Centres coordination

CO Dehydrogenase, also incorporating Fe and catalysing the oxidation of CO to CO2. The attachment of CO to a nickel centre coordinated to perhaps four 5-donors is postulated. 

A benzannulation reaction yielding the naphthoquinone 61 could also be performed with the ruthenium carborane-stabilised carbene 60 and 1-hexyne [56] (Scheme 36). The ruthenium carbene unit can be regarded as an 18-electron fragment containing a formal Ru(II) centre coordinated to a dianionic six-electron-donor cobaltacarborane ligand. 

It was found that the difference between the centre coordinates calculated from different sets of S5mimetry related reflections in the four-clicks method is less than 0.1 pixel which shows the quality of centre calculations by this simple method. 

A molecular structure, similar to that of hexasilylated benzene derivative 229, was obtained from tetracyclic hexasilylbenzene 230 by Kira, Sakurai and coworkers, where the six silicon centres are incorporated in three five-membered ring systems (Scheme 80) °. In dimetalated compound 231, two lithium centres, coordinated by a quinuclidine ligand each, are capping the phenyl ring plane from both sides in the solid state. Moreover, it could be found that compound 231 has a thermally accessible triplet state, investigated by temperature-dependent ESR spectroscopy °°. 

The structure of p-CsBeFs is usually described as consisting of chains of corner-connected Bep4 tetrahedra, with Cs atoms/ions interposed between them so that the Cs atoms are 8-coordinated by fluorine, but in a very irregular way. Figure 5 shows that, in fact, this Cs-centred coordination polyhedron is a rather irregular trigonal prism with two lopsided caps. 

The P— y-Ca2Si04 transformation is of some considerable interest, as it may be accompanied by dusting - a serious matter in cement, of which Ca2Si04 is a constituent. There is doubt as to whether the transformation is reconstructive or displacive. In one paper" it is decided that it is semi-reconstructive in another no decision was reached. Comparison of the two structures involved is, in conventional terms involving emphasis on cation-centred coordination polyhedra, a complex task. It would appear to be much simpler in terms of the approach adopted in the present paper. 

For the ROMP of the 5-alkylthiocyclooctenes (115), with R = Et, Bu, Hex, c-Hex, t-Bu, initiated by 12, the most reactive monomers are those with branched alkyl substituents on the sulphur atom for R = t-Bu, reaction is 95% complete in about 10 min. The variations in rate are likely to be connected with the strength of coordination of the sulphur atoms in the monomer and/or the propagating species to the tungsten centre. Coordination of the monomer to the metal centre through the sulphur atom will be impeded when R is t-Bu or c-Hex, allowing a higher equilibrium concentration of the precursor complex that leads to addition of monomer. For R = Bu the rate of polymerization is proportional to both monomer and initiator concentrations189,364. 

It is generally accepted in the theory of the cooperative Jahn-Teller effect to include the interaction with uniform strains in the way proposed by Kanamori [14], i.e., as additional terms of vibronic interaction at each Jahn-Teller ion. On the other hand, within one-centre-coordinate approach used here the vibronic interaction is fully described by means of one-centre active nuclear displacements qn. Therefore the interaction with uniform strains can be included implicitfy as additional terms in the Van Vleck expansion (3). Since phonons and uniform strains are independent degrees of freedom this new expansion is written as follows ... 

Fabbrizzi and co-workers have demonstrated the use of bis-copper(II) cryptates to sense ambidentate anions [49]. On titrating molecule 71 with NaN3 in aqueous solution, the colour changed from pale blue to bright green and an anion-metal LMCT absorption appeared at 400 nm. X-ray diffraction studies have shown that the azide anion is held colinear with the two Cu(II) centres, coordinated through the two terminal sp2-hybridised nitrogen atoms. Stability constants for 71 with a variety of anions in aqueous solution were calculated and the selectivity of this anion sensor was found to be controlled by the bite distance between the two copper atoms (Fig. 1). 

Scheme 1 Bronsted acidity arising from inductive effect of Lewis acid centre coordinated to a silica support...

