Carboxylate-bridged dimagnesium(II) complexes were synthesized and characterized by employing the dinucleating ligand XDK, where H(2)XDK is m-xylylenediamine bis(Kemp’s triacid imide). The reaction of 1 or 2 equiv of sodium diphenyl phosphate with [Mg-2(XDK)(CH3OH)(4)(H2O)(2)(NO3)](NO3), 1(NO3), afforded [Mg-2(XDK){mu-eta(2)-(PhO)(2)PO2}(CH3OH)(3)(H2O)(NO3)]. 3CH(3)OH(2 . 3CH(3)OH) and [Mg-2(XDK){mu-eta(2)-(PhO)(2)PO2}{eta(1)-(PhO)(2)PO2}(CH3OH)(3)(H2O)]. CH3OH(3 . CH3OH), respectively. These are the first structurally characterized phosphate ester-bridged dimagnesium(II) complexes. The reaction of 1 with 1 equiv of bis(4-nitrophenyl) hydrogen phosphate resulted in protonation of one of the carboxylate ligands and liberation of one magnesium(II) ion to give [Mg(HXDK)(2)(H2O)(2)](4), an octahedral complex containing two short, low barrier intramolecular OH ... O hydrogen bonds. The phosphate ester exchange rate of free and bound diphenyl phosphate in 3 in methanol solution was measured by variable-temperature P-31{H-1} NMR spectroscopy and compared to that of structurally analogous dizinc(II), [Zn-2(XDK){mu-eta(2)-(PhO)(2)PO2}{eta(1)-(PhO)(2)PO2}(CH3OH)(2)(H2O)] (5), and dicalcium(II), [Ca-2(XDK){mu-eta(2)-(PhO)(2)PO2}{eta(1)-(PhO)(2)PO2}(CH3OH)(3)(H2O)]. CH3OH), complexes. The synthesis and structural characterization of 6 is presented, along with a discussion of the differences between the carboxylate- and phosphate ester-bridged dimagnesium(II), dizinc(II), and dicalcium(II) centers. Crystallographic data are as follows. 1(NO3) : monoclinic, P2(1)/c, a = 11.240(3) Angstrom, b = 13.019(2) Angstrom, c = 30.208(7) Angstrom, beta = 99.11(1)degrees, V = 4365(2) Angstrom(3), Z = 4, R = 0.045, and R(w) = 0.054 for 5021 independent reflections with I > 3 sigma(I). 2 . 3CH(3)OH : monoclinic, P2(1)/c, a = 16.611(5) Angstrom, b = 16.059(6) Angstrom, c = 21.930(9) Angstrom, beta = 93.34(6)degrees, V = 5840(4) Angstrom(3), Z = 4, R = 0.069, and R(w) = 0.085 for 3759 independent reflections with I > 2 sigma(I). 3 . CH3OH : monoclinic, P(2)1/n, a = 18.912(4) Angstrom, b = 16.254(2) Angstrom, c = 21.646(5) Angstrom, beta = 112.26(2)degrees, V = 6158(2) Angstrom(3), Z = 4, R = 0.060, and R(w) = 0.072 for 5184 independent reflections with I > 3 sigma(I). 4 : monoclinic, P2(1)/n, a = 15.210(5) Angstrom, b = 15.772(3) Angstrom, c = 13.093(3) Angstrom, beta = 96.35(3)degrees, V = 3122(1) Angstrom(3), Z = 2, R = 0.069, and R(w) = 0.070 for 1840 independent reflections with I > 2 sigma(I). 6 . CH3OH : monoclinic, P2(1)/n, a = 16.5471(3) Angstrom, b = 24.3415(6) Angstrom, c = 16.5865(3) Angstrom, beta = 104.2530(10)degrees, V = 6475.1(2) Angstrom(3), Z = 4, R = 0.053, and wR(2) = 0.142 for 7858 independent reflections with I > 2 sigma(I).