eta(2)(pi)-cordination of a bis-phosphonio-benzophospholide cation: A novel complexation mode for unsaturated phosphorus heterocycles

The bis-phosphonio-benzo[c]phospholide tetraphenylborate 4[BPh4] reacts wit h CpCo(C2H4)(2) to form a chelate complex [Co(eta (5)-Cp)(kappa P-2,eta (2) (P=C)-4)][BPh4] (6[Bph(4)]) which was characterized by means of spectrosco pic techniques and a single crystal X-ray diffraction study. The observed e ta (2)(pi)-coordination of the benzophospholide moiety in the cation 6 is h ighly unusual for aromatic phosphorus heterocycles. The structural data sug gest a pronounced coordination-induced localization of pi -electrons in the condensed ring system.