Sarsfield reported the only three known examples of uranyl bis(imino-phosphorano)-methanides reported to date.52,53 Treatment of the tris-THF adduct of anhydrous uranyl dichloride with one molar equivalent of 2 resulted in the isolation of dimeric 101 when the recrystallisation was carried out in dichloromethane, but monomeric 102 when recrystallised from THF. Both 101 and 102, when treated with two and one equivalents, respectively, of potassium bis(trimethylsilyl)amide afford 103. Complexes 101-103 all exhibit uranyl-methanide contacts in the solid state and these represent noteable examples of out-of-equatorial-plane uranyl coordination. A DFT study concluded that the methanide centre coordinates to the uranyl centre with an orbital of p-type character by a highly polarised cr-type interaction. 

Aakeroy et al. were also able to prepare building blocks based upon Ag(I) centres coordinated in a trigonal planar manner by three isonicotinamide ligands. The isostructural perchlorate and tetrafluoroborate salts of these cationic complexes involve amide-amide hydrogen-bonded links from each cation to six others, resulting in 3D interpenetrated networks that resemble the network found in a-ThSi [53]. 

Figure 1.1 Active site of human carbonic anhydrase II with an inhibitor in place. The inhibitor, shown as a separate molecule in the centre, coordinates to the zinc atom via the group shown at the top. It is attached to other atoms in the cavity through non-bonded interactions. Molecules bound to active sites are often bonded via weak interactions such as hydrogen bonds and van der Waals interactions.

O Fig. 28.10 The structure of spinach plastocyanin (a) the backbone of the protein chain showing the position of the Cu(II) centre and (b) the coordination sphere of the Cu(II) centre, consisting of one methionine, one cysteine and two histidine residues. The structure of azurin from Pseudomonas putida (c) the backbone of the protein chain showing the position of the Cu(II) centre and (d) the Cu(II) centre, coordinated by a methionine, a cysteine and two histidine residues one O atom from the glycine residue adjacent to one of the histidines interacts weakly with the metal centre (the red hashed line). Hydrogen atoms are omitted colour code Cu, brown S, yellow C, grey N, blue O, red. 

Molybdoenzymes have an active molybdenum atom centre coordinated to sulfur-and oxygen-containing groups in the protein. A variant of zeolite A has been produced with molybdenum atoms coordinated to sulfur in complexes held inside the zeolite cavity. 

The racemization of the latter is believed to occur by pseudorotation of a five-coordinate species containing a weak H--AS --X interaction. Quaternary salts of the type [AsR4]X with X = C1, Br or 1 racemize > Br> Cl) in solvents promoting ion pair formation under certain conditions, and there is an example known of tertiary arsine centres coordinated to cobalt(lll) behaving similarly in boiling acetonitrile in the presence of chloride ions. ... 

A classical method for the preparation of enantiopure compounds is the resolution of racemate. However, it is much more effective to use the selective synthesis of the desired enantiopure substance via enantioselective approach. Stereoselective methods of synthesis have been widely developed in organic chemistry. The method of asymmetric synthesis has been known since the nineteenth century and asymmetric catalysis has witnessed an enormous amount of development in recent decades as shown in Chapter 3. In contrast, the asymmetric synthesis of coordination compounds has only recently become a subject of systematic investigation. This is no doubt related to the fact that the chirality of coordination compounds is a much more complex phenomenon than that of organic compounds, because of higher coordination and the multitude of possible central atoms. Furthermore, while in organic chemistry the chiral tetrahedral carbon centres can be prepared without racemization, in contrast T-4 metal centres are very often labile. In fact it is even difficult to prepare compounds with a metal centre coordinated to four different monodentate ligands, and thus the possibility of obtaining one enantiomer is excluded in most cases. 

A phthalocyanine may also have a chiral structure if the metallic centre coordinated by the nitrogen atoms is out of the plane defined by the Pc ligand. A subphthalocyanine (6.41) with C3 symmetry was resolved by HPLC. The CD showed dichroic effects at 560 and 570 nm (Qoo- band) and 280 and 300 nm (Soret band), being either positive or negative according to the enantiomer. Unfortunately, the authors were not able to assign configurations to the enantiomers. 

In the examples studied so far, the ligand is bound to the metal centre by only one of its atoms. The situation is different when several atoms of the ligand are boimd in an equivalent manner to the metal centre ( coordination). This is the case for tt complexes, in which the tt system of the ligand is oriented towards the metal. AH the tt orbitals of the ligand, both occupied and empty, must now be considered to describe the metal-ligand bonds. As an example, we shall treat an ) -ethylene complex in detail in Chapter 3 ( 3.4). 

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