Tetrahedral complexes carbon

Strychnine, the most celebrated member of the Strychnos alkaloids, possesses a complex polycyclic structure which is assembled from only twenty-four skeletal atoms. In addition to its obvious architectural complexity, strychnine s structure contains a contiguous array of six unsymmetrically substituted tetrahedral (asymmetric) carbon atoms of which five are included within one saturated six-membered ring. The intimidating structure of the strychnine molecule elicited the following remark by Sir Robert Robinson in 1952 For its molecular size it is the most complex substance known. 5... 

Tetrahedral complexes du not exhibit geometrical isomerism. However, they are potentially chiral just as is tetrahedral carbon. The simple form of optical isomerism exhibited by most organic enantiomers, namely four different substituents, is rarely observed because substituents in tetrahedral complexes are usually too labile10 for the complex to be resolved, i.e., they racemize rapidly. However, an interesting series of cyclopentadienyliron phosphine carbonyl compounds (see Chapter 15 for further... 

Unusual bridging (//-cyclopropyIidene)diiron complexes having a tetrahedral carbene carbon have been studied as model intermediates in carbon-carbon bond formation in the Fischer-Tropsch synthesis248. The cyclopropylidene complexes cis- and trans-[Cp(Co)Fe]2(/(-Co)(//-C,H4) were readily prepared by cyclopropanation in ether, of the corresponding cis- and mww-vinylidene complexes [CpCoFe](//-CO)(//-CH2) with diazomethane in the presence of CuCl (equation 181). Both isomers are air stable in the solid state. Solutions of the complexes are air stable for several hours, provided they are kept in the dark. The pure //-cyclopropylidene isomers slowly interconvert in solution, like their parent /z-vinylidene and other alkylidene complexes. The final equilibrium ratio cis .trans = 4.8 1 is reached after two weeks. 

In the case of ester hydrolysis the O of the water is an electron-rich substance and the carbonyl carbon of the ester is an electron-deficient substance. Ester hydrolysis occurs in three stages (1) the initial stage in which electrons flow from the water molecule to the ester (2) the intermediate stage in which the ester carbonyl forms a tetrahedral complex involving the hydroxyl group originating from the water molecule that releases a proton and (3) the final stage in which carboxylic acid and alcohol are formed. 

The solubility of AgCl is therefore sufficient to give [Ag(NH3)2]Cl on treatment with aqueous ammonium carbonate, while AgBr dissolves only in aqueous ammonia, in which Agl is poorly soluble. Aqueous pyridine and substituted pyridines form [Ag(py)]+ and [Ag(py)2]+ ions, but in non-aqueous conditions tetrahedral complexes, such as [Ag(py)4]C104, may be obtained. The tetrahedral acetonitrile adduct [Ag(NCMe)4]+ is also known and is quite stable. There are a number of argen-tate(I) complexes, such as [Ag(NCO)2] and [Ag(ON02)2]- they are linear, with 2-coordinate Ag+.4 Linear coordination is also found in the tetrameric silver amide [Ag N(SiMe3)2 ]4.5... 

Step II. The "planar" acetate (or thioacetate moity of the ACh (or ASCh) forms a "tetrahedral" complex with the attacking nucleophile methanol as the hybridi-2 zation of the substrate carbonyl carbon changes from sp to sp. ... 

The metal centre. In organic chemistry, chiral centres are usually associated with an asymmetric carbon atom, but this notion is of limited use for metal ions. Most tetrahedral metal ions are extremely labile, although pseudo-tetrahedral complexes such as (C5H5)MLL L" may be resolved into enantiomers. Octahedral centres with... 

Suitable reagents have two amino groups separated by a carbon atom. One amino group makes a nucleophilic attack on the electrophilic carbon atom of the keto group, whereas the other, by adding to the carbonyl group of the ester substituent, forms a tetrahedral complex that starts the exchange of—NH2 for—OEt. 

Earlier we reported the reaction of 1 with Ni(CO)4, in which two carbon monoxide molecules are displaced to yield the tetrahedral complex, (l)2Ni(CO)2 [7]. When bis(cyclooctadiene)nickel is treated with 1 or 2, both cyclooctadiene molecules are lost and the trigonal planar complexes (l) Ni and (2)3Ni are obtained [8]. The stracture of (l)3Ni is shown in Fig. 4. This trigonal structural arrangement for nickel complexes is unusual, but not unprecedented. 

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