AbstractThe seven‐, eight‐, and nine‐membered magnesium‐containing heterocycles magnesepin, C6H6Mg, 1,4‐dimagnesocin, C6H6Mg2, and 1,4,7‐trimagnesonin, C6H6Mg3, have been examined computationally at the density functional B3LYP/6‐311++G** level of theory. The MgH‐substituted benzene isomers of these heterocycles [C6H5MgH; 1,2‐; 1,3‐; and 1,4‐C6H4(MgH)2; and 1,2,3‐; 1,2,4‐; and 1,3,5‐C6H3(MgH)3], the mono‐ and di‐MgH‐substituted magnesepins [2‐; 3‐; and 4‐C6H5Mg(MgH) and 2,3‐; 2,4‐; 2,5‐; 2,6‐; 2,7‐; 3,4‐; 3,5‐; 3,6‐; and 4,5‐C6H4Mg(MgH)2], and the mono‐MgH‐substituted 1,4‐dimagnesocins [2‐; 5‐; and 6‐C6H5Mg2(MgH)] have also been investigated for comparative purposes. Magnesepin optimizes as a planar heterocyclic triene with no significant aromatic character, while both 1,4‐dimagnesocin and 1,4,7‐trimagnesonin optimize as nonplanar, twisted molecules. In all cases, the isomeric MgH‐substituted benzene systems were found to contain planar rings and were more stable than the isomeric magnesium‐containing heterocycles, but the nonplanar mono‐MgH‐substituted magnesepins were found to be less stable than 1,4‐dimagnesocin, and the nonplanar di‐MgH‐substituted 1,4‐dimagnesocins were generally less stable than the mono‐MgH‐substituted magnesepins and were more stable than 1,4,7‐trimagnesonin. Evidence for intramolecular, unsymmetrical μ2‐bridging hydrogen atoms was observed in those molecules containing adjacent MgH substituents.