Metal coordination complex

Four-coordinate metal complexes may have either of two different geometries (Figure 15.3). The four bonds from the central metal may be directed toward the comers of a regular tetrahedron. This is what we would expect from VSEPR model (recall Chapter 7). Two common tetrahedral complexes are Zn(NH3)42+ and C0CI42. ... 

Coordination number The number of bonds from the central metal to the ligands in a complex ion, 409,412t four-coordinate metal complex, 413 six-coordinate metal complex, 413-414 Copper, 412 blister, 539... 

Four-coordinate metal complex, 413 Franklin, Rosalind, 628 Frasch, Herman, 558... 

Single bond A pair of electrons shared between two bonded atoms, 167 Six-coordinate metal complex, 413-414 Skeleton structure A structure of a species in which only sigma bonds are shown, 168... 

Litde is known about the stability of these porphyrins in O2 reduction, how this peripheral substitution affects O2 affinity of the metalloporphyrin, how the peripheral metal complexes perturb the energetics of various intermediates, and/or the kinetics of various steps or the mechanisms of O2 reduction by these porphyrins. At present, it remains to be seen if the strategy of coordinating metal complexes on the periphery of a metalloporphyrin can be exploited in the rational design of new ORR catalysts. 

Thirty years ago Bacci and Sacconi [15] reported on five coordinate metal complexes of the tetradentate ligand l,4,7,10-tetraphenyl-l,4,7,10-tetraphos-phadecane (pppp) (3). 

There are examples where the metal ion may form high coordinate species with an unusual coordination number, for example, [Fe(H20)9]+ (97). The ability to form highly coordinated metal complex ions suggests that under suitable conditions, involving a high concentration of the ligand molecules, coordinative saturation may take place. 

Many two-coordinate metal complex ions [MX2]+ and [ML2]+ have been observed using several ionization techniques, with recent experiments favoring the electrospray technique. The ions [M02]+ and [M03]+ can often be more reactive than the M+ and [MO]+ ions, showing the role of the metal oxidation state in reactivity. Some of the [ML2]+ ions may be prepared by the reactions of metal ions with... 

C. Louis, and M. Che, Anchoring and grafting of coordination metal complexes onto oxide surfaces In Preparation of Solid Catalysts edited by G. Ertl, H. Knozinger, and J. Weitkamp (Wiley-VCH, Weinheim 1999) pp. 341-371. 

The solubility of many metal complexes is considerably limited due to their high polarity. As reported in literature, the solubihty-promoting factors in organometallic and coordination metal complexes can be summarized as follows ... 

Many of these systems employ charged polymers or polyelectrolytes that confer on them particular properties due to the existence of electrical charges in the polymer structure. Oyama and Anson [14,15] introduced polyelectrolytes at electrode surfaces by using poly(vinylpiridine), PVP, and poly-(acrylonitrile) to coordinate metal complexes via the pyridines or nitrile groups pending from the polymer backbone. Thomas Meyer s group at North Carolina [16, 17[ also employed poly(vinylpyridine) to coordinate Ru, Os, Re and other transition-metal complexes by generating an open coordination site on the precursor-metal complex. 

Coordination polymers can be prepared by a number of routes, among which the three most common being (1) preformed coordination metal complexes polymerized through functional groups where the actual polymer-forming step may be a condensation or addition reaction ... 

Since Fenton s work in the late nineteenth century, the role of transition metals in oxygen chemistry is known, but the formation of oxygen adducts with coordination metal complexes and their importance for O2 activation have been studied much later [1, 97]. The lively interest in ORR catalysis comes from its utmost importance to the development of fuel cells and this justifies that only a few studies have been done with metal complexes in solution most have been devoted to carbon electrodes modified by immobilization of a catalyst. The research for good catalysts that could be efficient substitutes for the expensive platinum naturally moved toward porphyrins. 

Reagent Reactivity. One of the most interesting aspects of substitution reactions of square planar complexes is that the reaction rates depend on the nature of the reagent. This permits a thorough investigation of the factors responsible for reagent reactivity towards these substrates. Note that this has not been possible for the reactions of most six-coordinated metal complexes, since their rates do not depend on the reagent. 

Ionic liquids offer a highly polar but noncoordinating environment for chemistry. It is difficult to dissolve catalysts in nonpolar, noncoordinating molecular solvents such as hexane. Polar solvents, such as acetonitrile, tend to coordinate metal complexes. Ionic liquids such as the tetrafluoroborates offer a straightforward replacement of a solvent with a polar solvent that is noncoordinating. 

The carbonate group retains planarity and the three copper ions deviate from this plane by 0.05 A. The Cu-0 bond lengths in the complex are typical of five coordinate metal complexes... 

The /wentfo-macrocyclic structure is also present in the solid state forms of some free ligands. [62] Overall, these are less planar than their 4-coordinate metal complexes, having a step conformation to reduce the repulsion between the central phenolic protons. Nevertheless, they are apparently well organized for complex formation and donor cavity sizes [63] give a good fit for divalent copper. [62]... 

The 5-hydrazinotetrazole 422 is known to be used in the preparation of energetic coordination metal complex systems, for example, the perchlorate complex of mercury(ll) with 5-hydrazinotetrazole 423 (cf. Section 6.07.12.2) (Equation 78) <2005MI21>. 

Reactions of NHC-coordinated Metal Complexes with Molecular Oxygen... 

Oxidation Reactions Catalyzed by NHC-Coordinated Metal Complexes... 

It is possible to classify the coordination metal complexes used as precursors for anchoring/grafting into three types ... 

The chemical reaction usually occurs as soon as the coordination metal complex is in contact with the support. The anchoring involves a condensation reaction between the precursor and the surface OH groups of the support ... 

The coordination metal complexes are often air-sensitive, so preparations involving nonaqueous and inert solvents or vapor phase reactions (chemical vapor deposition (CVD), Section A.4.9) are required. The reactions are usually performed between room temperature and a maximum that is limited by the decomposition temperature of the precursor for vapor phase reactions, and the solvent reflux temperature for liquid phase reactions. 

As shown in Eq. 1, the support acts as a macroligand for the metal. Depending on the number of bonds between the metal and the support (y = 1, 2 or 3), the metal can be considered as singly, doubly, or triply bonded to the support. The value of y may vary with the reactivity, concentration, and nature of both the OH and the coordination metal complex, and also with the reaction temperature. 

The anchoring of coordination metal complexes was first used to hctcrogcncizc homogeneous catalysts. 

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