Xenia Engelmann + , Shenglai Yao + , Erik R. Farquhar, Tibor Szilvμsi, Uwe Kuhlmann, Peter Hildebrandt, Matthias Driess,* and Kallol Ray* Abstract: The strikingly different reactivity of a series of homo- and heterodinuclear [(M III )( m -O) 2 (M III ) ’ ] 2 + (M = Ni; M ’ = Fe, Co, Ni and M = M ’ = Co) complexes with b -diket- iminate ligands in electrophilic and nucleophilic oxidation reactions is reported, and can be correlated to the spectroscopic features of the [(M III )( m -O) 2 (M III ) ’ ] 2 + core. In particular, the unprecedented nucleophilic reactivity of the symmetric [Ni III - ( m -O) 2 Ni III ] 2 + complex and the decay of the asymmetric [Ni III ( m -O) 2 Co III ] 2 + core through aromatic hydroxylation reac- tions represent a new domain for high-valent bis( m -oxido)di- metal reactivity.