An analysis of the electronic structure of the [(MnMnIII)-Mn-II(mu-OH)-(mu-piv)(2)(Me(3)tacn)(2)] (ClO4)(2) (PivOH) complex is reported. It displays features that include: (i) a ground 1/2 spin state; (ii) a small exchange (J) coupling between the two Mn ions; (iii) a mono-mu-hydroxo bridge, bis-mu-carboxylato motif; and (iv) a strongly coupled, terminally bound N ligand to the Mn-III. All of these features are observed in structural models of the oxygen evolving complex (OEC). Multifrequency electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) measurements were performed on this complex, and the resultant spectra simulated using the Spin Hamiltonian formalism. The strong field dependence of the Mn-55-ENDOR constrains the Mn-55 hyperfine tensors such that a unique solution for the electronic structure can be deduced. Large hyperfine anisotropy is required to reproduce the EPR/ENDOR spectra for both the Mn-II and Mn-III ions. The large effective hyperfine tensor anisotropy of the Mn-II, a d(5) ion which usually exhibits small anisotropy, is interpreted within a formalism in which the fine structure tensor of the Mn-III ion strongly perturbs the zero-field energy levels of the (MnMnIII)-Mn-II complex. An estimate of the fine structure parameter (d) for the Mn-III of -4 cm(-1) was made, by assuming the intrinsic anisotropy of the Mn-II ion is small. The magnitude of the fine structure and intrinsic (onsite) hyperfine tensor of the Mn-III is consistent with the known coordination environment of the Mn-III ion as seen from its crystal structure. Broken symmetry density functional theory (DFT) calculations were performed on the crystal structure geometry. DFT values for both the isotropic and the anisotropic components of the onsite (intrinsic) hyperfine tensors match those inferred from the EPR/ENDOR simulations described above, to within 5%. This study demonstrates that DFT calculations provide reliable estimates for spectroscopic observables of mixed valence Mn complexes, even in the limit where the description of a well isolated S = 1/2 ground state begins to break down.