Copper(I) Complexes with Bis(trimethylsilyl)acetylene: Role of Ancillary Ligands in Determining π Back-Bonding Interactions

We present syntheses and structures of Cu(I) complexes with bis(trimethylsilyl)acetylene (BTMSA) that demonstrate the role of the ancillary ligands in the copper coordination
sphere. The bonding interaction between the Cu(I) ion and the
coordinated alkyne is influenced by the ancillary ligand. The
ligands in this study are the bidentate ligands 1 and 2, which
form monomeric, trigonal planar Cu(I) complexes with olefins
and alkynes. In these complexes, the ancillary ligands vary from
a hard β-diketone 1 to a softer β-diketimine 2. Earlier studies
of Cu(I)−acetylene complexes have shown the Cu(I) ion to be
a poor π back-bonding ion, but the results here demonstrate
that the degree of back-bonding is strongly influenced by the
ancillary ligands. Although ligands 1 and 2 provide very similar
geometries about the Cu(I) ion, the interaction with the
coordinated BTMSA is considerably different. The degree of π
back-bonding for the complex with the β-diketimine ligand is
indicated by the spectroscopic properties of the coordinated
triple bond.