The synthesis of magnesium tetraarylporphyrins has been investigated using magnesium halides in noncoordinating solvents with hindered amine bases. The rate of reaction increases in the series MgCl2 << MgBr2 < MgBr2 . O(Et)(2) < MgI2. Considerable latitude exists in selecting among magnesium reagents (MgBr2, MgBr2 . O(Et)(2), MgI2), solvents (toluene, CH2Cl(2), CHCl3), and bases (triethylamine, diisopropylethylamine, 2,2,6,6-tetramethylpiperidine) for efficient metalation of tetraphenylporphyrin at room temperature. Thus treatment of a toluene, CH2Cl2, or CHCl3 solution of tetraphenylporphyrin with excess MgBr2, MgBr2 . O(Et)(2), or MgI2 and triethylamine at room temperature quantitatively affords the magnesium chelate in <10 min. Tetramesitylporphyrin is converted to the magnesium chelate with MgI2 and diisopropylethylamine in CH2Cl2 at room temperature in 10 min or by reaction with MgBr2 . O(Et)(2) and triethylamine in toluene at 60 degrees C for 1 h. [Tetrakis(2,6-dimethoxyphenyl)-porphinato]magnesium(II) was formed in similar fashion. The reaction conditions are compatible with porphyrins bearing (trimethylsilyl)ethynyl groups, and the reactions can be performed in the presence of zinc tetraphenylporphyrin without transmetalation. This approach is fundamentally distinct from that with DMF-MgCl2, which is designed to achieve high mutual solubility of the metal ion and free base porphyrin at elevated temperature. The facile magnesium insertion achieved with MgBr2, MgBr2 . O(Et)(2), or MgI2 at room temperature is attributed to the lability of their ligands, their partial organic solubility, and the limited stability of their crystal lattices relative to the porphyrin magnesium chelate. A noncoordinating milieu is essential to avoid forming octahedral complexes of magnesium that are more stable than the magnesium porphyrin. The ability to form magnesium tetraarylporphyrins under gentle conditions enables biomimetic studies where zinc porphyrins have previously been used.