METAL-CARBON MULTIPLE BONDS - HALF-SANDWICH PHENYLCARBYNE COMPLEXES OF CHROMIUM - SYNTHESIS, STRUCTURE, ELECTROCHEMISTRY AND REACTIONS WITHPME3

The chromium phenylcarbyne complexes (eta(5)-C5R5)(CO)(2)Cr=CPh (2a: R =H; 2b: R=Me) have been prepared and their structure, cyclic voltammet ry and reactions with PMe3 are reported. The compounds 2a and 2b are o btained as red and purple thermolabile solids respectively after treat ment of cis-Br(CO)(2)(pic)(2)Cr=CPh (pic=4-methylpyridine) (1) with Na Cp (Cp=C5H5) and KCp (Cp*=C5Me5) respectively. Complex 2a adds PMe3 a t the carbyne-carbon atom to give the green alpha-phosphoniocarbene co mplex Cp(CO)(2)Cr=C(PMe3)Ph (3), whereas 2b undergoes with PMe3 a carb yne-carbonyl coupling reaction to afford the green eta(2) ketenyl comp lex Cp(CO)(PMe3)Cr[C(Ph)CO] (4). Cyclic voltammetry studies show the phenylcarbyne complexes 2a and 2b and the carbene complex 3 to undergo a reversible one-electron oxidation in CH2Cl2 at E-1/2=0.39 V, E-1/2= 0.20 V and E-1/2=-0.52 V respectively (potentials vs. the ferrocene/fe rrocenium redox couple). In comparison, the ketenyl complex 4 is elect rochemically inactive in the potential range of -1.5 to 1.0 V. The cry stal structures of 2a and 3 are reported. (C) 1997 Elsevier Science S. A.