Tetrakis complex

Biuret (bu) co-ordinates transition metals in both neutral and anionic forms. Compounds containing neutral bu include both bis- and tetrakis-complexes. The latter are limited to the larger cations of the group II [e.g., [Sr(bu)4]2+ [9] and lanthanide groups e.g., [Sm(bu)4]2+ [ 10,11]. Only the former are considered in detail in this review, together with bis-complexes containing anionic bu as they have the greater potential for formation of 1-D chains and 2-D sheets. 

Other techniques, such as C.D. spectral change, have been used to demonstrate the presence of octa coordination for lanthanide ion in a system containing Eu(FOD)3 and alcohols or ketones (28). However, the anionic tetrakis complexes e.g. Eu(acac)i, Eu(benzac)i, Eu(DBM)i, Eu(BTFA)4, tend to dissociate into the tris-complex and L in alcoholic solution. The degree of dissociation depends on the complex as well as the polarity of the medium. In alcohol-DMF medium the dissociation is enhanced compared to the alcoholic solutions (29). The end product of these dissociation reaction may well be an octacoordinated species. Fluorescence emission from the coordinated europium ion was also helpful in estabhshing the nature of the species in solution 29). 

Fig. 23. The spanning of the g-edges by hexafluoroacetylacetonates in the tetrakis-complexes of the lanthanides and the actinides...

The tetrakis complexes are easily obtained by the addition of an excess of the appropriate pyridine, in absolute ethanol or methanol, to a solution in the same solvent of the anhydrous nickel salt.147 The bis adducts are prepared by using stoichiometric amounts of the anhydrous reactants783 or by heating the corresponding tetrakis complexes at temperatures in the range 80-110 °C.784 820 Thermal decomposition of the bis and tetrakis adducts leads to the mono adducts. Using hydrated nickel salts as starting material may cause the formation of complexes with coordinated water molecules. 

The effects of the substituents near the donor site is to reduce the number of coordinated ligand molecules from six to four or less. Tetrakis complexes with substituted pyrazoles and imidazoles are prepared using a large excess of the ligands, and, in the cases of 1,2-dimethylimidazole and 3,5-dimethylpyrazole derivatives, anhydrous reactants. Most of die mono and bis adducts may be conveniently prepared under anhydrous conditions and with the nickel salt in excess. 

New synthetic routes have therefore been developed in order to improve the purity of the precipitates. Thus pure tetrakis complexes (see fig. 55, bottom) could be isolated using ligand 57 in ethanol with a large excess of sodium ions in slightly basic conditions (method D, table 12), whereas method B seems to be the more appropriate synthetic way to obtain pure trimeric complexes. Pure tris complexes could not be isolated however mixtures with up to 75% of tris complex could be obtained by using as little ammonium hydroxide as possible and a mixture of dichloromethane and water as biphasic solvent (improved A method). 

Orange-red diamagnetic complexes of Pd and Pt with mercaptoacetic acid and 3-mercaptopropionic acid have been reported (see p. 390)48 Simple bis-complexes of N-pyrrolidyl monothiocarbamate with Pd and Pt are not formed,50 but i.r. studies indicate that the tetrakis-complexes are bonded through both O and S, although no definite structure has been proposed. Extraction of the metals Pd, Pt, Ag, and Au from aqueous solution by a large number of alkyl and alkene disulphides and sulphides has been reported (see p. 390).51,52... 

Solvent extraction technique has been used in the synthesis of tris chelates of l,l,l,5,5,6,6,7,7,7-decafluoro-2,4-heptanedione. An aqueous solution of lanthanide chloride is equilibrated with an ether solution of ammonium diketonate, with the condition of an excess of lanthanide in the aqueous phase to prevent the formation of the tetrakis complex [46]. 

With ethylenediamine complexes of the formula Ln(en)3X3 and Ln(en)4X3, where X = C1 , Br , NO, CIOJ have been characterized. IR data indicate that the tris and tetrakis complexes of the fighter lanthanides La-Sm, contain both ionic and coordinated nitrate groups. By contrast tetrakis complexes of heavier lanthanides, Eu-Yb contain ionic nitrate. This is possibly due to steric factors resulting from decreasing cationic radius that force the nitrate out of the coordination sphere of the lanthanides. A coordination number of 8 for tris complexes and a number of 9 for fighter lanthanide tetrakis complexes appears reasonable [234]. The thermodynamic parameters obtained show enthalpy stabilization for... 

The complexation with 1,2-propanediamine gives rise to both tris and tetrakis complexes with both coordinated and ionic nitrate groups. In the case of heavier lanthanides only ionic nitrate is indicated. A coordination number of 8 or 9 for the nitrate-containing species has been suggested [235]. 

Treatment of anhydrous lanthanide(III) bromides with the sodium salt of N,N-diethyldithiocarbamate (dtc) in absolute ethanol yielded Ln(dtc>3 for Ln = La-Lu-Pm. Reaction of Ln(dtc)3 with N, /V-diethy ldithiocarbamate and tetraethyl ammonium bromide in ethanol yielded [(C2Hs)4N][Ln(dtc)4]. All the tetrakis complexes are isostructural while two series of isostructural tris complexes exist, one for Ln = La-Nd and the other for Ln = Sm-Lu. The tris complexes are probably 6-coordinate, and the tetrakis complexes 8-coordinate [261,262]. 

Absorption spectra of Pr(M) and Nd(III) complexes with alkyl, aryl, fluorine substituted P-diketones and the associated hypersensitive transitions were studied in detail. Mono, bis, tris and tetrakis complexes in benzene, DMF, DMSO and ethanol were also studied. Spectra both in solids and solution permitted the interpretation of the state of p-diketones based on the trends in the intensity and energy interaction parameters [238,244]. The various complexes were isolated and their IR, 1H NMR, stability constants were studied as well. Octacoordination in the tetrakis complex and tris complex associated with two solvent molecules was noted. 

Tris- and tetrakis(triaryl phosphite)platinum(O) complexes have been prepared by reduction of platinum(II) phosphite derivatives with hydrazine.5 The tetrakis complexes have also been prepared by ligand exchange processes, and the synthesis described here is based on this latter procedure. The chemistry of platinum phosphite complexes has not been extensively studied. 

NIR luminescence intensity for the Nd(III) or Yb(III) species. The most intense NIR emitting 8-hydroxyquinolinate lanthanide(III) complexes are the tetrakis complexes with 5,7-dihalo-8-hydroxyquinoline ligands. 

The most common coordination numbers shown by RE + /3-diketonate complexes is eight, where the two most frequent chemical formulae are [RE(/3-diketonate)4] and [RE(/3-diketonate)3(unidentate)2] corresponding to dodecahedron and square antiprism polyhedra, respectively. It is noted that the number of crystal structures presented for the tetrakis complexes is very low compared with the tris complexes. The majority of the tetrakis complexes have square antiprism polyhedron structure [Ce(acac)4]. On the other hand, a number of rare earth /3-diketonate complexes has the dodecahedral coordination polyhedron. 

It has been suggested that the bulky ferf-butyl groups in thd complexes prevent more than three ligands from approaching the lanthanide ion (i). This hypothesis was advanced to account for unsuccessful attempts to synthesize tetrakis complexes of thd with trivalent lanthanides. The synthesis of Zr(thd)4 reported here demonstrates that it is possible to introduce a fourth thd group and indicates that there is still a significant amount of room available in the tris lanthanide complexes. Crowding must be considered only in relative terms, however, and this... 

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