Co-ordination Numbers Greater than Six

Complexes with Co-ordination Number Greater than Six 

The stability constants of the ternary complexes formed between [Eu(edta)] ion and ida , Meida , and nta ions are log =4.28, 4.33, and 5.05 respectively. The [Ln(edta)(ida)] ions show maximum stability in the middle of the lanthanide series log7sT= 3.27 (Pr), 4.28 (Eu), and 2.67 (Yb). Reactions of [Ln(edta)] ions with excess nta ion also involve the formation of [Ln(nta)2] , [Ln(edta)2] , and [Ln2(edta)a] ions. Ligand exchange reactions were investigated and the pseudo-first-order rate constants are collected in Table 1. Exchange of ida with [Pr(edta)(ida)] ion was too fast to measure.  

Parallel associative and dissociative pathways are reported for the reactions of [Eu(L)] and [Ho(L)] (L = methylthymol blue) with edta in ammonium acetate buffers.  

The chelate adducts formed between the lanthanide shift reagent [Eu(fod)a] and [Co(iS-diketonates)3] have relatively long lifetimes for dissociation (ca. 10- s).  

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The comparatively large water-exchange rates for these trivalent ions have been attributed to their having co-ordination numbers greater than six. 

All of the kinetic studies of substitution and related reactions of inert metal complexes with co-ordination numbers greater than six reported in the present volume are concerned with eight-co-ordinate complexes. The relevant references are collected together in Table 29. It is likely that the co-ordination numbers of the lanthanide and actinide cations whose complexes are mentioned in Sections 7 and 8 of this chapter are greater than six, but as knowledge of the precise co-ordination number in each case is lacking, these are not included in Table 29. 

The 1 1 complexes between the lanthanides and edta form stable associates with the anions of 8-hydroxyquinoline-5-sulphonic acid (oxs), iminodiacetic acid (ida), and nitrilotriacetic acid (nta) since the characteristic co-ordination number of the Ln ions is generally greater than six. In Figure 6 the free-energy changes associated with the formation of these... 

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