Mononuclear chelates

The adsorption behaviour of 1,2,4,5-tetra carboxylic and 2,3-dihydroxybenzoic acids most closely simulated that of NOM (Evanko and Dzombak, 1998). Another modelling study, this time with CD-MUSIC, suggested that adsorption of carboxylic acids on goethite involved two complexes located on different crystal planes, viz. a deprotonated outer sphere complex (pH 3-9) and an inner sphere mononuclear chelate present at ca. pH 6 (Boily et al., 2000). 

Dienes such as norbomadiene, 1,5-cyclooctadiene and Dewar hexamethylbenzene will form mononuclear chelate complexes such as PtX2(diene) (X = halogen).645 Alkene isomerization may occur. Thus whereas complexes of 1,4- and 1,5-cyclooctadiene have been obtained which involve no isomerization,646,647 under different experimental conditions phosphine complexes of... 

Pyridylethylphosphines react with [Pd2( J.-Cl)2R2(TITT)2J to generate mononuclear chelates 240 (R = C6F5, C6F3C12 R1 = R2 = Ph,... 

Like the mononuclear chelates,66 binuclear 55a,c,j (but not 55g ) undergo reversible ionization which is enhanced at low temperature, as is evident from the temperature-dependent 29Si NMR spectra (Fig. 24, Table XVIII). From the lowfield chemical shift already at 300 K (Table XVIII), relative to the mononuclear analogs 31a,c,j (Table XIV), it was concluded that significant ionization takes place at ambient temperature, to a much greater extent than in 31.69... 

Complexes with bridging bidentate phosphines tend to be dimers or tetramers with bridging phosphines, as in (18-I-IV). Phosphines with a large bite angle, as in (18-1-V), may however form mononuclear chelates. 

The coordination chemistry of dta towards carbonyl iron has been studied to find out if it was possible to prepare at least in situ mononuclear chelate [Fe(CO)3((7-S,(T-S -dta)] (dta = dithiooxamide) complexes (53) in order to investigate their 1,3-dipolar behaviour." This proved successful with dta a-d (Scheme... 

Our approach to the preparation of pure heteronuclear chelates is to establish the selectivity of the two coordination sites in a ligand such as H4(BAA)2en by determining the structure of some mononuclear chelates. Once these are unambiguously characterized, they are used as ligands to bind a second metal ion with a proven preference for the available site. The two metals are chosen so as to minimize competition for the same site within the molecule. 

Mononuclear Chelates of H4(BAA)2EN. Three well characterized complexes with the general formula M[H2(BAA)2en], where M is Ni(II), Cu(II), and VO (II), were prepared. Single-crystal x-ray structure determinations of the Cu(II) (18) and VO (II) (19) complexes revealed that the metal ions occupy the site with four oxygen donors under the preparative conditions used. Although the presence of a small amount of the other positional isomer cannot be discounted, sample homogeneity and crystallizability are indications that a significant quantity is not present in the crystalline product which is isolated in high yields. 

A structure determination of the other mononuclear chelate, Ni[H2-(BAA)2en], was deemed not necessary because of its unambiguous characterization by spectral and magnetic measurements. The visible spectrum of Ni[H2(BAA)oen] is essentially identical to that of Ni(Acac)2cn which is known to contain square planar Ni(II). The IR spectrum contains an intense unchelated carbonyl bond at 1700 cm which indicates that the nickel atom is coordinated to two nitrogens and two oxygens. In addition, Ni[H2(BAA)2en] is diamagnetic with Xg obs. = —266 X 10" cgs while Xg calculated from Pascal s constants is —246 X 10 cgs, which is further proof of square planar geometry about the Ni. 

It is obvious that the peaks around mass 275 are caused by doubly charged ions since the half-mass and full-mass peaks are all quite intense. The importance of the doubly charged ions is certainly the result of the presence of two metal centers, each of which could logically stabilize a positive charge. To pursue this point, the mass spectrum of the mononuclear chelate Ni[H2(BAA)2cn] was recorded. There is no evidence of formation of doubly charged ions by this compound, which constitutes strong indirect evidence that the maximum positive charge is determined by the number of metal atoms present. 

The basic types of complexes of ligating systems under consideration are presented by the mononuclear chelates 14-18. Di-, oligo-, mono-, and heteronuclear complexes containing hetarylazomethine ligands are also known. They are considered as a function of heterocyclic azomethine... 

Effect of the structure of the complex. The activities of the mononuclear chelates of Co(II) are quite different from that of the binuclear Co(II) chelate for butadiene polymerization. 

Figure 2 shows that the catalytic system PE-gr-PAAA Co(II)-Et2AlCl is more active than its homogeneous analog based on the chelate of Co(II) with N-acetylbenzamide. It is noteworthy that the immobilized mononuclear chelates of Co(II) with PE-gr-PAVK and PE-gr-PAAA have a comparable activity. This indicates that the nature of the chelate unit (N,0- and 0,0-, respectively) has inappreciable influence on the activity of the complexes in the polymerization of butadiene. At the same time, the binuclear Co(II) chelate with PE-gr-PSAA exhibits a lower (by a factor of 5-6) activity than its mononuclear analogs. 

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