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 t hat of ferrocene and no evidence has been found for a conformational p reference in the former. A smaller observed barrier for the ring rotat ion in the tetra-tert-butyl substituted diazaferrocene compared to tha t in the substituted ferrocene analog is attributed to more facile dis tortional modes in the heterocyclic system. Tetra-tert-butyl-diazanick elocene has been synthesized from NiCl2 and Li(C4H2tBu2N) and its crys tal structure determined. Small, albeit significant structural differe nces in the diazaferrocene, -cobaltocene, -nickelocene series are expl ained in terms of the sequential filling of pi-antibonding levels. In the bent Main Group species diazastannocene or -plumbocene an electron ic situation different from that for the normal metallocenes gives ris e to a conformational preference with both nitrogens pointing along th e direction of maximum opening of the aza-Cp ring planes.