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.