Eight-co-ordinate Complexes

Molybdenum.—There have been several suggestions as to the detailed mechanism of photoaquation of [Mo(CN)8]. The most recent investigation has proved that hydroxide ions are released in the primary photoaquation step, and that this step is inhibited by added cyanide. Thus, it supports the hypothesis that this photoaquation step is 

The four general approaches to mechanism determination to be mentioned here are the use of linear free energy plots and plots of activation enthalpies against activation entropies, the determination of volumes of activation, the search for transient intermediates, and the study of solvent variation on rates. The last topic is covered in Part II Chapter 6, the other three here. 

The third general approach is to seek evidence for transient intermediates, for example by trapping [Co(NH3)5] + to gain evidence for a dissociative mechanism for aquation of [CoX(NH3)5] +. Indeed, there is some evidence for the generation of [Co(NH3)5] + in aquation, particularly in nitrous acid assisted aquation of [Co(N3)(NH3)5] +. It therefore seemed reasonable to start the search for a comparable five-co-ordinate chromium(m) intermediate with the analogous reaction, nitrous acid A. Haim, Inorg. Chem., 1970, 9, 426. 

The fourth general approach, the study of solvent effects on reactivity, is amply illustrated in Part II Chapter 6. 

Stereochemical changes taking place in the course of substitution reactions of octahedral complexes have been reviewed. An improved method for estimating crystal field activation energies has been described.  

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New information for the discussion of the factors which govern the stereochemistry of eight-co-ordinate complexes of the early transition metals has been provided by the determination of the structures of the complexes M(NCS)4(bipy)2 (M = Zr or Nb). " Both complexes can be described in terms of gross distortions from dodecahedral D2d geometry, the distortion increasing with decreasing M—N bond length from Zr—N = 2.182(2) and 2.412(2) A, to Nb—N = 2.135(3) and 2.318(3) A (see also p. 73). 

The eight-co-ordinate complex [Co(napy)4](C104)2 (napy = 1,8-naph-thapyridine) has been isolated and its structure discussed in terms of a distorted dodecahedral structure similar to that reported for the analogous iron(ii) compound. The related complex [Co(napy)2(N03)2] has also been reported and appears to be the first example of a neutral mixed-ligand eight-co-ordinate complex of this metal. ... 

The eight-co-ordinate complex [Cu(napy)4](C104)2 (napy = naphthapyri-dine) has been isolated and its geometry discussed in terms of dodecahedral stereochemistry. The related compound [Cu(napy)2(N03)2] is probably polymeric with bridging nitrate groups and complexes [Cu(napy)2X2] (X = Cl or Br) are both dimeric in the solid state, the metal atom probably having square-pyramidal co-ordination. ... 

Reviews dealing with aspects of the stereochemistry of six-, seven-, and eight-co-ordinate complexes have appeared. ... 

Molybdenum(iv).—Further studies of photoaquation of the eight-co-ordinate complex [Mo(CN)8] have shown that naphthalene, phenan-threne, anthraquinone, and benzophenone are effective photosensitizers, but that biacetyl is not. The rates of photoaquation and of thermal aquation are little affected by the medium, which suggests some similarity between the excited state for the former and the transition state for the latter. The final products of aquation of [Mo(CN)g] - appear to remain uncharacterized, although this and other recent work confirms that the primary step in photoaquation is cyanide loss, which is reversible. ... 

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 new compounds M3[Nb(02)2F4],nH20 and M3[Ta(02)2F4],nH20 (M = NH4, n = 0 M = K or Na, n = 1) have been prepared, and i.r. and conductivity studies suggest that they involve eight-co-ordinate metal atoms. The stability constants of the niobium(v)-peroxide complexes formed in sulphuric acid solutions have been determined. ... 

Two vanadium(iv) ethylxanthate complexes have been prepared 440 their e.p.r. parameters are practically identical with those of the corresponding diethyldithio-carbamate complexes and on this basis analogous structures were proposed, i.e. a dodecahedral eight-co-ordinate structure with four bidentate ethylxanthate ligands and a distorted tetrahedral structure with four unidentate ligands. Addition of phosphorus(iii) ligands (L) such as trialkylphosphites to solution of the ethyl xanthate... 

According to their i.r. and n.m.r. spectra, the octaethyl porphyrin (OEP) diketonate complexes M(OEPXdik)2 (dik = acac or dbm) are eight-co-ordinate with bidentate dik ligands. ... 

Cyanides.—A concise review of the seven- and eight-co-ordinate molybdenum complexes and related molybdenum(vi) 0x0 complexes, with cyanide and isocyanide ligands has appeared and Group Via cyanide complexes have been reviewed within the broader scope of transition metal cyanides. ... 

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