Remarkable effects of axial pi* coordination on the Cr-Cr quadruple bond in dichromium paddlewheel complexes
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abstract
It is well-known that donation of electron density into the * 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 * orbitals. Some molecules have now been made that allow a definitive assessment of this axial * effect. A molecule has been designed to ensure that there is axial donation into the * orbitals but not onto the * 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 * orbitals but not the * 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 * orbitals and the Cr-Cr bond length is 2.265(1) . 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) . In this case the ligand is very similar to DPhIP except that it has no donor nitrogen atoms available for axial * donation. Thus, the cumulative effect of donation from four nitrogen atoms is very large, namely, 0.4 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, 1.940.01 ; these may be compared to that in the compound Cr2(mpa)4 (1.87 ] in which the bridging is quite similar but there are no tethered additional donor atoms.
