VARIATIONS IN THE CU(I)-CU(I) DISTANCES IN MULTINUCLEAR CLUSTERS WITHIDENTICAL COORDINATION GEOMETRIES - SHORT METAL-METAL CONTACTS INDUCED BY OLIGOMERIZATION

Factors contributing to the variations in the Cu(I)-Cu(I) distances in two clusters with identical ligand and coordination geometries have b een analyzed. While the hexamer, 4, exhibits metal-metal distances in the range 2.81-3.25 Angstrom, shorter contacts are found in the corres ponding tetramer, 3 (2.60-2.77 Angstrom). EHT calculations reveal rela tively little attractive interactions in the corresponding Cu-4(4+) an d Cu-6(6+) cores. Introduction of the ligands lowers the reduced overl ap populations between the metals further. MNDO calculations with mode l electrophiles have been carried out to determine the bite angle requ irements of the ligands. These are satisfactorily met in the structure s of both 3 and 4. The key geometric feature distinguishing 3 and 4 is the Cu-S-Cu angle involving the bridging S- unit. In 4, the correspon ding angles are about 90 degrees, while the values in 3 are smaller (7 0-73 degrees). Wider angles are computed to be energetically favored a nd are characterized by an open three-center bond and a long Cu-Cu dis tance. The bridging angles are suggested to be primarily constrained b y the mode of oligomerization. Implications of these results for the s tability and reactivity of these clusters and for short metal-metal di stances in d(10) systems in general are discussed.