Hafnium complexes hydrides

In contrast, exposure of 14-VE (diene)MCp Cl complexes (M = Zr, Hf) to CO (1 atm) results in the formation of cyclopentadienes70. The mechanism proposed for this transformation was elucidated with a carbon labeled CO ( CO) as requiring an initial coordination of CO to generate a (diene)MCp (CO)Cl complex 153 (Scheme 37). For the hafnium complex, the intermediate 153 (M = Hf) was observed by infrared spectroscopy. Insertion of CO into the a2, jt diene generates a metallacyclohexenone, which undergoes reductive elimination to generate the dimeric metallaoxirane species 154. -Hydride elimination from 154 (M = Zr, Hf) followed by 1,2-elimination produces substituted cyclopentadienes and the polymeric metal-oxide 155. Treatment of (diene)TiCp Cl with CO leads to isolation of the metallaoxirane complex 154 (M = Ti). 

Low temperature protonation of the zerovalent anion [> -CpTi(CO)3(PPh3)]" by acetic acid can be considered an oxidative addition The resultant hydride complex, > -CpTi(CO)3(PPh3)(H), is thermally unstable. Zerovalent (f -toluene)2M(PMe3) (M = Zr, Hf) undergoes an oxidative addition reaction with Me3SnSnMe3 to form M(/7 -toluene)2(SnMe3)2 the hafnium complex has been structurally characterized . 

Zirconium, titanium, and hafnium hydrides can activate the C-H bonds of several alkanes at low temperatures (even at room temperature) because they are very electrophilic and reactive. Moreover, the surface complex is immobilized by the strong metal-silica bonds, and this immobilization can prevent the coupling reactions leading to the deactivation of the complex. 

A number of transition metal complexes will catalyze the dehydrogenative coupling of organotin tin hydrides, R SnI I, to give the distannanes, RjSnSnRj.443 These metals include palladium,449 gold,450, hafnium,451 yttrium, and ruthenium.452 The catalyst that is most commonly used is palladium, often as Pd(PPh3>4, and the most active catalysts appear to be the heterobimetallic Fe/Pd complexes, in which both metals are believed to be involved in the catalysis.443... 

Zirconium and hafnium react with hydrogen at temperatures above 700 °C, yielding an a-phase and hydrides with a limiting M H ratio of 1 2 (see Hydrides Solid State Transition Metal Complexes). Under hydrogen pressure, absorption... 

Analytical Chemistry of the Transition Elements Coordination Numbers Geometries Coordination Organometallic Chemistry Principles Hydride Complexes of the Transition Metals Oxide Catalysts in Sohd-state Chemistry Periodic Table Trends in the Properties of the Elements Sol Gel Synthesis of Solids Structure Property Maps for Inorganic Solids Titanium Inorganic Coordination Chemistry Zirconium Hafnium Organometallic Chemistry. 

Gp sMs -HM -CDzCb 366. The crystal structure of the hafnium polyhydride complex reveals a fully asymmetric trinuclear structure with three widely differing Hf-Hf distances but the hydrides themselves could not be located. 

M. Yoshida, D. J. Crowther, R. F. Jordan, Synthesis, structure, and reactivity of a novel hafnium carbora-nyl hydride complex, Organometallics 1997,16, 1349. 

Mehrkhodavandi, P. and Schrock, R.R. (2001) Cationic hafnium alkyl complexes that are stable toward P-hydride elimination below 10 °C and active as initiators for the living polymerization of 1-hexene. Journal of the American Chemical Society, 123,10746-10747. 

The stepwise bond dissociation enthalpies Dj (Ti-Me) and D2(Ti-Me) in TiMe2Cp2 established from thermochemical data and extended Huckel calculations are compared to the respective halides and hydrides.34 The isolated C-H stretches of M(CH2D)2Cp2 (M = Ti, Zr, Hf) are lower than any other studied methyl complexes and the C-H bonds in the hafnium species are the longest and weakest yet characterised by this method.35... 

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