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

Abstract

AbstractThe electronic structures of the edge‐sharing bioctahedral chromium(III) dimers {C5H5Cr(CH3)(μ1‐Cl)}2 (2) and {Me5C5Cr(CH3)(μ1‐CH3)}2 (4* (Cp* derivative), 4 (Cp derivative)), the trinuclear complex {C5H5Cr(μ1‐Cl)}3(μ1‐CH) (3), and the face‐sharing bioctahedral chromium(III) compounds {Me5C5Cr}2(μ1‐CH2) (6*, 6) [{Me5C5Cr}2(μ1‐CH3)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, μeff (2.53 – 2.05 – 1.53 μB 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 μB 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 S0, 1, 2) while for the latter both σ* and π* are destabilized beyond electron occupation (leaving four MOs for six electrons, possible spin multiplicities S0, 1). 4*, 6* and 7* are unusual examples of metal – metal bonded complexes containing octahedral CrIII ions.