dysprosium( iii ) metallocenium salts, [Dy(Cp iPr4R ) 2 ][B(C 6 F 5 ) 4 ] (R = H ( 1 ), Me ( 2 ), Et ( 3 ), iPr ( 4 )), was synthesized by reaction of DyI 3 with the corresponding known NaCp iPr4R (R = H, iPr) and novel NaCp iPr4R (R = Me, Et) salts at high temperature, followed by iodide abstraction with [H(SiEt 3 ) 2 ][B(C 6 F 5 ) 4 ]. Variation of the substituents in this series results in substantial changes in molecular structure, with more sterically-encumbering cyclopentadienyl ligands promoting longer Dy–C distances and larger Cp–Dy–Cp angles. Dc and ac magnetic susceptibility data reveal that these structural changes have a considerable impact on the magnetic relaxation behavior and operating temperature of each compound. In particular, the magnetic relaxation barrier increases as the Dy–C distance decreases and the Cp–Dy–Cp angle increases. An overall 45 K increase in the magnetic blocking temperature is observed across the series, with compounds 2–4 exhibiting the highest 100 s blocking temperatures yet reported for a single-molecule magnet. Compound 2 possesses the highest operating temperature of the series with a 100 s blocking temperature of 62 K. Concomitant increases in the effective relaxation barrier and the maximum magnetic hysteresis temperature are observed, with 2 displaying a barrier of 1468 cm −1 and open magnetic hysteresis as high as 72 K at a sweep rate of 3.1 mT s −1 . Magneto-structural correlations are discussed with the goal of guiding the synthesis of future high operating temperature Dy III metallocenium single-molecule magnets.