Some unusual stereochemistries

While it remains true that tetrahedral and octahedral coordination modes are the predominant stereochemistries adopted by the group 13 metals, nevertheless increasing diversity is being achieved by carefully selecting appropriate electronic and geometric features to enhance the stabilization of unusual stereochemistries. Some representative examples follow. 

Trigonal planar A1 is found in the [AlSb3] anions in [Cs6K3Sb(AlSb3)], which is formed by heating a stoichiometric mixture of 6Cs, 3KSb and AlSb in a sealed Nb ampoule at 677° 

The Ga analogue was prepared similarly. The planar anions are embedded between columns of condensed icosahedra (Cs6K6/2) which in turn are centred by the remaining unique monatomic Sb anion. 

The indium molybdate InnMo4o062, prepared by heating the appropriate mixture of In, Mo and M0O2 at 1100°C, features novel quasi-linear chain cations. Ins + and Ing + in channels 

Seven-coordinate pentagonal-bipyramidal In has been found in the chloroindium complex of 1,4,7-triazacyclononanetriacetic acid 

6 pm from the best plane of the five Mn atoms. Note also that the ligand is isolobal with cyclopentadienyl, C5H5. 

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The complex coordination chemistry of Pb" is also beginning to be actively explored and some unusual stereochemistries are emerging. Thus, the mononuclear ( ) -nitrato)bis(phenanthrolene)(A-thiocyanato) complex [Pb(phen)2(NCS)( ) -NO3)] has 7-coordinate Pb" with a large vacancy... 

A wide variety of these are known, and the principal preparative routes to most of them are indicated in Scheme 12. The examples amongst these of seven- and even eight-coordination — very unusual stereochemistries for osmium—is noteworthy, as is the fact that, in a formal sense at least, some of them contain osmium(IV) or osmium(VI), very high oxidation states for phosphine-containing species. 

Epoxidation of most substrates follows the empirical rules described in Scheme 11, but some substrates show unusual stereochemistry, especially when the substrates carry a free hydroxy group in the vicinity of the allylic alcohol. Coordination of the non-allylic hydroxy group to the titanium ion influences the conformation of the loaded substrate at the transition state and affects the sense... 

The enhancement of the enantiomeric excess of the mono-epoxide can be expected also in the epoxidation of the meso-secondary diallylic alcohol 31. Epoxidation of 32 gives the desired anfi-epoxide of high enantiomeric excess in good yield [72]. However, some meso-substrates bearing two hydroxy groups in close vicinity to each other may show unusual stereochemistry as discussed in the preceding section. 

Another characteristic of modem coordination chemistry is the increasing reliance upon physicochemical methods unknown to Werner and his contemporaries. Simultaneously with an introduction of these newer techniques, emphasis shifted from preoccupation with qualitative studies of stmcture and stereochemistry to quantitative studies of thermodynamics, kinetics, and reaction mechanisms. Some areas of current research interest include unusual ligands, oxidation states and coordination numbers, solid-state chemistry, photochemistry, relationship between stmcture and reactivity, variable oxidation state, chelates, heteropoly complexes, organometalhc... 

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