Metals stereochemistry

Crystal fields, 34 177 ESR spectra of metal ions in, 13 197-204 influence on transition-metal stereochemistry, 2 12-33... 

Two- or three-atom bridges on the edges of the M3 triangle allow closer approach to square-planar metal stereochemistry in structure (3c). Examples are Pd3(OAc)650 with near D3h symmetry and the M atoms inside their 04 planes, and Pd3(OAc)3(ONCMe2)351 with two-atom acetoxime bridges on one side of the Pd3 triangle and acetate bridges on the other. 

IV. The Influence of Crystal Fields on Transition-Metal Stereochemistry. 12... 

Such considerations indicate that the conventional ideas of transition metal stereochemistry and co-ordination number should be applied with great caution to organometallic complexes. 

CHR) , formed, e g. from the reaction of diazomethane and alcohols or hydroxylamine derivatives in the presence of boron compounds or with metal compounds. Poly-methylene is formally the same as polyethene and the properties of the various polymers depend upon the degree of polymerization and the stereochemistry. 

The stereochemistry of metal-ammonia reduction of alkynes differs from that of catalytic hydrogenation because the mechanisms of the two reactions are different The mechanism of hydrogenation of alkynes is similar to that of catalytic hydrogenation of alkenes (Sections 6 1-6 3) A mechanism for metal-ammonia reduction of alkynes is outlined m Figure 9 4... 

Hydrogenation of alkynes may be halted at the alkene stage by using special catalysts Lindlar palladium is the metal catalyst employed most often Hydrogenation occurs with syn stereochemistry and yields a cis alkene... 

Gold Compounds. The chemistry of nonmetallic gold is predominandy that of Au(I) and Au(III) compounds and complexes. In the former, coordination number two and linear stereochemistry are most common. The majority of known Au(III) compounds are four coordinate and have square planar configurations. In both of these common oxidation states, gold preferably bonds to large polarizable ligands and, therefore, is termed a class b metal or soft acid. 

Catalytic hydrogenation of the 14—15 double bond from the face opposite to the C18 substituent yields (196). Compound (196) contains the natural steroid stereochemistry around the D-ring. A metal-ammonia reduction of (196) forms the most stable product (197) thermodynamically. When R is equal to methyl, this process comprises an efficient total synthesis of estradiol methyl ester. Birch reduction of the A-ring of (197) followed by acid hydrolysis of the resultant enol ether allows access into the 19-norsteroids (198) (204). 

Extensive efforts have been made to develop catalyst systems to control the stereochemistry, addition site, and other properties of the final polymers. Among the most prominant ones are transition metal-based catalysts including Ziegler or Ziegler-Natta type catalysts. The metals most frequentiy studied are Ti (203,204), Mo (205), Co (206-208), Cr (206-208), Ni (209,210), V (205), Nd (211-215), and other lanthanides (216). Of these, Ti, Co, and Ni complexes have been used commercially. It has long been recognized that by varying the catalyst compositions, the trans/cis ratio for 1,4-additions can be controlled quite selectively (204). Catalysts have also been developed to control the ratio of 1,4- to 1,2-additions within the polymers (203). 

Cobalt exists in the +2 or +3 valence states for the majority of its compounds and complexes. A multitude of complexes of the cobalt(III) ion [22541-63-5] exist, but few stable simple salts are known (2). Werner s discovery and detailed studies of the cobalt(III) ammine complexes contributed gready to modem coordination chemistry and understanding of ligand exchange (3). Octahedral stereochemistries are the most common for the cobalt(II) ion [22541-53-3] as well as for cobalt(III). Cobalt(II) forms numerous simple compounds and complexes, most of which are octahedral or tetrahedral in nature cobalt(II) forms more tetrahedral complexes than other transition-metal ions. Because of the small stabiUty difference between octahedral and tetrahedral complexes of cobalt(II), both can be found in equiUbrium for a number of complexes. Typically, octahedral cobalt(II) salts and complexes are pink to brownish red most of the tetrahedral Co(II) species are blue (see Coordination compounds). 

The copper(I) ion, electronic stmcture [Ar]3t/ , is diamagnetic and colorless. Certain compounds such as cuprous oxide [1317-39-1] or cuprous sulfide [22205-45 ] are iatensely colored, however, because of metal-to-ligand charge-transfer bands. Copper(I) is isoelectronic with ziac(II) and has similar stereochemistry. The preferred configuration is tetrahedral. Liaear and trigonal planar stmctures are not uncommon, ia part because the stereochemistry about the metal is determined by steric as well as electronic requirements of the ligands (see Coordination compounds). 

The stereochemistry of hydrogen-deuterium exchange at the chiral carbon in 2-phenylbutane shows a similar trend. When potassium t-butoxide is used as the base, the exchange occurs with retention of configuration in r-butanol, but racemization occurs in DMSO. The retention of configuration is visualized as occurring through an ion pair in which a solvent molecule coordinated to the metal ion acts as the proton donor... 

On treatment with potassium metal, cij-bicyclo[6.1.0]nona-2,4,6-triene gives a mono-cyclic dianion. The trams isomer under similar conditions gives only a bicyclic monoanion (radical anion). Explain how the stereochemistry of the ring junction can control the course of these reductions. 

The stereochemistry of Mg and the heavier alkaline earth metals is more flexible than that of Be and, in addition to occasional compounds which feature low coordination numbers (2, 3 and 4), there are many examples of 6, 8 and 12 coordination, some with 7, 9 or 10 coordination, and even some with coordination numbers as high as 22 or 24, as in SrCdn, BaCdn and (Ca, Sr or Ba)Zni3. " Strontium is 5-coordinate on the hemisolvate [Sr(OC6H2Bu3)2(thf)3]. jthf which features a distorted trigonal bipyramidal structure with the two aryloxides in equatorial positions. ... 

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. 

The structural chemistry of the Group 14 elements affords abundant illustrations of the trends to be expected from increasing atomic size, increasing electropositivity and increasing tendency to form compounds, and these will become clear during the more detailed treatment of the chemistry in the succeeding sections. The often complicated stereochemistry of compounds (which arises from the presence of a nonbonding electron-pair on the metal) is... 

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