The reactions of the water-soluble chelating phosphines 1,2-bis(bis(hydroxyalkyl)phosphino) ethane (alkyl = n-propyl, DHPrPE; n-butyl, DHBuPE; n-pentyl, DHPePE) with FeCl2.4H(2)O and FeSO4.7H(2)O were studied as routes to water-soluble complexes that will bind small molecules, dinitrogen in particular. The products that form and their stereochemistry depend on the solvent, the counteranion, and the alkyl chain length on the phosphine. In alcoholic solvents, the reaction of FeCl2.4H(2)O with 2 equiv of DHBuPE or DHPePE gave trans-Fe(L-2)(2)Cl-2. The analogous reactions in water with DHBuPE and DHPePE gave only cis products, and the reaction of FeSO4.7H(2)O with any of the phosphines gave only cis-Fe(L-2)(2)SO4. These results are interpreted as follows. The trans stereochemistry of the products from the reactions of FeCl2.4H(2)O in alcohols is suggested to be the consequence of the trans geometry of the Fe(H2O)(4)Cl-2 complex, i.e., substitution of the water molecules by the phosphines retains the geometry of the starting material. The formation of cis-Fe(DHPrPE)(2)Cl-2 is an exception to this result because the coordination of two -OH groups forms two six-membered rings, as shown in the X-ray structure of the molecule. DHBuPE and DHPePE reacted with FeSO4.7H(2)O in water to initially yield cis-Fe(P-2)(2)SO4 compounds, but subsequent substitution reactions occurred over several hours to give sequentially trans-Fe(DHBuPE)(2)(H2O)(SO4) and then trans-[Fe(DHBuPE)(2)(H2O)(2)]SO4. The rate constants and activation reactions for these aquation reactions were determined and are consistent with dissociatively activated mechanisms. The cis-and trans-Fe(L-2)(2)X (X = (Cl)(2) or SO4) complexes react with N-2, CO, and CH3CN to yield trans complexes with bound N-2, CO, or CH3CN. The crystal structures of the cis-Fe(DHPrPE)(2)SO4, trans-Fe(DHPrPE)(2)(CO)SO4, trans- Fe(DHBuPE)(2)Cl-2, trans-[Fe(DHBuPE)(2)(CO)(Cl)][B(C6H5)(4)], trans-Fe(DMeOPrPE)(2)Cl-2, trans-Fe(DMeOPrPE)(2)Br-2, and trans-[Fe(DHBuPE)(2)Cl-2]Cl complexes are reported. As expected from using water-soluble phosphines, the complexes reported herein are water soluble (generally greater than 0.5 M at 23 degreesC).