Square-planar complexes, isomerization

In the cis isomer, the two CH3 groups (or the two H atoms) are as close to one another as possible. In the trans isomer, the two identical groups are farther apart. The two forms exist because there is no free rotation about the carbon-to-carbon double bond. The situation is analogous to that with cis-trans isomers of square planar complexes (Chapter 15). In both cases, the difference in geometry is responsible for isomerism the atoms are bonded to each other in the same way. 

Isomerization involving a square planar complex is also known. Because of the trans effect, it is easier to synthesize the trans isomer of many complexes than it is to prepare the cis complex. The following reactions lead to the formation of an unusual platinum complex ... 

One of the effective catalysts for isomerization of alkenes is a square planar complex of Rh1 +. That... 

G. K. Anderson and R. J. Cross, Chem. Soc. Revs. 9, 185 (1980) for a comprehensive and incisive review of isomerization mechanisms of square-planar complexes. 

Square planar complexes of the formula [Ma,B2l may exhibit cis-trans isomerism ... 

Stereochemistry and Isomerism. Metals having a coordination number of four usually give square-planar complexes, whereas metals having a coordination number of six are octahedrally surrounded by the functional groups of ligands. 

The crystal structure of the square planar complex [Ir(Cl)(Cu)(Ph2PC6H4NMe2)] (53) has been determined. IR and31P NMR spectra for complexes (53), (54) and (55) have suggested that all three complexes possess the same stereochemistry.138 It was also observed that these complexes are more effective than Vaska s complex for the isomerization of 1-hex-l-ene under hydrogenation conditions.138... 

DiaUcyl as well as diaryl tellurides behave as monodentate ligands. Consistent with their soft character, their best explored group of complexes involves Pd(II) and Pt(II) centers. A variety of techniques have been applied to the characterization of their solid state and solution stractures. In such square-planar complexes, [MX2(TeR2)2] (M = Pd, Pt X = Cl, Br, I), cis - trans isomerization, intramolecular ligand exchange and tellurium inversion processes have all been observed by detailed far-infrared and variable-temperature NMR studies (see Refs. 14,15,17,3 5 for a more detailed review of the subject). 

However, stereochemical effects cannot be inferred from the present data, i.e. whether the total hydrolytic stabilities of the isomeric square-planar complexes [Ni(ox)(en)] and cis- or fran.y-[Ni(glyc)2] (ox = oxalate... 

Addition of racemfe-Mej [ 14) aneN,-HjO to nickel diacetate in methanol produces the blue-violet six-coordinate acetato complex [Ni(rocem c-Me5 [14] ane N4)(CH3COO)] CIO4 in which the macrocycle is coordinated in a folded form. Addition of perchloric acid to this acetato complex in water produces the yellow square planar complex a-[Ni(racemic-Me l 14]aneN4)] (004)2. which isomerizes in solution to the (3-racemic form, by way of inversion of two nitrogen atoms. 

Mechanisms for the isomerization of square-planar complexes are either intra- or intermolecular. The simplest intramolecular process is a twist mechanism where the initial square-planar complex rearranges to its isomer via an intermediate or transition state having a T configuration ... 

The number of possible diastereomers depends on the variety of ligands and sometimes requires use of the one-letter code (cis/trans is noted c/t). This nomenclature may be applied to square planar complexes and to square planar pyramidal and octahedral complexes, but not to tetrahedral complexes where a given position is equivalent to any other. Moreover, geometric isomerism often implies the existence of optical isomerism. 

However, for such well-known reactions 45> as positional isomerization and bimolecular substitution in simple square planar coordination compounds, very recently promissing attempts have been reported to derive selection rules on the basis of the orbital symmetry conservation principle 189> and state correlation diagrams 521> e.g. the cis-trans isomerization of square planar complexes has been predicted as a thermally forbidden and photochemically allowed process, in accordance with experiments 28,31,370) [see aiso section FI]. 

CH2=CH2 R = Ph, L = MeCN). These complexes exhibit square-planar coordination with a bidentate Tp ligand, with the exception of [PtMe(CH2=CH2)(Tp )] trigonal bipyramidal. [PtMe(Tp )(CO)] is present in solution in both isomeric forms the four-coordinate square-planar and five-coordinate trigonal-bipyramid species rapidly interconvert in solution. The structures of [Pt(Me)(MeCN)(ic2-HTp )][BArF], [Pt(Me)(K2-HTp )(L)][BArF] (L = CO, CH2=CH2, PMe2Ph), fr ws-[Pt(Me)(K1-HTp )(PMe2Ph)2][BArF], and of the neutral square planar complex [PtMe(Tp )(SMe2)] have been determined by single-crystal X-ray study.520... 

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