Metal alkoxides: models for metal oxides. 17. Reductive cleavage of carbon monoxide to carbide and oxide by ditungsten and tetratungsten alkoxides. Crystal and molecular structures of W4(.mu.4-C)(OCH2-c-Pen)14, W4(.mu.4-C)(O)(OCH2-tert-Bu)12 and W4(.mu.4-C)(O)(O-i-Pr)12