A family of magnesium and calcium salen-derivatives was synthesized and characterized for use as subterranean fluid flow monitors. For the Mg complexes, di-n-butyl magnesium ([Mg(Bu-n)(2)]) was reacted with N,N'-ethylene bis(salicylideneimine) (H-2-salen), N,N'-bis(salicylidene)-1,2-phenylenediamine (H-2-saloPh), N,N'-bis(3,5-di-t-butylsalicylidene)-ethylenediamine (H-2-salo-Bu-t), or N,N'-bis(3,5-di-t-butylsalicylidene)-1,2-phenylenediamine (H-2-saloPh-Bu-t), and the products were identified by single-crystal X-ray diffraction as [(kappa(3)-(O,N,N'),mu-(O')saloPh)(mu-(O),(kappa(2)-(N,N'),mu-(O')saloPh)(2)(mu-(O),kappa(3)-(N,N',O')saloPh')Mg-4] 2tol (1.2tol; saloPh' = an alkyl-modified saloPh derivative generated in situ), [(kappa(4)-(O,N,N',O')saloPh)Mg(py)(2)] py (2 py), [(kappa(4)-(O,N,N',O')salo-Bu-t)Mg(py)(2)] (3), [(kappa(4)-(O,N,N',O')saloPh-Bu-t)Mg(py)(2)] tol (4 tol), and [(kappa(3)-(O,N,N'),mu-(O')saloPh-Bu-t)Mg](2) (5), where tol = toluene; py = pyridine. For the Ca species, a calcium amide was independently reacted with H-2-salo-Bu-t and H-2-saloPh-Bu-t to generate the crystallographcially characterized compounds: [(kappa(4)-(O,N,N',O')salo-Bu-t)Ca(py)(3)] (6), [(kappa(4)-(O,N,N',O')saloPh-Bu-t)Ca(py)(3)]py (7py). The bulk powders of these compounds were further characterized by a number of analytical tools, where 2-7 were found to be distinguishable by Fourier transform infrared and resonance Raman spectroscopies. Structural properties obtained from quantum calculations of gas-phase analogues are in good agreement with the single-crystal results. The potential utility of these compounds as taggants for monitoring subterranean fluid flows was demonstrated through a series of experiments to evaluate their stability to high temperature and pressure, interaction with mineral surfaces, and elution behavior from a loaded proppant pack.