Terminal and Bridging Modes

Owing to their conformational flexibility, bridging and/or chelating coordination modes are typical for polychalcogenide ligands. As a result, very few 

Pg-Te3 units. Each of the Te atoms in the latter tritelluride anions coordinates two indium atoms in a binding mode also found in the phase [NEt4]4 [Au(Agi tAu42Sn2Te9] (x = 0.32) synthesized by solvent extraction of an intermetallic alloy. A further example for the versatility of polytelluride bridging is provided by K2Cu2Te5, in which Cu atoms are linked by intralayer 

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Nitric oxide can bind to metals in both terminal and bridging modes to give metal nitrosyl complexes. Depending upon the stereochemistry of the complexes, NO may exhibit within one given complex either NO+ or NO-character, as illustrated in Table 15.2.2. 

One aspect of metal carbonyl chemistry that should be mentioned in surveying the more commonly found modes of CO coordination is the stereochemical nonrigidity of carbonyl clusters. This aspect has received considerable attention over the past decade, especially as 13C nmr instrumentation has become more readily available. In many carbonyl clusters, terminal and bridging carbonyls as established by x-ray structural studies are equilibrated on the nmr time scale (37, 39-41). The manner of equilibration takes place in a concerted way in order that each metal center maintains a constant electron count. For example, bridge terminal interconversion, (1), proceeds via complementary unsymmetrical CO bridges. 

Fig. 1. OR-ligands terminal and bridging bonding mode - depending on steric and electronic factors...

Table VIII shows the characteristic ranges of frequencies for stretching and deformation modes for terminal and bridging oxy and nitrido systems. In each case, the frequencies for the stretching modes are higher for nitrides than for oxides. Although the nitrogen atom is lighter than oxygen, it is likely that this effect is caused in large part by the fact that...

A number of V complexes of various nuclearities are now known that contain persulfide groups in either terminal or bridging modes, mostly in the III and IV oxidation states, but at least two pentavalent vanadium compounds have been characterized,33 namely (Me3NCH2Ph)2[VS2(S2)(SPh)] and (NEt4)[VO(S2)2(bipy)]. 

CS and CSe are similar to CO in their bonding modes in that they behave as both o- donors and tt acceptors and can bond to metals in terminal or bridging modes. Of these two ligands, CS has been studied more closely. It usually functions as a stronger or donor and tt acceptor than CO. " ... 

IR can be useful in two respects. The number of IR bands, as discussed in Chapter 4, depends on molecular symmetry consequently, by determining the number of such bands for a particular ligand (such as CO), we may be able to decide among several alternative geometries for a compound or at least reduce the number of possibilities. In addition, the position of the IR band can indicate the function of a ligand (e.g., terminal vs. bridging modes) and, in the case of TT-acceptor ligands, can describe the electron environment of the metal. 

The far-IR and Raman spectra of Cd2X4(tpa)2, where tpa = tris(2-pyridyl) amine, X = Cl, Br or I, yielded assignments to terminal and bridging Cu-X stretching modes.437... 

Chloride is just one example of a ligand which may coordinate to a metal centre in a terminal or bridging mode other ligands may be equally versatile. For example, [Co(acac)2] is prepared from C0CI2, Hacac and Na[02CMe] in aqueous methanol. In the solid state, the blue anhydrous salt is tetrameric with a structure related to that of the trimer [ Ni(acac)2 3] (see Figure 21.27b). 

XY,and XY denote terminal and bridging XY stretching modes, respectively. When these two modes couple strongly, distinction between them is not clear (see Ref. 1381). 

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