A Tetranuclear Mixed-Valent Mn$\rm{^{II}_{3}}$MnIV Compound with a (μ4-O)Mn4 Core This work was carried out in the frame of WG0010 of D21 COST action. The authors thank Prof. V. L. Pecoraro for his valuable suggestions. V.T. thanks Dr. Claudio Sangregorio and the Department of Chemistry, Florence, Italy for taking the magnetic measurements.