Lewis acid kinetically labile

Applications involving ring transfer or loss. The kinetic lability, volatility, and Lewis acidity of heavy alkaline earth metallocenes have been the properties most important to their applications. The gas-phase decomposition of volatile metallocenes is useful in the preparation of thin films of alkaline earth-containing materials and in doping semiconductors. Reviews are available on the use of group 2 organometallic compounds as precursors for chemical-vapour deposition (CVD).2 3... 

In spite of the above mentioned Co(EII) compounds, kinetically labile metal complexes may provide fast product/substrate exchange and some of these systems show real catalytic activity. In native dinuclear phosphatases Mg(II), Mn(II), Fe(II/III), or Zn(II) ions are present in the active centers. Although the aqua complexes of the weakest Lewis acids, Mg(H) and Mn(II), show measurable acceleration of e.g. the transesterification of 2-hydroxypropyl p-nitrophenyl phosphate HPNP, [Mn(II)] = 0.004 M, kobs/ uncat = 73 at pH 7 and 310 K, [38] or the hydrolysis of S -uridyluridine (UpU) [39], only a few structural [40] but no functional phosphatase-mimicking dinuclear complexes have been reported with these metal ions. 

In order to destabilize the likely unproductive 6-membered chelate structure of type K (Scheme 17) that might be formed if the catalyst reacts with the 4-pen-tenoate entity, the cyclization was run in the presence of a Lewis acid which competes with the evolving carbene for the Lewis basic ester group. Such an additive has to be compatible with the RCM catalyst, should provoke a minimum of acid-catalyzed side reactions, and must undergo a kinetically labile coordination with the relay substituent. Ti(OiPr)4 was found to meet these stringent requirements... 

The chelate ligand in dionato complex 3 is planar and it is particularly stabilized by 71-delocalization. In addition to this thermodynamic stability, the iron center has 17 valence electrons in an octahedron, hence its coordination sphere is kinetically labile. By ligand exchange, the acceptor 41a is coordinated at a vacant site to form species 44 (Scheme 8.18). The function of the center metal is not only to hold the acceptor in proximity to the donor. Additionally, the acceptor is activated by Lewis acidity of the center metal. Subsequently, the nucleophilic carbon atom of the dionato ligand is... 

Thus, to summarize, zinc is a good Lewis acid, especially in complexes with lower coordination numbers it lowers the pK of coordinated water and is kinetically labile, and the interconversion among its four-, five-, and six-coordinate states is fast. All of these properties make zinc quite suitable for biological catalysis. ... 

Studies of Lewis acid catalysis have been carried out with kinetically labile metal ions (Cu(II), Ni(II), Zn(II) etc.) and with kinetically inert metal centres such as cobalt(III). The use of inert cobalt(III) complexes in kinetic studies presents many advantages as the metal complex is well defined and it is often possible to obtain more detailed mechanistic information from such systems. 

Work has been carried out in three broad areas using (a) labile metal ions particularly copper(Il), (b) metal ions of intermediate lability such as Pd(ll) which is a powerful Lewis acid and (c) kinetically inert cobalt(III) complexes. 

Since platinum(II) and - to a lesser extent - palladium(II) complexes are kinetically stable, most of the complexes described herein are prepared by reacting a preformed platinum(n) or paUadium(II) complex with a salt of a d ° or s cation. In some cases, the Lewis-acidic moiety is also a preformed complex containing labile ligands. There are some examples of more complicated reactions in which coordination to a Lewis acid induces changes in the coordination sphere of Pt to form oligomeric structures that cannot be isolated otherwise (see Sect. 4.2.1). 

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