The insight into the nature of the decay of 9 in solution came serendipitously, when a single crystal of the Mncontaining product 10 was isolated from the solution of complex 9 used for tracing its decay by UV-vis spectroscopy. The XRD structure of this complex (Figure ) shared much similarity with those of 8•H2O and 9. Yet, the Mn-O bond length of 1.885(4)Å in it lay between those of the Mn(II)-(OH2) and the Mn(IV)-oxo species, and was consistent with the values reported for typical monomeric Mn(III)-OH complexes (1.80-1.90Å). The (+3) oxidation state assignment of this EPR-silent complex was further supported by the Evans method, which yielded μeff = 4.74 BM. This value is very close to the theoretical spin-only magnetic moment of 4.89 BM for a high spin d 4 system. Thus, it appeared that the Mn(IV)-oxo core of 9 decays via a HAA reaction, with the solvent molecules (DCM) -small enough to fit in the cavity -being the most likely HA donors. Indeed, the close approach of the DCM molecule, observed in the XRD structure of 9, might suggest a similar geometry of the corresponding transition state (vide infra).