Chromocenium: Structural and Electronic Characterization of a Forgotten Member of the Unsubstituted Metallocenes, [Cr(η ⁵ -C ₅ H ₅ ) ₂ ] ⁺

Simple metallocenes of the general formula [MCp2]0,+, where M = a wide range of 3, 4, 5d 2+ or 3+ ions and Cp– = η5-cyclopentadienide anion, have been known for more than half a century and are paradigms of organometallic chemistry. Chromium(III) is perhaps the most common oxidation state of chromium. Yet the corresponding metallocene, chromocenium, [CrCp2]+, has been relatively little examined, despite having been first prepared in 1962 (as an iodide salt) by E. O. Fischer who was a pioneer of metallocene chemistry. We report here the synthesis of [CrCp2][BArF], (BArF = tetrakis[(3,5-trifluoromethyl)phenyl]borate) and its structural and spectroscopic characterization. Spectroscopic methods comprised X-band electron paramagnetic resonance (EPR) and high-frequency and -field EPR (HFEPR) in conjunction with magnetic circular dichroism (MCD). The S = 3/2 ground state of [CrCp2][BArF] gives spin Hamiltonian parameters: D = +4.82(1) cm–1, E = 0, gx(⊥) = gz(||) = 2.00(1). The axial electronic symmetry is expected for this 5-fold symmetric complex and the positive sign of D is as found earlier for isoelectronic vanadocene, [VCp2], but the magnitude of D here is larger, qualitatively indicating the difference between isoelectronic CrIII and VII ions. Ligand-field theory (LFT) and quantum chemical theory (QCT) are used to analyze quantitatively the electronic structure of [CrCp2]+ on its own and in the context of its better known 3d3 congener [VCp2].