Heteronuclear Chains of Four Metal Atoms Including One Quadruply Bonded Dimetal Unit. Dichromium and Dimolybdenum Compounds with Appended Copper(I) Atoms

It is shown that the M2(DPhIP)4 molecules (M = Cr, Mo; HDPhIP = 2,6-di(phenylimino)piperidine) can each
capture two CuI atoms to form molecules with linear Cu···MM···Cu chains. In these chains the CuI atoms
have only weak interactions with the M atoms (Cr···Cu = 2.628 Å; Mo···Cu = 2.615 Å) even though their
introduction causes marked decreases in the MM distances (from 2.265 to 1.906 Å for Cr and from 2.114 to
2.078 Å for Mo). These seemingly contradictory facts are explained by noting that in the M2(DPhIP)4 molecules
there are strong destabilizing (to the MM bonds) donor interactions of the dangling nitrogen atoms with the π*
orbitals. When these are eliminated by the introduction of the CuI atoms, the MM bonds become stronger and
shorter.