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 M-2(DPhIP)(4) molecules (M = Cr, Mo; HDPhIP = 2,6-di(p henylimino)piperidine) can each capture two Cu-I atoms to form molecules wi th linear Cu ... M-M-4 ... Cn chains. In these chains the Cu-I atoms have o nly weak interactions with the M atoms (Cr ... Cu = 2.628 Angstrom; Mo ... Cu = 2.615 Angstrom) even though their introduction causes marked decreases in the M-M-4 distances (from 2.265 to 1.906 Angstrom for Cr and from 2.114 to 2.078 Angstrom for Mo). These seemingly contradictory facts are explain ed by noting that in the M-2(DPhIP)(4) molecules there are strong destabili zing (to the M-M-4 bonds) donor interactions of the dangling nitrogen atoms with the pi* orbitals. When these are eliminated by the introduction of th e Cu-I atoms, the M-M-4 bonds become stronger and shorter.