Stepwise Insertion of Alkylisocyanides into the Metal‐Alkyne Bond of Half‐Sandwich Type Rhodium Complexes: Synthesis and Structural Characterization of Metallacyclobutenes and Metallacyclopentenes

The cyclopentadienyl complexes [C 5 H 5 Rh(RCCR′)(Sb i Pr 3 )] ( 5–8 ), which were prepared from trans ‐[RhCl‐(RCCR′)(Sb i Pr 3 ) 2 ] ( 1–4 ) and NaC 5 H 5 and which contain a labile Rh–Sb i Pr 3 bond, reacted with CO and CNR″ (R″ = Me, t Bu) to give the carbonyl and isocyanide derivatives [C 5 H 5 Rh(RCCR′)(CO)] ( 9–11 ) and [C 5 H 5 Rh(RCCR′)‐(CNR″)] ( 12–16 ), respectively. On treatment of 12 (R = R′ = Ph; R″ = Me) with Sb i Pr 3 , the metallacyclobutene complex [C 5 H 5 Rh{κ 2 ( C,C }‐ C ( = NMe)CPh= C Ph}(Sb i Pr 3 )] ( 17 ) was formed; it reacts with excess CNMe or CN t Bu to yield the metallacyclopentenes [C 5 H 5 Rh{κ 2 ( C,C)‐C (NMe)CPhCPh C ‐(NR)}(CNR)] ( 18, 19 ). Similar compounds 20–23 containing a five‐membered RhC 4 metallacycle were prepared either from [C 5 H 5 Rh(RCCR′)(Sb i Pr 3 )] ( 7, 8 ) or [C 5 H 5 Rh‐(PhCCPh)(CN t Bu)] ( 14 ) and excess isocyanide. The crystal and molecular structures of 17 and 18 (R = Me) have been determined.