Square Planar Coordination Spheres

The three iridium complexes 72d, 72f and 72g were analyzed by X-ray diffraction. Unfortunately the iridium complex 72a, the most efficient in many reactions, failed to give suitable crystals for analysis but the corresponding crystalline rhodiiun complex 73 coifld be analyzed. According to the results obtained, the coordination sphere of the Ir atom and of the Rh atom can be described as pseudo-square planar (Fig. 12). 

Thermolysis of 1-phenyl-3,4-dimethylphosphole in alcoholic solvents in the presence of NiCl2 leads to the synthesis of the racemic biphospholene complex (222).653,654 Upon reaction of the bromo derivative with AgBF4, the meso and racemic diastereomers of (223) are formed, which can be separated by fractional crystallization.655 In both (222) and (223) the coordination sphere is slightly distorted from square planar. 

Thiosemicarbazones attached to an amide moiety rather than an aldehyde or ketone moiety show better solubility. For example, the Ni11 complex with 2-pyridineformamide N(4)-methylthio-semicarbazone is square planar with N,N,S ligand coordination.1251 The coordination sphere is completed by an O-bound acetate coligand. 

The [Pt(Se4)2]2 complex (133) has been prepared by the reduction of the platinum(IV) species [Pt(Se4)3]2 with excess borohydride.328 The X-ray structure of the complex shows that each of the five-membered rings adopts a half-chair conformation, with the platinum coordination sphere exhibiting slight distortion from a square-planar geometry.329... 

V,/V-bis(2-hydroxy-di-3,5-/-butylphenyl)amine forms complexes of zinc which have ligand-based redox processes with four oxidation levels of the coordinated anion.864 2 1 and 1 1 complexes are formed in the presence of zinc with the 2 1 complex coordinated in an octahedral geometry and the 1 1 complex square planar with a triethylamine ligand completing the coordination sphere. The complexes, at the different redox levels, have been investigated by EPR, spectro-electrochemistry, l I NMR, and magnetochemistry, as appropriate. 

Sc2 anions. Highly asymmetric opposite Se4-Se3/Se4-Se5 distances of 242.5(4)/298.9(4) pm are observed in the square-planar coordination sphere of the central Se4 atom. 

In the end, while computations on these alternative pathways, oxidative addition and cr-bond metathesis, have provided some insight, the question by which way does a particular reaction proceed cannot yet be answered definitively. It is very interesting, however, that both mechanisms involve the same Pt(II) intermediate in which the hydrocarbon binds in the square-planar coordination sphere of the metal. 

The difference in behaviour has been attributed to the preferred formation, by cadmium(II) and mercury(II), of [MX2]oo chains in which pairs of chloride anions link metal centres to give a square planar MX/ arrangement, the octahedral coordination sphere then being completed by two monodentate bu molecules [MX2]oo chain formation does not occur for copper(II) and zinc(II) and so the anions only occupy two of the six available co-ordination sites allowing bu to act as a bidentate chelating ligand. 

The reagent PhI=NTs can be drawn in the resonance form Phi-NTs, where its resembalnce to CIO-becomes clear. Moreover, the issues of the square planar coordination sphere of the Mn(salen) complex don t exist with Cu(II), so a very simple mechanism can be drawn coordination of the N of the reagent to Cu(II), displacement of Phi by a lone pair on Cu to give a Cu(IV)=NTs reagent, [2 + 2] addition to the alkene, and reductive elimination. 

These systems are also described as normal copper proteins due to their conventional ESR features. In the oxidized state, their color is light blue (almost undetectable) due to weak d-d transitions of the single Cu ion. The coordination sphere around Cu, which has either square planar or distorted tetrahedral geometry, contains four ligands with N and/or 0 donor atoms [ 12, 22]. Representative examples of proteins with this active site structure (see Fig. 1) and their respective catalytic function include galactose oxidase (1) (oxidation of primary alcohols) [23,24], phenylalanine hydroxylase (hydroxy-lation of aromatic substrates) [25,26], dopamine- 6-hydroxylase (C-Hbond activation of benzylic substrates) [27] and CuZn superoxide dismutase (disproportionation of 02 superoxide anion) [28,29]. 

In the metal-binding site (Figure 12) the Cu(II) ion is coordinated by four histidines in a distorted square-planar coordination sphere and by a fifth axial water ligand, while the tetrahedral zinc ion is coordinated by three histidines, one of which is deprotonated and bridges to the copper, and by a carboxylate group °. 

Several four-coordinate selenium(U) complexes with bidentate ligands, all of which involve highly distorted square planar SeS4 or SeSe4 coordination spheres, and some tellurium analogues have been described.11... 

Finally, polydentate ligands can affect the geometry of a complex merely as a result of their own steric requirements. For example, we find some tetradentate ligands such as tris(2-dimethylaminoethyl)amine, [Me6tren = ((CHj NCHjCH N], form only five-coordinate complexes (Fig. 12.10), apparently because the polydentate ligand cannot span a four-coordinate tetrahedral or square planar complex and cannot conform ( fold ) to fit a portion of an octahedral coordination sphere. 

By analogy to the zinc structure, the coordination sphere around the silver(II) ion was anticipated to contain four sulfur atoms from two dialkyl dithiocarbamates in a square planar arrangement. Dimerization would occur by interaction via two Ag—S bridges (8S).538... 

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