Lead complexes sandwich-type

In accord with the expected trans influence of the 71-acceptor aUenylidene unit [212], substitution of the chloride ligand by different anionic nucleophiles in complexes frans-[MCl(=C=C=CR R )(Pi-Pr3)2] (M = Rh, Ir) is favored, affording new aUenylidene derivatives frans-[MX(=C=C=CR R )(P/-Pr3)2] (X = I, F, OH, N3, etc.) (see reactivity studies below). Of particular interest is the behavior of the Rh(I) species frans-[RhCl(=C=C=CPh2)L2] (L = Pf-Pr3, f-Pr2AsCH2CH20Me) towards NaCsHs since the reactions lead to the clean formation of complexes 40 (Scheme 13), the only half-sandwich-type Group 9 allenylidenes presently known [206, 209]. 

The nature and extent of /-orbital participation in the bonding of uranocene and other bis(cyclooctatetraenyl) actinides has never been satisfactorily established, although a good deal of effort has been expended on it. The X-ray structures do not resolve the issue because an ionically bonded model would also lead to a sandwich-type structure (for example, MgCp2 has essentially the same structure as ferrocene). Other physical techniques have been used, but the complexity of the electronic structures often leads to ambiguous interpretations. 

However, ab initio calculations [QCISD-(T)/6-31G //UMP2/6-31G ] on ethylene and its radical cation support an anti -7i-complex, in which the two components are joined by one long bond (190 pm), rather than the sandwich -type 71 complex. The complex is connected to two transition states leading to a (rhombic) cyclobutane radical cation (see above) or, by 1,3-H-shift, to 1-butene radical... 

NaBPh4140. This complex has a sandwich-type structure and the macrocycles show a folded conformation, in which two oxygen atoms lie on the opposite side of the N4 plane from the lead atom (Figure 93). The perchlorate complex has a perchlorate anion as a ligand. If the perchlorate anions are replaced by non-coordinating tetraphenylborate anions, the coordination sphere of the Pb(II) ion becomes unsaturated thus the Pb2+ ion may prefer to coordinate two macrocyclic molecules. 

Visibly luminescent, NIR (980 nm) excitable Er/Yb-doped nanoparticles have been used for microscopic detection in DNA microarrays [65]. The complete absence of endogenous upconversion luminescence leads to very low backgrounds and enhanced contrast. An NIR luminescent ytterbium(III) complex based on the fluorescein-isothiocyanate motif was demonstrated to be an adequate luminescent label in a sandwich-type in vitro diagnostic test [27]. 

Recently, Lacour, Sauvage and coworkers were able to show that the association of chiral [CuL2] complexes (L=2-R-phen,6-R-bpy and2-iminopyridine) with TRISPHAT 8 leads to an NMR enantiodifferentiation, which allows the determination of the kinetics of racemization of the complexes (bpy=2,2 -bipyri-dine phen=l,10-phenanthroline) [119]. This type of application has recently been reported in conjunction with chiral sandwich-shaped trinuclear silver(l) complexes [122]. Several reports, independent from Lacour s group,have confirmed the efficiency of these chiral shift agents [123-127]. Finally, TRISPHAT can be used to determine the enantiomeric purity of (r] -arene)chromium complexes. These results broaden the field of application of 8 to chiral neutral, and not just cationic, species [114,128,129]. 

In the case of the trinuclear [ t-N1,C2-bzimAu]3 (bzim = benzylimidazolate), in addition to the extended structures that form with other metals (see Section 6.3), it also forms supramolecular networks, acting as an electron donor with small organic acids [48]. For example, it reacts with TCNQ (tetracyanoquinodimethane) giving rise to a columnar structure in which each TCNQ molecule is sandwiched between two units of the trinuclear complex in a face-to-face manner. Thus, the repetition of this pattern leads to a stacking of the type (Au3)(Au3)( t-TCNQ)(Au3)... 

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