Facile substitution reactions of the two water ligands in the hydrophilic tetradentate phosphine complex cis-[Fe\{({HOCH2)P\{CH2N(CH2P(CH2OH)(2))CH2\}(2)P(CH2OH)\}(H2O)(2)](SO4)} ( abbreviated to [Fe(L-1)({H2O)(2)](SO4)},1) take place upon addition of Cl-, {NCS-}, N-3(-), {CO32-} and {CO} to give [Fe(L-{1)X-2]} (2, X = Cl; 4, X = {NCS;} 5, X= N-3), [Fe(L-1)(kappa(2)-{O2CO)]}, 6 and [Fe(L-1)({CO)(2)](SO4)}, 7. The unsymmetrical mono-substituted intermediates [Fe(L-1)({H2O)(CO)](SO4)} and [Fe(L-1)({CO)(kappa(1)-OSO3)]} (8/9) have been identified spectroscopically en-route to 7. Treatment of 1 with acetic anhydride affords the acylated derivative [Fe\{({AcOCH2)P\{CH2N(CH2P(CH2OAc)(2))CH2\}(2)P(CH2OAc)\}(kappa(2)-O2SO2)]} (abbreviated to [Fe(L-2)(kappa(2)-{O2SO2)]}, 10), which has increased solubility over 1 in both organic solvents and water. Treatment of 1 with glycine does not lead to functionalisation of L-1, but substitution of the aqua ligands occurs to form [Fe(L-1)({NH2CH2CO2-kappa} N-2, O)]({HSO4)}, 11. Compound 10 reacts with chloride to form [Fe(L-{2)Cl-2]} 12, and 12 reacts with {CO} in the presence of {NaBPh4} to form [Fe(L-{2)Cl(CO)](BPh4)} 13b. Both of the chlorides in 12 are substituted on reaction with {NCS-} and N3- to form [ Fe( L2)( {NCS)} 2] 14 and [ Fe( L2)( N3) 2] 15, respectively. Complexes 2 (.) {H2O}, 4({.)2H(2)O}, 5({.)0.812H(2)O}, 6({.)1.7H(2)O}, 7 (.) {H2O}, 10({.)1.3CH(3)C(O)CH3}, 12 and 15({.)0.5H(2)O} have all been crystallographically characterised. ; 5