Molecules to a metal

Bond lengths and infrared spectra support the multiple-bond character of the M—CO bonds. Coordination of a CO molecule to a metal center can change the C—O bond order. According to the description of ( - and TT-bonding given herein, increased ( -bonding between a metal and CO results in a... 

Evidence of attachment of an organic molecule to a metal surface by a particular group in the molecule is not only provided from surface tension data on the surface of liquid mercury but is also to be noted in the phenomenon of displacement of one liquid by another from a surface. The hydrocarbons adhere to metals much less tenaciously than molecules containing polar groups such as —COOH and —OH. Thus we find that a hydrocarbon may be removed from a metal surface by displacement with alcohol and also that the lubricating properties of a hydrocarbon are materially affected by the addition of hydrocarbon compounds containing polar groups (Hardy, Proc. Roy. 8oc. A, c. 650, 1922, A, ci. 487, 1923). 

We saw in Chapter 2 that co-ordination of a water molecule to a metal ion modifies the pK and can make the water considerably more acidic. This stabilisation of the hydroxide anion is rationalised in terms of transfer of charge from the oxygen to the metal in the coordinate bond. Some typical pKa values of co-ordinated water molecules are given in Table 5-2. 

Figure 3.24 Two possible chiral coordinations of a propylene molecule to a metal atom re (rectus) - right-handed si (sinister) - left-handed...

Figure 20. Schematic diagram for the bonding of an isolated CO molecule to a metal atom (right hand side) and a free two-dimensional array of CO molecules (left panel) to a metal surface (middle).

Metal ions are vital to the function of many enzymes that catalyze hydrolytic reactions. Coordination of a water molecule to a metal ion alters its acid-base properties, usually making it easier to deprotonate, which can offer a ready means for catalyzing a hydrolytic reaction. Also, the placement of a metal center in the active site of a hydrolytic enzyme could permit efficient delivery of a catalytic water molecule to the hydrolyzable substrate. In fact, the first enzyme discovered, carbonic an-hydrase, is a metalloenzyme that requires a Zn2+ center for its catalytic activity (32). The function of carbonic anhydrase is to catalyze the hydrolysis of carbon dioxide to bicarbonate ... 

Coordination polymerization is yet another variation on the same theme. Here, polymerization is initiated by attachment of a monomer molecule to a metal complex. The polymer grows by successive insertion of monomer molecules at the metal. Growth stops when the metal complex detaches itself or the reactive center becomes deactivated by some intended or inadvertent event. Stereo-specific polymers can be produced. 

Transition metal complexes are widely used in aromatic chemistry and organic synthesis in general. They are of particular value because com-plexation of an organic molecule to a metal centre often modifies its reactivity. The metal can subsequently be removed. After discussing the ways by which these complexes react, we will discuss their use in the synthesis of aromatic compounds. 

The forward process describes the transfer of an electron from a reduced species, which is in equilibrium with the solvent molecules, to a metal phase, with electrochemical potential //. this results, eventually, in an oxidized species in equilibrium with the solvent molecules. When electronic equilibrium is reached between the metallic phase and the solution, the electrochemical potential in both phases is equal and given by... 

Schunack M, Petersen L, Kuhnle A, Laegsgaard E, Stensgaard 1, Johannsen 1, Besenbacher E (2001) Anchoring of organic molecules to a metal surface HtBDC on Cu(l 10). Phys Rev Lett 86 456... 

Coordination of the water molecules to a metal center is a Lewis acid/base reaction in which charge is transferred from the ligands (water) to the metal. This makes a coordinated water molecule more acidic than a noncoordinated one, i.e., dissociation of a proton from a coordinated water is easier than from a noncoordinated water. Depending on the magnitude of the charge transfer, the equilibrium concentrations of aquo complexes, hydroxo species, and oxo species (Equation (3)) are different. The formation of metal hydroxide species from aquo complexes is called hydrolysis. 

The metal-diazo compound interaction. Several modes are conceivable for coordination of a diazoalkane molecule to a metal atom [9]. Coordination modes x]2 (N,N) X, g3 XII, and g2 (C,N) XIV demand metal complexes possessing two available adjacent coordination sites whereas for coordination modes nKN) XI and nKC) XIII one coordination site per metal atom is sufficient. 

Fig. 5.2 An example of reaction involving the formation of monolayers based on organosilicon compounds the anchoring of trimethoxyorganosilane molecules to a metal oxide surface is depicted (Reproduced from Ref. [11] with the permission of Wiley-VCH)...

Step 1 of the dual mechanism involves adsorption of a paraffin molecule to a metal site, followed by reversible dehydrogenation to form an olefin. In Step 2, the olefin migrates to an acid site, where it reacts with a proton to form a carbenium ion. The carbenium ion can rearrange into a more-stable carbenium ion (Step 3), which explains why products from hydrocrackers are relatively rich in iso-paraffins. In Step 4, P-scission of the carbenium ion produces an olefin and a smaller carbenium ion. The olefin can undergo further cracking on an acid site, or it can react with hydrogen at a metal site... 

It eonsists in eoordinating two ethylene molecules to a metal center that then undergo oxidative coupling, then p-elimination followed by reductive elimination of 1-butene. It has been developed by hauvin at the Institut Fran ais du hole using... 

Coordination of the urea molecule to a metal atom may occur through either the oxygen or the nitrogen atoms. The electronic structure of urea may be considered as a hybrid of the three resonance structures ... 

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