Recent studies have shown that the coordination modes of the acetylide dianion (C 2 2À , IUPAC name acetylenediide) can be classified into three categories: 1) linear end-to-end Scheme 5. a) TiCl 4 , Et 2 O, 0 8C, 88 %; b) 2-(trimethylsilyloxy)furan, BF 3Ó Et 2 , CH 2 Cl 2 , À 78 8C; c) Burgess reagent, PhH, 55 8C, 80 % (two steps); d) 1n LiOH aq , 238C; 16 % HF aq , 238C, 93 % (1:1.4, 1/21); e) 8 n HCl aq , THF, 23 8C, quant.