Tetrahedral complexes, absolute

Consider now spin-allowed transitions. The parity and angular momentum selection rules forbid pure d d transitions. Once again the rule is absolute. It is our description of the wavefunctions that is at fault. Suppose we enquire about a d-d transition in a tetrahedral complex. It might be supposed that the parity rule is inoperative here, since the tetrahedron has no centre of inversion to which the d orbitals and the light operator can be symmetry classified. But, this is not at all true for two reasons, one being empirical (which is more of an observation than a reason) and one theoretical. The empirical reason is that if the parity rule were irrelevant, the intensities of d-d bands in tetrahedral molecules could be fully allowed and as strong as those we observe in dyes, for example. In fact, the d-d bands in tetrahedral species are perhaps two or three orders of magnitude weaker than many fully allowed transitions. 

Schurig, V. and Leyrer U. (1990) Semi-preparative enantiomer separation of l-chloro-2,2-dimethylaziridine by complexation gas chromatography - absolute configuration and barrier of inversion, Tetrahedr. Asymm, 1,865-868. 

Fig. 7 One-bond metal-ligand reduced nudear spin-spin coupling constants (absolute values) for 5d metal complexes. Scalar relativistic ZORA DFT results (VWN functional) versus experimental values. A few data points for Pb are also included. Data taken from Refs. [45,123]. The line is not a fit but indicates where Ka c = Kexp. The different markers indicate the different metals. For not octahedrally or not tetrahedrally coordinated metal centers, the first coordination shell was completed with solvent molecules, as described in Ref. [124]...

The convention used to describe the absolute configurations of tetrahedral centres was originally developed for carbon atom centres (see Ref. 13 and Section P-91 of Ref. 1) but can be used for any tetrahedral centre. There is no need to alter the rules in treating tetrahedral metal complexes. 

The absorption and c.d. spectra of 4,4 -(i -propylene-di-iminato)di(3-penten-2-one)-copper(ii) (133) has been interpreted to show that the complex is tetrahedrally distorted with the absolute configuration A 2f-ray diffraction has confirmed this. The synthesis of new binucleating agents (134) and their dimeric cupric complexes has been reported. The new macrocyclic quadridentate ligands of type (135) form... 

Bis(oxazoline)-magnesium complex 20 (10mol%) catalyzes the indicated cycloaddition (Scheme 22) to give 19 (2R) in 82% yield and 91% ee endolexo= 97 3) [74]. The absolute stereochemistry of the product is consistent with biden-tate activation of the substrate through a tetrahedral metal geometry with reaction out of the s-cis conformer. Complex 21, derived from the opposite enantio-... 

While copper and iron Lewis acids are the most prominent late transition metal Diels-Alder catalysts, there are reports on the use of other chiral complexes derived from ruthenium [97,98],rhodium [99],andzinc [100] in enantioselective cycloaddition reactions, with variable levels of success. As a comparison study, the reactions of a zinc(II)-bis(oxazoline) catalyst 41 and zinc(II)-pyridylbis(ox-azoline) catalyst 42 were evaluated side-by-side with their copper(II) counterparts (Scheme 34) [101]. The study concluded that zinc(II) Lewis acids catalyzed a few cycloadditions selectively, but, in contrast to the [Cu(f-Bubox)](SbFg)2 complex 31b (Sect. 3.2.1), enantioselectivity was not maintained over a range of temperatures or substitution patterns on the dienophile. An X-ray crystal structure of [Zn(Ph-box)] (01)2 revealed a tetrahedral metal center the absolute stereochemistry of the adduct was consistent with the reaction from that geometry and opposite that obtained with Cu(II) complex 31. 

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