HETEROCYCLES AS LIGANDS .19. THE ELECTRONIC-STRUCTURE OF 1,1'-DIAZAMETALLOCENES AND THE SYNTHESIS AND CRYSTAL-STRUCTURE OF 2,2',5,5'-TETRA-TERT-BUTYL-1,1'-DIAZANICKELOCENE

The electronic structure of diazaferrocene is shown to be similar to that of ferrocene and no evidence has been found for a conformational preference in the former. A smaller observed barrier for the ring rotation in the tetra-tert-butyl substituted diazaferrocene compared to that in the substituted ferrocene analog is attributed to more facile distortional modes in the heterocyclic system. Tetra-tert-butyl-diazanickelocene has been synthesized from NiCl2 and Li(C4H2tBu2N) and its crystal structure determined. Small, albeit significant structural differences in the diazaferrocene, -cobaltocene, -nickelocene series are explained in terms of the sequential filling of pi-antibonding levels. In the bent Main Group species diazastannocene or -plumbocene an electronic situation different from that for the normal metallocenes gives rise to a conformational preference with both nitrogens pointing along the direction of maximum opening of the aza-Cp ring planes.