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...M[symbol: see text] M...Cu chains. In these chains the CuI atoms have only weak interactions with the M atoms (Cr...Cu = 2.628 A; Mo...Cu = 2.615 A) even though their introduction causes marked decreases in the M[symbol: see text]M distances (from 2.265 to 1.906 A for Cr and from 2.114 to 2.078 A for Mo). These seemingly contradictory facts are explained by noting that in the M2(DPhIP)4 molecules there are strong destabilizing (to the M[symbol: see text]M bonds) donor interactions of the dangling nitrogen atoms with the pi* orbitals. When these are eliminated by the introduction of the CuI atoms, the M[symbol: see text]M bonds become stronger and shorter.