Metal‐Metal Bonding in Paramagnetic Chromium(III) Complexes – An MO‐Theoretical Case Study

The electronic structures of the edge‐sharing bioctahedral chromium(III) dimers {C 5 H 5 Cr(CH 3 )(μ1‐Cl)} 2 ( 2 ) and {Me 5 C 5 Cr(CH 3 )(μ1‐CH 3 )} 2 ( 4* (Cp* derivative), 4 (Cp derivative)), the trinuclear complex {C 5 H 5 Cr(μ1‐Cl)} 3 (μ1‐CH) ( 3 ), and the face‐sharing bioctahedral chromium(III) compounds {Me 5 C 5 Cr} 2 (μ1‐CH 2 ) ( 6*, 6 ) [{Me 5 C 5 Cr} 2 (μ1‐CH 3 ) 3 ] + ( 7*, 7 ) have been studied MO‐theoretically by the extended Hückel method. Proceeding from 2 over 3 to 4* , or from 6* to 7* a 3‐center/4‐electron chloride or methylene bridge is replaced by a 3‐center/2‐electron methyl group (a methylidyne cap in the trimer). The 3c/2e bridges give rise to an increased metal – metal overlap population due to Cr – Cr bonding within the core levels. In the series of 2 – 3 – 4* this is accompanied by a decrease in the Cr – Cr distance (329 – 284 – 260 pm) and effective magnetic moment, μ eff (2.53 – 2.05 – 1.53 μ B per Cr at room temp.). The latter can be ascribed to an increased splitting within the chromium d‐block because of the shorter metal‐metal separation. The bioctahedra 6* and 7* feature even shorter Cr – Cr contacts than 4* (239 and 242 pm), primarily because of their face‐sharing geometry. However, their effective magnetic moments (per Cr) are 2.33 and 1.32 μ B at room temp. The high magnetic moment for 6* can be understood from the orbital interactions in the frontier orbital metal d block: A face‐sharing bioctahedron shows one σ, two δ, and no π type overlap, while an edge‐sharing bioctahedron ( 4 ) has σ, π, and δ interaction between the metals. In the former this destabilizes only σ* sufficiently to prevent occupation by electrons (leaving five orbitals for six electrons, possible spin multiplicities S0, 1, 2) while for the latter both σ* and π* are destabilized beyond electron occupation (leaving four MOs for six electrons, possible spin multiplicities S0, 1). 4* , 6* and 7* are unusual examples of metal – metal bonded complexes containing octahedral Cr III ions.