Coordinate neutral

The majority of U(V1) coordination chemistry has been explored with the trans-ddo s.o uranyl cation, UO " 2- The simplest complexes are ammonia adducts, of importance because of the ease of their synthesis and their versatihty as starting materials for other complexes. In addition to ammonia, many of the ligand types mentioned ia the iatroduction have been complexed with U(V1) and usually have coordination numbers of either 6 or 8. As a result of these coordination environments a majority of the complexes have an octahedral or hexagonal bipyramidal coordination environment. Examples iuclude U02X2L (X = hahde, OR, NO3, RCO2, L = NH3, primary, secondary, and tertiary amines, py n = 2-4), U02(N03)2L (L = en, diamiaobenzene n = 1, 2). The use of thiocyanates has lead to the isolation of typically 6 or 8 coordinate neutral and anionic species, ie, [U02(NCS)J j)/H20 (x = 2-5). 

Methyl-3-formylpyrazole A-phenylthiosemicarbazone, 45, coordinates as a deprotonated NNS tridentate ligand to form the diamagnetic, spin-paired [Ni(45-H)NCS] complex [200]. However, the complexes formed with other nickel(II) salts have stoichiometry [Ni(45)2]A2, where A = Cl, Br, NO3, CIO4, and BF4. These octahedral complexes NNS coordinated neutral hgands have Dq values between 1086 and 1150cm" ... 

Four-coordinate neutral complexes with anionic ligands derived from deprotonated amines, imines,... 

The redox properties of the quinone-pyrocatecholate system are shown in Scheme 23. If the species NiL2+, NiL and Nil4 are taken into account, they can be assumed to contain nickel(II) ion and coordinated neutral benzoquinone, mononegative semiquinone and dinegative pyrocatecholate, respectively. 

Reactions of Pt(O,0-MeCOCHCOMe)(MeCOCH2COMe)X (X = Cl, Br) with a wide variety of donors, including phosphines, arsines, amines, alkenes, diamines, etc. have been examined. In all cases the coordinated neutral acetylacetone is displaced. When monodentate ligands, L, are employed, products are of the type Pt(0,0-MeCOCHCOMe)XL. Bidentate ligands, LL, yield Pt(C-MeCOCHCOMe)X(LL) and bridging alkenes form PtCl(0,0-MeCOCHCOMe) (dialkene).494... 

In the solid state, [HB(C6F5)2]2 is dimeric, but is distinguished from other diorganoboranes in that observable amounts of the monomeric species HB(C6F5)2 appear in solution. This is clearly demonstrated by the nB NMR spectra in benzene, which show a minor peak centered around 60 ppm (three coordinate boron) and a major peak at 18 ppm (four coordinate, neutral boron).35 Recent quantitative analysis of this monomer/dimer equilibrium using variable temperature 19F NMR spectroscopy36 at various temperatures confirmed our earlier, more qualitative assessment that the dissociation enthalpy in benzene is much less (4.6 kcal mol-1) than that observed for [(Mes)2BH]2 (16.7 kcal mol-1),37 the only other borane for which this monomer/dimer equilibrium is assessable in solution. As expected, the AS° for this equilibrium is positive (15.3 cal K-1 mol-1), but less so than that observed for dissociation of [(Mes)2BH]2... 

Radical A-exo cyclizations can also be catalyzed without recourse to the Thorpe-Ingold effect by applying the concept of template catalysis (Fig. 24) [157]. Here, 20 mol% of a cationic titanocene(IV) precatalyst 82 is applied, which contains a coordinating neutral tether. After reductive opening of epoxide 81, a cationic titanocene(III) complexed radical 83 is formed, in which both the epoxide and the xfi-un saturated carboxamide radical acceptor are coordinated. This provides the template to accelerate the slow A-exo radical cyclization step considerably. Cyclobutanes 84 were isolated in 46-84% yield with mostly good / / [Pg.145]

A third effect of halide promoters is to enhance the relative nucleophilici-ties of the metal center. This generally occurs by halide displacement of a coordinated neutral ligand or by bridge-splitting reactions to generate anionic metal species. In cases where this type of halide promotion has been independently demonstrated, the effect of the increase in electron density on the reactivity is very large. 

The iron atom in haems and haemoproteins is usually five- or six-coordi-ftate, since it can bind ligands at the axial positions. Haems such as tron(II) protoporphyrin IX will readily coordinate neutral bases such as NHj and pyridine, small unsaturated molecules like CO, and some anions. In haemoproteins, at least one of the axial ligands is provided by the polypeptide, and with the exception of some cytochromes, this is the only linkage between the polypeptide and the prosthetic group. Where only We axial position is occupied by the polypeptide, the other is thought to be taken up by a water molecule in ferric haemoproteins. This is readily replaced by other ligands. Ferrous haemoproteins, in the absence of potential ligands such as CO, can remain five-coordinate. 

Seven-coordinate neutral complex sensitive to hydrolysis Seven-coordinate neutral complex sensitive to hydrolysis... 

Examples oftetra-coordinate boronium ions (41) have been known for many years. The quest for low-coordinate, neutral borane compounds also stimulated interest in the formation of three-coordinate borenium ions (42) and two-coordinate borinium ions (43). ... 

A few neutral ntixed-Ugands compounds were also reported. These are cis and trani-Tc(CO)3Cl(CNtBu)2 and Tc(NO)Br2(CNrBu)3. Two five-coordinate Tc(III) compounds Tc(CMPr)(L) where L is a tetradentate ligand containing some P and S atoms have been studied. More recently, three five-coordinate neutral compounds with the formula Tc(S3NC6Hi2)(CNR) where R=cyclohexyl, benzyl or CH2COOEt were also reported. 

Divalent group 14 cations [Cp "Ge]+ and [Cp Sn]+ form crystalline salts containing isolated Csv cations. The cations will coordinate neutral (pyridine) or anionic (halide) ligands to form slipped complexes. Bis-cyclopentadienyl... 

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