We report a solid-state Mg-25 NMR spectroscopic study of two magnesium-containing organic compounds: monopyridinated aqua( magnesium) phthalocyanine (MgPc, H2O, Py) and chlorophyll a (Chla). Each of these compounds contains a Mg(II) ion coordinating to four nitrogen atoms and a water molecule in a squarepyramidal geometry. Solid-state Mg-25 NMR spectra for MgPc, H2O, Py were obtained at 11.7 T (500 MHz for H-1) for a Mg-25-enriched sample (99.1% Mg-25 atom) using both Hahn-echo and quadrupole Carr-Purcell Meiboom-Gill (QCPMG) pulse sequences. Solid-state Mg-25 NMR spectra for Chla were recorded at Mg-25 natural abundance (10.1%) at 19.6 T (830 MHz for 1H). The Mg-25 quadrupole parameters were determined from spectral analyses: MgPc, H2O, Py, C-Q = 13.0 +/- 0.1 MHz and A(Q) = 0.00 +/- 0.05; Chla, C-Q = 12.9 +/- 0.1 MHz and A(Q) = 1.00 +/- 0.05. This work represents the first time that Mg( II) ions in a square-pyramidal geometry have been characterized by solid-state 25Mg NMR spectroscopy. Extensive quantum mechanical calculations for electric-field-gradient (EFG) and chemical shielding tensors were performed at restricted Hartee-Fock (RHF), density functional theory (DFT), and second-order Miller-Plesset perturbation theory (MP2) levels for both compounds. Computed Mg-25 nuclear quadrupole coupling constants at the RHF and MP2 levels show a reasonable basis-set convergence at the cc-pV5Z basis set (within 7% of the experimental value); however, B3LYP results display a drastic divergence beyond the cc-pVTZ basis set. A new crystal structure for (MgPcH2OPy)-H-.-Py-. is also reported.