Transition structure group

Attempts to classify carbides according to structure or bond type meet the same difficulties as were encountered with hydrides (p. 64) and borides (p. 145) and for the same reasons. The general trends in properties of the three groups of compounds are, however, broadly similar, being most polar (ionic) for the electropositive metals, most covalent (molecular) for the electronegative non-metals and somewhat complex (interstitial) for the elements in the centre of the d block. There are also several elements with poorly characterized, unstable, or non-existent carbides, namely the later transition elements (Groups 11 and 12), the platinum metals, and the post transition-metal elements in Group 13. 

Compounds with Sc, Y, lanthanoids and actinoids are of three types. Those with composition ME have the (6-coordinated) NaCl structure, whereas M3E4 (and sometimes M4E3) adopt the body-centred thorium phosphide structure (Th3P4) with 8-coordinated M, and ME2 are like ThAsi in which each Th has 9 As neighbours. Most of these compounds are metallic and those of uranium are magnetically ordered. Full details of the structures and properties of the several hundred other transition metal-Group 15 element compounds fall outside the scope of this treatment, but three particularly important structure types should be mentioned because of their widespread occurrence and relation to other structure types, namely C0AS3,... 

The predominantly ionic alkali metal sulfides M2S (Li, Na, K, Rb, Cs) adopt the antifluorite structure (p. 118) in which each S atom is surrounded by a cube of 8 M and each M by a tetrahedron of S. The alkaline earth sulfides MS (Mg, Ca, Sr, Ba) adopt the NaCl-type 6 6 structure (p. 242) as do many other monosulfides of rather less basic metals (M = Pb, Mn, La, Ce, Pr, Nd, Sm, Eu, Tb, Ho, Th, U, Pu). However, many metals in the later transition element groups show substantial trends to increasing covalency leading either to lower coordination numbers or to layer-lattice structures. Thus MS (Be, Zn, Cd, Hg) adopt the 4 4 zinc blende structure (p. 1210) and ZnS, CdS and MnS also crystallize in the 4 4 wurtzite modification (p. 1210). In both of these structures both M and S are tetrahedrally coordinated, whereas PtS, which also has 4 4... 

The formation of monomer and dimer of (salen)Co AIX3 complex can be confirmed by Al NMR. Monomer complex la show Al NMR chemical shift on 5=43.1 ppm line width =30.2 Hz and dimer complex lb 5=37.7 ppm line width =12.7 Hz. Further instrumental evidence may be viewed by UV-Vis spectrophotometer. The new synthesized complex showed absorption band at 370 nm. The characteristic absorption band of the precatalyst Co(salen) at 420 nm disappeared (Figure 1). It has long been known that oxygen atoms of the metal complexes of the SchifT bases are able to coordinate to the transition and group 13 metals to form bi- and trinuclear complex [9]. On these proofs the possible structure is shown in Scheme 1. 

The important features of this transition structure are (1) the chelation of the methoxy group with the lithium ion, which establishes a rigid structure (2) the interaction of the lithium ion with the bromide leaving group, and (3) the steric effect of the benzyl group, which makes the underside the preferred direction of approach for the alkylating agent. 

Fig. 7.4. Tricyclic transition structures for aminoalcohol catalysts syn and anti refer to the relationship between the transferring group and the bidentate ligand cis and trans refer to the relationship between the aldehyde substituent and the coordinating zinc. Reproduced from J. Am. Chem. Soc., 125, 5130 (2003), by permission of the American Chemical Society.

These results were attributed to a preference for an eight-membered chelated transition structure that was lost in the presence of excess BF3 because of coordination of a second BF3 at the ester group. What objections would you raise to this explanation What alternative would you propose ... 

In Chap. 3 the elementary structure of the atom was introduced. The facts that protons, neutrons, and electrons are present in the atom and that electrons are arranged in shells allowed us to explain isotopes (Chap. 3), the octet rule for main group elements (Chap. 5), ionic and covalent bonding (Chap. 5), and much more. However, we still have not been able to deduce why the transition metal groups and inner transition metal groups arise, why many of the transition metals have ions of different charges, how the shapes of molecules are determined, and much more. In this chapter we introduce a more detailed description of the electronic structure of the atom which begins to answer some of these more difficult questions. 

Figure 7.1 Diagram showing the crystal structure groups of the transition metals.

In Sn2 reactions, substituents at the central atom that can stabilise cationic character will also stabilise the SN2 transition state leading to products. As well as lowering the energy, such stabilisation moves SN2 saddle points in the direction of cationic species resulting in incipient cationic character in the transition structure and longer bonds to both the nucleophile and the leaving group.162 165... 

Models accounting for the observed selectivities can be obtained from simple analysis of transition structures, according to the work of Spellmeyer and Houk [37]. These authors have calculated the transition states for cyclizations of radicals to be those shown in Scheme 12.19, but with a hydrogen atom being replaced by the CH2OTiCp2Cl group. 

The presence of this bulky group leads to a higher diastereoselectivity than in the unsubstituted case because interactions of the alkene with the titanocene group lead to the exclusive formation of one diastereoisomer, presumably through the most favored transition structure shown in Scheme 12.19, in which steric interactions should be minimized. 

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