Remarkably Short Metal-Metal Bonds: A Lantern-Type Quintuply Bonded Dichromium(I) Complex

摘要

The field of quadruply bonded dinuclear complexes in which two metal atoms are embraced by eight ligands has been considered mature. The bonding and electronic structures of these compounds have been well understood ever since the discovery of the first dimetal species containing a quadruple bond, [Re2Cl8]~(2-), over 40 years ago. The quest for thermally stable and isolable dinuclear complexes with higher bond orders is one aim of chemists in this field. From the synthetic point of view, quintuply bonded dinuclear complexes have become the focus in the past few years. On the theoretical side, the hypothetical RMMR molecules (M = Cr, Mo, W, U; R = H, F, Cl, Br, CN, Me) have been highlighted, particularly the trans-bent structures. These feature a quintuple bond between two metal atoms, and for RCrCrR, the calculated Cr-Cr bond lengths are in the range of 1.64-1.78 A. Experimentally, a recent landmark advance was the isolation of a quintuply bonded dichro-mium(I) complex supported by two monodentate bulky carbyl ligands [Ar'CrCrAr'] (Ar' = C6H3-2,6-(C6H3-2,6-iPr2)2), in which, coincidentally, a trans-bent (C_(2h)) geometry is adopted and the two chromium atoms share five electron pairs in five bonding molecular orbitals according to computational analyses. Moreover, D_(2h)-symmetric [Cr2(μ-η~2-~HL~(iPr))2] (~HL~(iPr) = N, N'-bis(2,6-diisopropylphenyl)-1,4-diaza-diene) was recently reported to feature a very short Cr-Cr distance of 1.8028(9) A and calculated to display some degree of fivefold bonding.