Nickel complexes, lability

Polymerization of alkynes by Ni" complexes produces a variety of products which depend on conditions and especially on the particular nickel complex used. If, for instance, O-donor ligands such as acetylacetone or salicaldehyde are employed in a solvent such as tetrahydrofuran or dioxan, 4 coordination sites are available and cyclotetramerization occurs to give mainly cyclo-octatetraene (cot). If a less-labile ligand such as PPhj is incorporated, the coordination sites required for tetramerization are not available and cyclic trimerization to benzene predominates (Fig. A). These syntheses are amenable to extensive variation and adaptation. Substituted ring systems can be obtained from the appropriately substituted alkynes while linear polymers can also be produced. 

There is a difference between the thermodynamic terms stable and unstable and the kinetic terms labile and inert. Furthermore, the differences between the terms stable and unstable, and labile and inert are relative. Thus, Ni(CN)4 and Cr(CN)6 are both thermodynamically stable in aqueous solution, yet kinetically the rate of exchange of radiocarbon-labeled cyanide is quite different. The half-life for exchange is about 30 sec for the nickel complex and 1 month for the chromium complex. Taube has suggested that those complexes that react completely within about 60 sec at 25°C be considered labile, while those that take a longer time be called inert. This rule of thumb is often given in texts, but is not in general use in the literature. Actual rates and conditions are superior tools for the evaluation of the kinetic and thermodynamic stability of complexes. 

The mechanism suggested for this reaction is based on the identification of the species shown in Eqs. 49 and 50, the results obtained with the Rh and Ru complexes [Rh(H)(CO)(S4)] and Ru(H)(PCy3)(S4) ", the finding that the related [Ni(N3)(S3)l complex can add CO (cf. Section V.C), and the well-known fact that many four-coordinate nickel complexes add a labile fifth ligand. Scheme 35 illustrates the mechanism. 

In the field of Inorganic Chemistry, size exclusion chromatography (SEC) has been used out only for separating complexes for analyses but also for purifying complexes a note-worthy example in the latter context is the separation of the labile nickel complexes of the phosphorous esters like... 

A series of thio- 46 [51,168] and seleno-carboxylato complexes 47 [175] with tmns-configurations have been synthesized from the reaction of NiCl2(R3P)2 with alkali metal or 0-trimethylsilyl selenocarboxylates (Scheme 4). Telluro-carboxylato nickel complexes 48 Ni(RCOTe)2(R3P)2 are too labile to isolate, although their formation has been confirmed by their H, and Te NMR and IR spectral data [175]. The anionic thiocarboxylato nickel complex 49 is... 

Acetonitrile solvent exchange at the five-coordinate complex ions [ML(AN)] (with M = Co or Ni and L = l,4,8,ll-tetramethyl-l,4,8,ll-tetraazacyclote-tradecane) appears to follow an la mechanism, with large, negative entropies of activation (-69.5 and —47.8 J K" mol" respectively) and low enthalpies of activation (19.5 and 20.3 kJ mol" ). Interestingly, the nickel complexes [NiL(AN)] and [NiL(OH2)] are significantly more labile than the cobalt(II) analogs. 

It can be shown experimentally, that four coordinate complexes of palladium in general are more labile than platinum but more inert than nickel complexes. This reflects the greater crystal field stabilization energy of 4d and 5d elements, that is, A increases with increasing quantum number. 

The Variation of the Lability of Coordinated Water in Nickel(II) Complexes... 

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