Lead coordination complexes

Four-coordinate complexes provide good examples of the early use of preparative methods for establishing stereochemistry. For complexes of the type [Ma2b2], where a and b are unidentate ligands, a tetrahedral structure cannot produce isomerism whereas a planar structure leads to cis and trans isomers (see below). The preparation of 2 isomers of [PtCl2(NH3)2], for instance, was taken as good evidence for their planarity. ... 

Another stndy on binding to NHC complexes, that combined experiments and DFT (density functional theory) calculations was recently reported on a ruthenium system. This study shows the reversible binding of oxygen to the tetra-NHC complex [Ru(NHC) H)][BAr/] 6 (BAr/ = B (3,5-CF3) C H3 ), which leads to complex 7 (Scheme 10.2) [12]. Unexpectedly, the chemical shift of the hydride ligand undergoes a large downfield shift upon coordination to (from -41.2 ppm... 

COD)Ru(acetate)2 and (COD)Ru(trifluoroacetate)2 were not successful. In all cases, full conversion of the ligand with the precursor was determined via NMR, leading to mixtures of several, partly unidentified species. Furthermore, P-NMR experiments proved that in those cases no formation of any PP-coordinated complexes was achieved. 

Although in most of the above reports structural information is based on X-ray data, Au Mossbauer spectroscopy has also been successfully employed for the investigation of two-, three-, and four-coordination in gold(I) complexes.2551 An increase in coordination numbers leads to a decrease in the isomer shift (IS) by 1-2mm s 1 (three-coordination) or 2-4mm s 1 (four-coordination) relative to two-coordination. For the same ligands, the quadrupole splitting (QS) for three-coordinated complexes is expected to be very similar to that for the two-coordinate derivatives, while that for complexes with Td symmetry should be zero. 

The chloride ligand in OsHCl(CO)(P,Pr3)2 can be also replaced by acetate, acetylacetonate, or iodide (Scheme 49). The reactions with the O-donor ligands lead to six-coordinate complexes OsH(K2-02CMe)(CO)(P, Pr3)2 and OsH(acac) (CO)(P Pr3)2,12 respectively, whereas the treatment of OsHCl(CO)(FPr3)2 with Lil affords the five-coordinate derivative Osffl(CO)(P, Pr3)2.67 Complex OsH... 

Aryl derivatives of the heavier group 6 derivatives can also serve for the synthesis of arylmercury compounds.52 For instance, treatment of the diaryltellurium complex 30 with HgCl2 leads to the formation of a coordination complex which undergoes a slow dismutation reaction to afford the arylmercury chloride derivative 31 and the corresponding tellurium species (Scheme 3).53... 

Linear coordination complexes are also observed with carbenes rather than carbonyl ligands. However, recent structural work carried out on bis(carbene) complexes indicates that coordination of the counteranion can lead to significant deviation of the Ccalbene-Hg-Gcalbene angle from linearity. In 70, for example, the two chloride anions are... 

Figure 4.69 The geometry and leading 7tcc->nM donor-acceptor interaction for the HfH4- C2H4 dative coordination complex.

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