Isomers four-coordinate 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. ... 

Four-coordinate complexes exhibit lower isomer shifts than six-coordinate compounds. Metal-ligand bonds are shorter and more covalent if the coordination number is smaller because of less steric hindrance and less overlap with antibonding 2g orbitals in the case of four as compared to six bonds. 

In the same way, the existence of two isomers, yellow and orange, of Pt(NH3)2Cl2 showed that these complexes are square planar, rather than tetrahedral (Fig. 13.2) the yellow one has no electric dipole moment and therefore is the trans isomer, whereas the orange does and is cis. A tetrahedral Pt(NH3)2Cl2 would have just one isomer (with a dipole moment). Four-coordinate complexes of platinum(II), palladium(II), and gold(III) are virtually always square planar, but tetrahedral complexes such as the purple... 

The square-planar geometry (see Figs. 8.18b,c) is common for four-coordinate complexes of Au, Ir, Rh", and especially common for ions with the valence electron configurations Ni ", Pd ", and for example. The Ni " ion forms a few tetrahedral complexes, but four-coordinate Pd " and Pp" are nearly always square planar. Square-planar complexes of the type MA2B2 can have isomers, as illustrated in Figures 8.18b and c for cis- and trd ws-[Pt(NH3)2Cl2]. The cis form of this compound is a potent and widely used anticancer drug called cisplatin, but the trans form has no therapeutic properties. 

Four-coordinate complexes can present tetrahedron or square-planar geometries. While square-planar complexes allow cis/trans isomers, the same is not possible for tetrahedron complexes. Again, Werner was correct in his conclusions concerning the tetrahedron structure he stated that only one isomer would be produced for this composition, since, if one of the four corners is occupied than the three remaining ones are equivalent. Werner studied a series of four-coordinate complexes of palladium and platinum and two isomers had been actually iso-... 

Scheme 5.40 depicts the plausible mechanism of the isomerization of trans-dimethylpalladium complex with phosphine ligands to its cis isomer [205]. The three-coordinated dimethylpalladium complex, which is formed via dissociation of a phosphine ligand, reacts with the four-coordinated complex to cause intramolecular methyl ligand exchange. The reaction of trans and cis complexes affords two cis complexes as shown in the above scheme. This promoting effect of the cis complex in the isomerization of trans to cis complex gives autocatalytic type kinetics of the reaction. 

A four-coordinate complex MA2B2 is prepared and found to have two different isomers. Is it possible to determine fiom this information whether the complex is square planar or tetrahedral If so, which is it ... 

As was stated earlier, four-coordinated complexes have either a tetrahedral or a square planar structure. The geometry observed in a particular system depends on the metal and also on the ligands. Thus complexes of beryllium(ii) are always tetrahedral, whereas nickel(ll) forms tetrahedral [NiBr4] and square planar [Ni(CN)4] ". For certain ligands the stabilities of the two structures may not differ greatly in such cases both forms may be obtained and these are called conformational isomers. Examples of such isomers for nickel(ii) and cobalt(ii) are... 

With the rules discussed above, it is easy to predict the stereochemistry of a four-coordinate complex if we can make a good guess of its electron configuration. Actually, it is important to realize that several electron configurations may exist in two or three different stereo-spin isomers (stereospinomers) and the problem we face is to know which of them is most stable or, alternatively, whether more than one stereospinomer could be isolated. 

Geometrical isomerism with respect to the central atom is impossible in tetrahedral complexes. The observation of geometrical isomers is, therefore, a way of deciding that a certain four-coordinate complex is square-planar rather than tetrahedral. 

Two other publications on Ir (73 keV) Mossbauer spectroscopy of complex compounds of iridium have been reported by Williams et al. [291,292]. In their first article [291], they have shown that the additive model suggested by Bancroft [293] does not account satisfactorily for the partial isomer shift and partial quadrupole splitting in Ir(lll) complexes. Their second article [292] deals with four-coordinate formally lr(l) complexes. They observed, like other authors on similar low-valent iridium compounds [284], only small differences in the isomer shifts, which they attributed to the interaction between the metal-ligand bonds leading to compensation effects. Their interpretation is supported by changes in the NMR data of the phosphine ligands and in the frequency of the carbonyl stretching vibration. 

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. 

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