Transitions unsymmetrical ligands

Let us return then to the problem of low symmetry in transition metal complexes. The most direct and unassuming approach would be to write a symmetry-based expansion of the ligand field potential in terms of spherical harmonics. For a completely unsymmetrical molecule (C,) this would be written as... 

In the numerous octahedral bis- and tris-bidentate complexes of this type, the configurational chirality can be associated simply with the array of chelate-spanned edges, and is designated A or A, as in (19a) and (19b), as discussed previously. The nature of the donor atoms is immaterial in this regard, although their consideration is necessary in the analysis of diastereomers with unsymmetrical bidentate ligands and in the assessment of any quantitative measures of chirality, such as the rotational strengths of electronic transitions.121,149... 

According to these findings it was clear that an optically active spirophosphorane of that type was only possible if at least one unsymmetrically substituted 2,2 -biphenylylene ligand was present. Only then, as is shown in the next scheme, the Berry process involving tetragonal-pyramidal transition state C will not lead to raeemization, since now the trigonal bipyramids B, B are no longer enantiomers but diastereomers. 

Some homoleptic unsymmetrical (dmit/mnt, dmit/tdas) dithiolene nickel complex-based D-A compounds with D = TTF and EDT-TTF also exhibit metal-like conductivity (see Table I) (101). Their molecular structure is shown in Scheme 3. The unsymmetrical tetraalkylammonium salts [MLjLJ- (M = Ni, Pd, Pt) have been prepared by ligand exchange reaction between tetraalkylammonium salts of MLj and ML21 (128, 129) and the D-A compounds have been synthesized by electrooxidation. Among these complexes, only the Ni derivatives exhibit metallic-like properties, namely, TTF[Ni(dmit)(mnt)] (metallic down to --30 K), a-EDT-TTF[Ni(dmit)(mnt)] (metallic down to 30 K), TTF[Ni(dmit)(tdas)] (metallic down to 4.2 K), and EDT-TTF[Ni(dmit)(tdas)] (metallic down to --50 K) (see Table I). The complex ot-EDT-TTF-[Ni(dmit)(mnt)J is isostructural (130) to a-EDT-TTF[Ni(dmit)2)] [ambient pressure superconductor, Section II.B.2 (124)]. Under pressure, conductivity measurements up to 18 kbar show a monotonous decrease of the resistivity but do not reveal any superconducting transition (101). 

Gregory A. M. and F. A. Walker. Electronic Effects in Transition Metal Porphyrins. 5. Thermodynamics of Axial Ligand Addition and Spectroscopic Trends of a Series of Symmetrical and Unsymmetrical Derivatives of Tetrephenylporphinatozinc(II). Inorganica chimica Acta. 91 (1984) pp. 95-102. 

As chiral ligands for transition metal complex-catalyzed asymmetric reactions, a variety of novel chiral ferrocenylchalcogen compounds, which possess a planar chirality due to the 1,2-unsymmetrically disubstituted ferrocene structure, have been prepared from chiral ferrocenes (Scheme 1). Thus, chiral diferrocenyl dichalcogenides bearing an optically active dimethylaminoethyl or p-tolyl-sulfoxide moiety 1-10 were prepared by lithiation of the corresponding chiral ferrocenes, highly diastereoselectively, in moderate to high chemical yields. 

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