From high field and frequency electron paramagnetic resonance (EPR) powder spectra of the high spin chromium(II) complex trans-[Cr(NCCH 3 ) 4 (FBF 3 ) 2 ] (1) recorded at T=5–40K the EPR parameters were found to be D=−2.19cm −1 , E≈0cm −1 , and g ‖ =g ⊥ =1.9833. The reaction between a MeCN solution of 1 and dioxygen gave dark green crystals of [(CH 3 CN) 5 CrOCr(NCCH 3 ) 5 ](BF 4 ) 4 ·2CH 3 CN (2). From the X-ray crystal structure of 2 at T=122K a Cr–O–Cr angle of 180° and a Cr–O distance of 1.7556(5)Å was found. This is the shortest among the known Cr–O distances in dinuclear complexes containing a CrOCr 4+ core, and this can partly explain the strong antiferromagnetic coupling between the two chromium(III) centers in 2.