2-Azaallenylidene complexes: syntheses and structures of chromium and tungsten complexes with a (cyclopropenylidene)-2-azaethenylidene ligand

The reactions of NEt4[(CO)5M(CN)] (M=Cr, W) with chloro-bis(diisopropylamino)cyclopropenylium salts [ClC3(NiPr)2]X, (1)X (X=ClO4, OSO2CF3), afford cationic 2-azaallenylidene complexes {[(CO)5M=C=N=C3(NiPr)2]+A↔[(CO)5MCNC3(NiPr)2]+B} (M=Cr, 2a; M=W, 2b), in which C3(NiPr)2 represents the bis(diisopropylamino)cyclopropenylidene or bis(diisopropylamino)cyclopropenylium moiety, respectively. The IR and NMR spectroscopic characterization of the chromium complexes (2a)X (X=ClO4, OSO2CF3) and of the tungsten complex (2b)[(CO)5W(CN)] reveals an exceptionally high π-acceptor ability of the 2-azaallenylidene ligand and a high degree of electron delocalization within the metallaheterocumulene system. The X-ray structure determination of (2b)[(CO)5W(CN)] indicates that the structure is best described as a resonance hybrid between two canonical forms, the cumulene form A and the isocyanide form B.