The synthesis and magnetochemical properties of K-4[Mn18O16(O(2)CPh)(22)(phth)(2)(H2O)(4)]. 10MeCN (5a) are reported. This complex was prepared by the reaction of (NBu(4)(n))[Mn4O2(O(2)CPh)(9)(H2O)] with potassium hydrogen phthalate, KH(phth). Complex 5a crystallizes in monoclinic space group C2/c with the following cell parameters at -167 degrees C : a = 22.673(5) Angstrom, b = 33.426(7) Angstrom, c = 27.991(8) Angstrom, beta = 105.06(1)degrees, V = 20.484 Angstrom(3), and Z = 4. The structure was refined employing 13 479 unique reflections with F > 3 sigma(F) to give R = 0.0607 and R(w) = 0.0606. Complex 5a has eighteen Mn-III ions held together by sixteen mu(3)-O2- ions to give a [Mn18O16](22+) core. The two phthalate (phth(2-)) groups each bridge a total of four Mn atoms with each of their oxygen atoms terminally ligated. The [Mn18O16](22+) core can be described as arising from the fusion of five [Mn-4(mu(3)-O)(2)](8+) units possessing either a planar or bent ("butterfly") Mn-4 unit. Three such units are fused body-to-body to give the central [Mn10O6](18+) unit. Two additional [Mn4O2](8+) units are attached to the [Mn10O6](18+) unit at the six "wing-tip" positions. Variable temperature and variable-field de magnetic susceptibility data were collected for a polycrystalline sample of the MeCN-desolvated complex 5. In a 10.0 kG field mu(eff) per molecule varies gradually from 15.1 mu(B) at 320 K to 1.73 mu(B) at 2.00 K. Alternating current (ac) magnetic susceptibility data measured in the 2.0-60.0 K range in zero de field with a 1.0 G ac field oscillating at 50 Hz nearly superimpose with the de susceptibility data. As the temperature approaches zero, chi(M)T approaches zero. Thus, the Mn-18(III) complex 5 has a S = 0 ground state. This is unusual for polynuclear Mn-III complexes, which generally have high-spin ground states. The S = 0 ground state for complex 5 is explained in terms of spin frustration that is likely present in the [Mn4O2](8+) butterfly building blocks.