We report the resonance Raman (RR) spectra of iron complexes containing the Fe-2(mu-O)(2) core. Frozen CH3CN solutions of the (FeFeIV)-Fe-III intermediate [Fe-2(mu-O)(2)L-2](ClO4)(3) (where L = TPA, 5-Me-3-TPA, 5-Me-2-TPA, 5-MeTPA, 5-Et-3-TPA, or 3-Me-3-TPA) show numerous resonance-enhanced vibrations, and among these, an oxygen-isotope-sensitive vibration around 667 cm(-1) that shifts ca. 30 cm(-1) when the samples are allowed to exchange with (OH2)-O-18, and whose Raman shift does not vary with methyl substitution of the TPA ligand. Spectra of iron-isotope-substituted samples of [Fe-2(mu-O)(2)(L)2(])(ClO4)(3) (Fe-54 and Fe-57 for L = TPA, and Fe-54 and Fe-58 for L = 5-Me-3-TPA) show that this vibration is also iron-isotope sensitive. These isotopic data taken together strongly suggest that this vibration involves motion of the Fe-2(mu-O)(2) core that is isolated from motions of the Ligand. A frozen CH3CN solution of the diiron(III) complex [Fe-2(mu-O)(2)(6-Me-3-TPA)(2)](ClO4)(2) shows one intense resonance-enhanced vibration at 692 cm(-1) that shifts -30 cm(-1) with O-18 labeling. Normal coordinate analysis of the Fe-2(mu-O)(2) core in [Fe-2(mu-O)(2)(5-Me-3-TPA)(2)](ClO4)(3) supports the assignment of the Fermi doublet centered around 666.2 cm(-1) as an A(1) vibration of this core. Furthermore, we propose that this unique feature found in the region between 650 and 700 cm(-1) is indicative of a diamond core structure and is the Raman signature of an iron cluster containing this core.