Remarkable Effects of Axial π* Coordination on the Cr−Cr Quadruple Bond in Dichromium Paddlewheel Complexes

It is well-known that donation of electron density into the σ* orbital of a Cr−Cr quadruple bond causes major lengthening of the Cr−Cr distance, and there is some prior evidence that a similar lengthening is caused by dative interaction with the π* orbitals. Some molecules have now been made that allow a definitive assessment of this axial π* effect. A molecule has been designed to ensure that there is axial donation into the π* orbitals but not onto the σ* orbital; ligands have been used in which the donor atoms are tethered to the bridging ligands in such a way that they can reach only the π* orbitals but not the σ* orbital. The ligands used for this purpose are the anions of 2,6-di(phenylimino)piperidine (DPhIP) and 2,2‘-dipyridylamine (dpa). In the compound Cr₂(DPhIP)₄ four imino nitrogen lone pairs are suitably positioned to donate to the π* orbitals and the Cr−Cr bond length is 2.265(1) Å. For direct comparison, the compound Cr₂(PhIP)₄ (PhIP is the anion of 2-(phenylimino)piperidine) was made and found to have a Cr−Cr distance of 1.858(1) Å. In this case the ligand is very similar to DPhIP except that it has no donor nitrogen atoms available for axial π* donation. Thus, the cumulative effect of donation from four nitrogen atoms is very large, namely, 0.4 Å in the Cr−Cr distance. The Cr₂(dpa)₄ molecule occurs in three different crystalline compounds, in all of which there are slightly different conformations, but the same Cr−Cr distance, 1.94 ± 0.01 Å; these may be compared to that in the compound Cr₂(mpa)₄ (1.87 Å) in which the bridging is quite similar but there are no tethered additional donor atoms.