Binding modes, transition metal

Figure 3. Possible modes of acetylene binding to transition metal sulfur sites.

For the purposes of this chapter, which focuses on comparisons of isocyanide binding in transition metal complexes and isocyanide adsorption on metal surfaces, we first summarize known modes of isocyanide binding to one, two and three metals in their complexes. In such complexes, detailed structural features of isocyanide attachment to the metals have been established by single-crystal X-ray diffraction studies. On the other hand, modes of isocyanide attachment to metal atoms on metal surfaces are proposed on the basis of comparisons of spectroscopic data for adsorbed isocyanides with comparable data for isocyanides in metal complexes with known modes of isocyanide attachment. 

The interest in these ligands has stemmed largely from their proven involvement in the chemistry of coordinated dinitrogen. However, they are of considerable intrinsic interest as their electronic flexibility permits them to bind to transition metals with a wide variety of bonding modes. 

Allyl ligands bind to transition metals most commonly by one of the two monomeric structures shown in Figure 3.10. In addition, the ir-allyl ligand can bridge two metals in either a symmetrical Tj -mode or the unusual i) ,Tf) - or r)VV niod s shown on the right in... 

The Dewar-Chatt-Duncanson model that takes into account this bonding mode, including the n backbonding with the CO and C2H4 ligands, is represented below for classic metal-carbonyl and metal-ethylene bonds. Of course, the other unsaturated hydrocarbons bind the transition-metals according to the same n backbonding model. 

Figure 6.38. Potential energy diagram for the hydrogenation of ethylene to the ethyl (C2H5) intermediate on a palladium(m) surface. The zero of energy has been set at that of an adsorbed H atom, (a) Situation at low coverage ethylene adsorbed in the relatively stable di-cr bonded mode, in which the two carbon atoms bind to two metal atoms. In the three-centered transition state, hydrogen and carbon bind to the same metal atom, which leads to a considerable increase in the energy...

The versatile binding modes of the Cu2+ ion with coordination number from four to six due to Jahn-Teller distortion is one of the important reasons for the diverse structures of the Cu-Ln amino acid complexes. In contrast, other transition metal ions prefer the octahedral mode. For the divalent ions Co2+, Ni2+, and Zn2+, only two distinct structures were observed one is a heptanuclear octahedral [LnM6] cluster compound, and the other is also heptanuclear but with a trigonal-prismatic structure. 

In view of the fact that early transition metal alkyls insert CO under very mild conditions (2, 5.) we chose to examine the reactions of electron-rich metal hydrides ( ) with the resultant dihapto acyl complexes. Such acyls obviously benefit from reduction of the CO bond order from three (in OO) to two. More significantly, the dihapto binding mode will significantly enhance the electrophilic character of the acyl carbon. 

Rurack K, RadegliaR (2000) Transition metal ion complexes of 2,2 -bipyridyl-3,3 -diol and 2,2 -bipyridyl-3-ol spectroscopic properties and solvent-dependent binding modes. Eur J Inorg Chem 2271-2282... 

Fig. 5.20. Modes of coordination of transition metal ions with /3-lactam antibiotics. Complex A In penicillins, the metal ion coordinates with the carboxylate group and the /3-lactam N-atom. This complex stabilizes the tetrahedral intermediate and facilitates the attack of HO-ions from the bulk solution. Complex B In benzylpenicillin Cu11 binds to the deprotonated N-atom of the amide side chain. The hydrolysis involves an intramolecular attack by a Cu-coordinated HO- species on the carbonyl group. Complex C In cephalosporins, coordination of the metal ion is by the carbonyl O-atom and the carboxylate group. Because the transition state is less stabilized than in A, the acceleration factor of metal ions for the hydrolysis of cephalosporins is lower than for penicillins. Complex D /3-Lactams with a basic side chain bind the metal ion to the carbonyl and the amino group in their side chain. This binding mode does not stabilize the tetrahedral transition complex and, therefore, does not affect the rate of... 

Polyphosphazenes and cyclophosphazenes are almost unique as carrier molecules for transition metals because of the wide range of binding sites that can be incorporated into the phosphazene structure. The substitutive mode of synthesis described earlier allows a structural diversity that is not found, for example, in polystyrene, polyphenylene oxide, or other organic carrier polymers. 

Although isocyanides bind strongly to various transition metal surfaces, there is still much to be learned about their modes of adsorption and factors that influence these bonding modes. 

In the following sections we will review the copper complexes of the various classes of aminoglycosides described earlier. Discussion will focus on complexes that are formed at physiological pH (7.4) but will take into account the differing modes of coordination at other pH values, as well as accounting for the distinct binding modes of other transitions metal ions. 

FIGURE 17. Modes of tert-butylperoxy ligand binding to selected transition metal fragments in the solid state... 

The examples summarized above demonstrate that organometalhc derivatives of early transition metals can and will form dioxygen complexes, even though the stability of these adducts varies widely. The availability of some d-electrons is required i.e., d°-complexes do not show this mode of reactivity, presumably because binding of O2 requires some degree of electron transfer (oxidation of the metal). 

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