The syntheses, characterizations, and physical properties of asymmetric polyethyl-substituted mixed-valence 1’,2’,1triple prime-triethylbiferrocenium triiodide (15), 1’,3’,1triple prime-triethylbiferrocenium triiodide (16), and 1’,2’,1triple prime,3triple prime-tetraethylbiferrocenium triiodide (17) are described. Complexes 15-17 were characterized by electrochemical measurements and by near-IR, Fe-57 Mossbauer, and paramagnetic H-1 NMR spectroscopy. These complexes are found to be localized on the Fe-57 Mossbauer time scale (electron-transfer rates less than 10(7) s(-1)). The cations in complexes 15-17 are not in equivalent environments. This asymmetry results in a nonzero zero-point energy barrier for intramolecular electron transfer. Analysis of the sign of contact shifts suggests that the electron delocalization in 15-17 is based on competing a and re delocalization mechanisms. The X-ray structure of 15 has been determined at 298 K : monoclinic, P2(1)/n, a = 9.962(3) Angstrom, b = 20.610(3) Angstrom, c = 13.326(3) Angstrom, beta = 92.74(2)degrees, Z = 4, D-calcd = 2.029 g cm(-3), R(F) = 0.033, R(wF) = 0.036. The neutral compound 1’,2’,1triple prime-triethylbiferrocene crystallizes in the triclinic space group P (1) over bar with two molecules in a unit cell with dimensions a 7.465(3), b = 8.419(2), c = 18.581(4) Angstrom and alpha = 87.83(2), beta = 87.98(3), gamma = 64.27(2)degrees; R(F) = 0.021 and R(wF) = 0.029.