Mono complexes hafnium

In contrast to the vast number of mono- and multinuclear binary carbonyl complexes of the transition metals, no isolable binary carbonyls of titanium, zirconium, or hafnium have been reported. 

Monoanionic boratabenzene ligands are isoelectronic to cyclopentadienide anion and can be nsed to synthesize Zr equivalent metallocenes (eqnation 29). A more congested complex of 9-phenyl-9-borata-anthracene (C 1-synunetry) was obtained by a similar way. Mono and bis boratabenzene complexes can also be prepared by reaction of a nentral boratabenzene-PMes adduct with tetrabenzyl zirconium or hafnium. Boratabenzene ansa metallocenes with CH2-CH2 or Si(CH3)2 bridges as well as Cp-boratabenzene mixed complexes or CG complexes were synthesized. 

The second strategy involves stabilization of the mono-Cp r/°-non-benzyl dialkyl metal cation with 7t-arene coordination. The arene coordination becomes the primary stabilization factor enabling the detection or isolation of the mono-Cp dimethyl cations. Thus, the reaction of Cp MMe3 (M = Zr, Hf) with 1 equiv. of B(C6Fs)3 in toluene/hexanes (1 10) solutions at ambient temperature affords the cationic arene complexes [(Gp M(Me)2(7]6-PhMe)][MeB(C6Fs)3] 304 as the solvent-separated ion pairs stabilized by the coordination of the aromatic solvent (Equation (22)).244 The crystal stmeture of the hafnium complex [(Cp Hf(Me)2(7]6-PhMe)][MeB(C6Fs)3] confirms the formation of the separated, discrete ion pairs in which the bent-sandwich cation is coordinated to an 7]6-toluene ligand.245... 

Salt metathesis was employed to synthesize half-sandwich zirconium and hafnium dichloride complexes 331 incorporating the bidentate, mono-anionic benzamidinate ligand (Equation (26)). The corresponding zirconium dimethyl and dibenzyl complexes have also been prepared using appropriate alkylating reagents.260 The zirconium dichloride complex (R= H), upon activation with MAO, are active for both polymerizations of ethylene... 

Wang, Q. Quyoum, R. Gillis, D. J. Tudoret, M.-J. Jeremic, D. Hunter, B. K. Baird, M. C. Ethylene, styrene, and a-methylstyrene polymerization by mono(pentamethylcyclopentadienyl) (Cp ) complexes of titanium, zirconium, and hafnium Roles of cationic complexes of the type [Cp MR2] (R = alkyl) as both coordination polymerization catalysts and carbocationic polymerization initiators. Organometallics 1996,15, 693-703. 

The relative stability of a number of mono(cyclopentadienyl) complexes has been assessed. Hafnium complexes are stable to 160-180°C, but hydrolyze slowly in air (625), while titanium complexes, which are stable up to 150°C, are also air stable, but hydrolyze slowly in solution (626). 

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