Homoleptic Organometallic Anions of Ti, Zr, Hf, and Nb

Reactions of C 6 H 5 Li and 4‐CH 3 C 6 H 4 Li with halides of Ti, Ir, Hf, and Nb lead to the formation of homoleptic organometallic anions of these metals. Owing to their thermal instability and their sensitivity towards H 2 O and O 2 , these compounds are characterized by single‐crystal structure determinations at low temperature, whereas other physical data could only be obtained occasionally. Three pentacoordinate complex anions [Ti(C 6 H 5 ) 5 ] − , [Ti(4‐CH 3 C 6 H 4 ) 5 ] − , and [Zr(C 6 H 5 ) 5 ] − have square‐pyramidal structures that display only slight deviations from the ideal geometry, in contrast to the already known structures of [Ti(CH 5 ) 5 ] − . The hexacoordinate complex anions [Zr(C 6 H 5 ) 6 ] 2− , [Zr(4‐CH 3 C 6 H 4 ) 6 ] 2− , [Nb(C 6 H 5 ) 6 ] 2− , and [Nb(4‐CH 3 C 6 H 4 ) 6 ] 2− all have trigonal‐prismatic structures, in accord with the known hexamethyl complex dianions. In contrast, the hexacoordinate complex anion [Hf(C 6 H 5 ) 6 ] 2 − has an octahedral or close to octahedral structure, in contrast to the known trigonal‐prismatic structures of [Ta(C 6 H 5 ) 6 ] − and [Ta(4‐CH 3 C 6 H 4 ) 6 ] − . A qualitative explanation for this structural variability is given.