Coordination chemistry of sterically hindered thiolate ligands. Preparation and structural characterization of the oligomeric homoleptic complexes [o-trimethylsilylbenzenethiolato]copper dodecamer and [o-trimethylsilylbenzenethiolato]silver octamer and a comparison to the structure of the cadmium mononuclear species (Et4N)2[Cd(SC6H4-o-SiMe3)4]