The high-spin S = 2 Mn(III) complex [Mn{(OPPh2)(2)N}(3)] (1(Mn)) exhibits field-induced slow relaxation of magnetization (Inorg. Chem.2013, 52, 12869). Magnetic susceptibility and dual-mode X-band electron paramagnetic resonance (EPR) studies revealed a negative value of the zero-field-splitting (zfs) parameter D. In order to explore the magnetic and electronic properties of 1(Mn) in detail, a combination of experimental and computational studies is presented herein. Alternating-current magnetometry on magnetically diluted samples (1(Mn)/1(Ga)) of 1(Mn) in the diamagnetic gallium analogue, [Ga{(OPPh2)(2)N}(3)], indicates that the slow relaxation behavior of 1(Mn) is due to the intrinsic properties of the individual molecules of 1(Mn). Investigation of the single-crystal magnetization of both 1(Mn) and 1(Mn)/1(Ga) by a micro-SQUID device reveals hysteresis loops below 1 K. Closed hysteresis loops at a zero direct-current magnetic field are observed and attributed to fast quantum tunneling of magnetization. High-frequency and -field EPR (HFEPR) spectroscopic studies reveal that, apart from the second-order zfs terms (D and E), fourth-order terms (B-4(m)) are required in order to appropriately describe the magnetic properties of 1(Mn). These studies provide accurate spin-Hamiltonian (sH) parameters of 1(Mn), i.e., zfs parameters vertical bar D vertical bar = 3.917(5) cm(-1), vertical bar E vertical bar = 0.018(4) cm(-1), B-4(0) = B-4(2) = 0, and B-4(4) = (3.6 +/- 1.7) x 10(-3) cm(-1) and g = [1.994(5), 1.996(4), 1.985(4)], and confirm the negative sign of D. Parallel-mode X-band EPR studies on 1(Mn)/1(Ga) and CH2Cl2 solutions of 1(Mn) probe the electronic-nuclear hyperfine interactions in the solid state and solution. The electronic structure of 1(Mn) is investigated by quantum-chemical calculations by employing recently developed computational protocols that are grounded on ab initio wave function theory. From computational analysis, the contributions of spin-spin and spin-orbit coupling to the magnitude of D are obtained. The calculations provide also computed values of the fourth-order zfs terms B-4(m), as well as those of the g and hyperfine interaction tensor components. In all cases, a very good agreement between the computed and experimentally determined sH parameters is observed. The magnetization relaxation properties of 1(Mn) are rationalized on the basis of the composition of the ground-state wave functions in the absence or presence of an external magnetic field.