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 sigma* 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 pi* orbitals. Some molecules have now been made that allow a definitive assessment of this axial pi* effect. A molecule has been designed to ensure that there is axial donation into the pi* orbitals but not onto the sigma* 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 pi* orbitals but not the sigma* 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 Cr-2(DPhIP)(4) four imino nitrogen lone pairs are suitably positioned to donate to the pi* otbitals and the Cr-Cr bond length is 2.265(1) Angstrom. For direct comparison, the compound Cr-2(PhIP)(4) (PhIP is the anion of 2-(phenylimino)piperidine) was made and found to have a Cr-Cr distance of 1.858(1) Angstrom. In this case the ligand is very similar to DPhIP except that it has no donor nitrogen atoms available for axial pi* donation. Thus, the cumulative effect of donation from four nitrogen atoms is very large, namely, 0.4 A in the Cr-Cr distance. The Cr-2(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 Angstrom; these may be compared to that in the compound Cr-2(mpa)(4) (1.87 Angstrom) in which the bridging is quite similar but then are no tethered additional donor atoms.