Carboxylate-bridged high-spin diiron(II) complexes with distinctive electronic transitions were prepared by using 4-cyanopyridine (4-NCC5H4N) ligands to shift the charge-transfer bands to the visible region of the absorption spectrum. This property facilitated quantitation of water-dependent equilibria in the carboxylate-rich diiron(II) complex, [Fe-2(mu-O2CArTol)(4)(4-NCC5H4N)(2)] (1), where (-O2CArTol) is 2,6-di-(p-tolyl)benzoate. Addition of water to 1 reversibly shifts two of the bridging carboxylate ligands to chelating terminal coordination positions, converting the structure from a paddlewheel to a windmill geometry and generating [Fe-2(mu-O2CArTol)(2)(O2CArTol)(2)(4-NCC5H4N)(2)(H2O)(2)] (3). This process is temperature dependent in solution, rendering the system thermochromic. Quantitative treatment of the temperature-dependent spectroscopic changes over the temperature range from 188 to 298 K in CH2Cl2 afforded thermodynamic parameters for the interconversion of 1 and 3. Stopped flow kinetic studies revealed that water reacts with the diiron(II) center ca. 1000 time faster than dioxygen and that the water-containing diiron(II) complex reacts with dioxygen ca. 10 times faster than anhydrous analogue 1. Addition of {H(OEt2)(2)}{BAr'4}, where BAr'(-)(4) is tetrakis (3,5-di(trifluoromethyl)phenyl)borate, to 1 converts it to [Fe-2(mu-O2CArTol)(3)(4-NCC5H4N)(2)]-(BAr'(4)) (5), which was also structurally characterized. Mossbauer spectroscopic investigations of solid samples of 1, 3, and 5, in conjunction with several literature values for high-spin iron(II) complexes in an oxygen-rich coordination environment, establish a correlation between isomer shift, coordination number, and N/O composition. The products of oxygenating 1 in CH2Cl2 were identified crystallographically to be [Fe-2(mu-OH)(2)(mu-O2CAr (Tol)) (2)(O2CArTol)(2)(4-NCC5H4N)(2)]center dot 2(HO2CArTol) (6) and [Fe-6(mu-O)(2)(mu-OH)(4)(mu-O2CArTol)(6)(4-NCC5H4N)(4)Cl-2] (7).