Ligand centred transitions

A more eflicient and general synthetic procedure is the Masamune reaction of aldehydes with boron enolates of chiral a-silyloxy ketones. A double asymmetric induction generates two new chiral centres with enantioselectivities > 99%. It is again explained by a chair-like six-centre transition state. The repulsive interactions of the bulky cyclohexyl group with the vinylic hydrogen and the boron ligands dictate the approach of the enolate to the aldehyde (S. Masamune, 1981 A). The fi-hydroxy-x-methyl ketones obtained are pure threo products (threo = threose- or threonine-like Fischer formula also termed syn" = planar zig-zag chain with substituents on one side), and the reaction has successfully been applied to macrolide syntheses (S. Masamune, 1981 B). Optically pure threo (= syn") 8-hydroxy-a-methyl carboxylic acids are obtained by desilylation and periodate oxidation (S. Masamune, 1981 A). Chiral 0-((S)-trans-2,5-dimethyl-l-borolanyl) ketene thioketals giving pure erythro (= anti ) diastereomers have also been developed by S. Masamune (1986). 

Ligand-centred (LC) transitions between bonding and antibonding ligand-centred MOs. These transitions are expected for aromatic ligands with extended n- and it -orbitals. 

Ligand-to-metal charge transfer (LMCT) transitions between the bonding ligand-centred MOs and antibonding metal-centred MOs. Such transitions are found where a ligand is easily oxidised and the metal is easily reduced. 

Information on the solid-state structures of cyclosilaphosphanes is lacking and there have been limited studies of their chemical behaviour. As might be expected, these ring systems may act as P-mono, -di- or -tridentate ligands towards transition metal centres, as illustrated by the examples given in Figure 10.10. ... 

Figure 4.75 Schematic representation of the charge transfer in various excited states of a metal complex. M is the metal centre and L stands for a ligand. LF is a ligand field transition, CTs are the charge transfer transitions, LL is an intraligand transition, and CTTS is a charge transfer to solvent...

Recent band structure calculations have confirmed that the increase in electrical conductivity of doped polymer phthalocyanines is the result of both the existence of partially filled bands and the decrease of the M—M distance.104 They also confirm that the conduction process for the doped polymers changes from metal-centred (M = Cr, Mn, Fe and Co), to ligand and metal (M = Ni) and to ligand-centred (Cu and Zn) with increasing electronegativity of the metal atom along the first transition series. 

The 1,3-diketonate anions which are formed are excellent didentate chelating ligands for transition metals. In general, the formation of a diketonate complex is so favourable that simply treating a metal salt with the 1,3-diketone in the presence of a mild base results in the formation of a complex of the deprotonated ligand. In some cases, it is not necessary to add an external base - another ligand co-ordinated to the metal centre may be capable of acting as the base (Fig. 5-3). 

Figure 11.4 Energy level diagram for an octahedral transition metal complex showing the various kinds of electronic transition. MC = metal-centred, LC = ligand-centred, MLCT = metal-to-ligand charge transfer, LMCT = ligand-to metal-charge transfer.

Transitions from -bonding to 7r-antibonding ligand orbitals, called ligand-centred (LC) or internal ligand (IL)... 

Figure 17.6 Photophysical processes of the ruthenium(ll) polypyridine complexes. MC, metal centred MLCT, metal-to-ligand charge transition. (Adapted from Moucheron et al. [199])...

CO acts as both a a-donor (via the lone pair of electrons on carbon) and a 7i-acceptor ligand in transition metal complexes. CO is usually depicted as having a triple bond (one a- and two ti-) between the C and the O as well as lone pairs on both the C and the O. The lone pair on C is used for donation into a suitable metal centred o-orbital. However, the strongest M-CO bonds are formed (in simple terms) when some of the electron density donated by the carbon to the metal is directed back from a filled metal (i-orbital of the correct symmetry into an antibonding ti of the CO. Thus the M-CO bond has two parts, the forward (C M) donation, and the (M C) back donation (Figure 1). 

E20.22 The blue-green colour of the Cr ions in [Cr(H20) ] is caused by spin-allowed but Laporte-forbidden ligand field transitions. The relatively low-molar-absorption coefficient, , which is a manifestation of the Laporte-forbidden nature of the transitions, is the reason that the intensity of the colour is weak. The oxidation state of chromium in tetrahedral chromate dianion is CifVI), which is d . Therefore, no ligand field transitions are possible. Ilte intense yellow colour is due to LMCT transitions (i.e., electron transfer from the oxide ion ligands to the Cr(VI) metal centre). Charge transfer transitions are intense because they are both spin-allowed and Laporte-allowed. 

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