Zirconium complexes bridging ligands

Zirconium and hafnium complexes are highly fluxional. NMR studies indicate stereochemical nonrigidity, facile exchange of terminal and bridging ligands, and rapid intermolecular ligand exchange. 

Stereoblock polypropylene is synthesized by unbridged metallocenes such as bis(neomenthyl)zirconium dichloride (7) with cyclopentadienyl or phenyl substituted indenyl ligands [55, 56]. The stereoblock length increases at lower polymerization temperatures. Products containing isotactic and atactic blocks are elastomeric if the isotactic block length is short. Rieger [57] obtained similar polypropylenes by bridged fluorenyl-substituted indenyl zirconium complexes. 

The bridging zirconium phosphinidene complexes in Fig. 7.30 are expected to resonate upheld from the terminal ones in Fig. 7.29, since terminal phosphinidene complexes are far better suited for. -donor interactions of the phosphorus atom towards the metal than are the bridging ligands. 

Quite recently, Fryzuk and co-workers observed the N-H bond formation when a dinuclear zirconium complex with a side-on bridging dinitrogen ligand was treated with molecular dihydrogen. However, no ammonia was formed (a) M. D. Fryzuk, J. B. Love, S. J. Rettig, V. G. Young, Science 275 (1997) 1445. (b) H. Basch, D. G. Musaev, K. Morokuma, M. D. Fryzuk, J. B. Love, W. W. Seidel, A. Albinati, T. F. Koetzle, W. T. Klooster, S. A. Mason, J. Eckert, J. Am. Chem. Soc. 121 (1999) 523. 

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