Tetra-atomic complex

The complexation of anionic species by tetra-bridged phosphorylated cavitands concerns mainly the work of Puddephatt et al. who described the selective complexation of halides by the tetra-copper and tetra-silver complexes of 2 (see Scheme 17). The complexes are size selective hosts for halide anions and it was demonstrated that in the copper complex, iodide is preferred over chloride. Iodide is large enough to bridge the four copper atoms but chloride is too small and can coordinate only to three of them to form the [2-Cu4(yU-Cl)4(yU3-Cl)] complex so that in a mixed iodide-chloride complex, iodide is preferentially encapsulated inside the cavity. In the [2-Ag4(//-Cl)4(yU4-Cl)] silver complex, the larger size of the Ag(I) atom allowed the inner chloride atom to bind with the four silver atoms. The X-ray crystal structure of the complexes revealed that one Y halide ion is encapsulated in the center of the cavity and bound to 3 copper atoms in [2-Cu4(//-Cl)4(//3-Cl)] (Y=C1) [45] or to 4 copper atoms in [2-Cu4(/U-Cl)4(/U4-I)] (Y=I) and to 4 silver atoms in [2-Ag4(/i-Cl)4(/i4-Cl)] [47]. NMR studies in solution of the inclusion process showed that multiple coordination types take place in the supramolecular complexes. 

The sulfide group forms a large number of complexes where it is in chelation with a different heteroatom. Among the common heteroatoms are N, P and As. These complexes are too numerous to list here, but individual complexes can be found from Table 9 or from refs. 1224 and 1667. It is also possible to synthesize compounds which will form bi-, tri- and tetra-dentate complexes to platinum(II), where sulfur, selenium and tellurium.are the only atoms which coordinate to the metal. A review of complexes formed from ligands of the type RS(CH2) SR has been recently published.1734 This article outlines the synthesis, reactions and spectroscopy of these complexes, and allows the complexes of platinum to be placed in context with those of other transition metals. 

In the presence of excess TC, soft acidic Cd2+ forms tetra-thiocyanato complexes in which four TC coordinate to Cd2+ with their soft basic S atoms as expected from the high stability constant of complex formation. 

Two-electron reduction of the 18-electron [M(T)5-L)(Tj5-C5Me5)]2+ (M = Ir, L = 2,5-dimethylthiophene (55) M = Rh, L = 2,3,4,5-tetra-methylthiophene) complexes gives the corresponding r)4-bound thiophene species in which the ligand has lost its aromaticity and now acts as a 4-electron donor via the diene as shown in Scheme 13 for the Ir complex. This was first demonstrated in the crystal structure of [Ir(774-2,5-dimethylthiophene)(T75-C5Me5)] (56), where the thiophene ring was found to be nonplanar with the sulfur atom lying 0.905 A out of the plane of the... 

It has proved convenient to classify centers other than trapped electrons as monatomic, diatomic, triatomic, tetra-atomic and penta-atomic (9), these classes being expanded to include more complex species having similar basic structures. Thus, the radicals R2NO and (S03)2N02 were classed with the corresponding simple AB3 tetra-atomic radicals. 

State geometries involves analysis of the rotational fine structure of the electronic bands. Since the spectra of polyatomic molecules are complex, most analyses of this sort have been carried out on the triatomic or tetra-atomic molecules. For molecules whose spectra contain unresolved fine structure, estimates of the excited state geometry can be obtained by vibrational analyses. 

Simple cleavages of a molecnlar ion conld lead to a rather complex mass spectrum (10 ions for a tetra-atomic molecule 212 ions for an arrangement of 20 atoms). In reality, certain bonds are more likely to break than others, a phenomenon that leads to the formation of relatively few abundant ions. 

---