Square planar structure isomerism

Square planar structures can show cis-trans isomerism. Examples are drawn following, with the m-isomer drawn on the left, and the /ra/w-isomer drawn on the right. 

Treatment of [Rh°(ri -TMPP)2] with terf-butylisonitrile yields a new paramagnetic species [Rh°(ri -TMPP)2(CNt-Bu)2] (16). X-ray diffraction revealed that [Rh°(ri -TMPP)2(CNt-Bu)2] " " adopts a distorted square-planar structure with mutually trans phosphine and isocyanide ligands. Somehow a cis-trans isomerization has taken place. The trans Rh—P bonds are slightly longer than the cis Rh P bonds in the starting material, as a result of a stronger trans influence of a phosphine donor compared to an ether. 

The associative pathway of ligand exchange starts from the addition of a nucleophile to the positively charged iodine atom of a X -iodane with the initial formation of a trans hypervalent 12-1-4 square-planar species. This intermediate species isomerizes to the cis 12-1-4 square-planar intermediate and eliminates the ligand L to afford the final product (Scheme 1.4). Such a mechanism has been validated by the isolation and X-ray structural identification of several stable 12-1-4 species. For example, the interaction of ICI3 with chloride anion affords tetrachloroiodate anion, ICl4, which has a distorted square-planar structure as established by X-ray analysis of the trichlorosulfonium salt, CI3S+ ICU [209]. 

The second series of related materials was based on the (3,4-didodecyloxyphenyl)diaminodi-oxime derivative (73). Vicinal dioximes are capable of coordinating through N,N or N,0 sites of the oxime groups. Upon the complexation, the ligands fold back, and the transition metal complexes are V,V-coordinated with a square-planar structure. Both isomeric forms were selectively isolated for the nickel complexes, but only the anii-isomer for the palladium complex. The two uwti-complexes complexes exhibited a Colh phase ((73) M = Ni Cr 78 Colh H7 I M = Pd Cr 80 Colh 131 I). Most astonishing, however, was the observation of a Colh phase for the isomeric amphi-mckel complex ( , Z-isomer), between 66 °C and 145 °C. Not only was the existence of mesomorphism surprising and unexpected, but the mesophase stability also increased with respect to the anti-complex. 

The most common coordination number is six and such complexes have an octahedral structure. The next most common four-coordinated systems have either tetrahedral or square planar structures. Other complexes are known having different coordination numbers and structures. The stereochemistry of metal complexes is a fascinating subject. Several different types of isomeric structures are possible and have been demonstrated in these systems. For our purpose here it is sufficient to cite examples of geometrical (ds-trans) and optical isomerism. This can readily be iUustrated by the cis (III) and trans (IV) isomers of QCo(en)2Cl2]+. Note that the... 

Geometric isomerism A type of isomerism that arises when two species have the same molecular formulas but (Efferent geometric structures, 413 octahedral planar, 415 square planar 414 trans isomer, 414... 

The trans compound melts at approximately 90 °C, and continued heating leads to isomerization to the cis structure. Geometrical isomerizations can also lead to a change in structure of the complex. For example, a change from square planar to tetrahedral structure has been observed for the complex [Ni(P(C2H5)(C6H5)2)2Br2]. 

This is the first coordination number io be discussed that has an important place in coordination chemistry. It is also the first for which isomerism is to be expected. The structures formed with coordination number 4 can be convenienlly divided into tetrahedral and square planar forms allhough intermediate and distorted structures are common. 

The metallation of corrole with Rh(CO)2Cl 2 gives an isomeric mixture of Rh HjCorXCO) 244 According to an X-ray structure analysis of the N-23,N-24 complex (N2l-Mc,N22-H), the two CO molecules coordinate to the approximately square planar Rh atom on the same side of the corrole which is considerably distorted and serves as a bidentate ligand. The N-23.N-24 isomer may be thermally equilibrated with the N-21,N-23 isomer (Scheme 79). 

EXAMPLE 8 Geometrical isomerism in compounds with square symmetry is illustrated in Fig. 9-18. Because of the rigidity of the square planar arrangement in the Pt, N, and Cl atoms there are two distinct forms. Notice that the chlorines in (a) are opposite each other, but the chlorines in (b) are adj acent. This is because the four locations around the central metal are different, but if the structure had been tetrahedral, the locations of the atoms would not have been different and there would be no isomerism. 

Chemistry is stereochemistry. After development of the fundamental synthetic methods and the structural characterization of many important compounds in such an expanding field as organometallic chemistry during the past 25 years, the problems demanding solution become more subtle, so that more elaborate techniques, e.g., kinetic and spectroscopic measurements, as well as stereochemical methods, gain in importance. Stereochemical information can be obtained by a variety of techniques. Hence, the study of such stereochemical aspects as cis-trans isomerism in square-planar and octahedral transition-metal compounds was a consequence of the increasing availability of IR spectrometers, and temperature-dependent NMR spectroscopy proved to be an extremely valuable technique for the study of nonrigid systems. 

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