Inorganic Isomerism

Structural thinking in inorganic chemistry, particularly in the field of coordination compounds, is of more recent development and stems chiefly from the massive body of research by A. Werner in the early twentieth century it was Werner who established the importance of the coordination number, and by chemical means alone gave us our present picture of the shapes of coordination compounds. 

We may divide the large number of known cases (and the even greater number of hypothetical cases) of inorganic isomerism into two very broad classes (1) position isomers having the same sets of atoms but different sets of bonds (as is the ease with ethyl alcohol and dimethyl ether) and (2) stereoisomers having the same atoms, the same sets of. bonds, but differing somehow in the orientation of these bonds. Werner and many workers after him recognized a number of subclasses of position isomerisms. 

The pair of compounds Co(NH3)5SOtBr and Co(NH3) 6Br s+SO are said to be ionization isomers. The first of these, in which the sulfate ion occupies one of the octahedral positions about the cobalt atom, reacts only slowly with Ba2+ but gives an immediate precipitate with Ag+. The reverse is true of the second compound which contains a Co—Br bond. 

Hydrate isomerism is quite analogous to ionization isomerism. The compound Cr(H20)5Ci2+ (C1 )2 H20 contains one Cr—Cl bond and five Cr—O bonds per complex whereas its isomer, Cr(H20)4Clt C1 2H20, has two Cr—Cl bonds and only four Cr—O bonds per complex. The first of these yields three ions per gram-atom of chromium when dissolved in water (as may be checked by freezing-point measurements), whereas the second yields but two ions per complex. Those water molecules not directly coordinated to chromium are probably present as lattice water.  

The compounds Co(NH3)t Cr(CN)8 and Cr(NH3)6Co(CN)6 are termed coordination isomers. The first contains Co— N and Cr—C bonds whereas the second contains Cr—N and Co—C bonds. There are four additional coordination isomers related to these two compounds (Exercise 2). Pairs of compounds such as PtCI2(NH3)2 and Pt(NH )4+PtCl are sometimes called polymerization isomers, but may just as logically be regarded as coordination isomers in which both centers of coordination hold the same atom. 

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Addition of aqueous barium chloride to the potassium salt yields the bluish insoluble barium trioxalatochromate (III). Many other heavy metal ions and coordination cations give insoluble chromioxalates which are interesting from the standpoint of inorganic isomerism. 

From these discoveries branched his work in hydrogenation (both organic and inorganic), isomerization, polymerization, condensation, cyclization, alkylation, and related reactions... 

Octahedral models of the cyclic amine show that four possible strain-free planar conformations are possible, as well as two bent arrangements which lead to cis orientation of the remaining two coordination positions. Because each of these six unhindered configurations is distinct from one another, mirror images of each form are possible—even for the trans structures. The latter phenomenon would be without precedent, and further investigation may open a new chapter in inorganic isomerism. 

In fact, Werner played such a central and almost monopolistic role in coordination chemistry that his name is virtually synonymous with the field. Even today, almost 75 years after his death in 1919, coordination compounds, particularly metal-ammines, are still colloquially called Werner complexes. The coordination theory not only provided a logical explanation for known "molecular compounds, but also predicted series of unknown compounds, whose eventual discovery lent further weight to Werner s controversial ideas. He showed how ammonia could be replaced by water or other groups, and he demonstrated the existence of transition series between ammines, double salts, and hydrates. Werner recognized and named many types of inorganic isomerism such as coordination isomerism, polymerization isomerism, ionization isomerism, hydrate isomerism, salt isomerism, coordination position isomerism, and valence isomerism. He also postulated explanations for polynuclear complexes, hydrated metal ions, hydrolysis, and acids and bases. His view of the two types of chemical... 

INORGANIC COMPLEXES. The cis-trans isomerization of a planar square form of a rt transition metal complex (e.g., of Pt " ) is known to be photochemically allowed and themrally forbidden [94]. It was found experimentally [95] to be an inhamolecular process, namely, to proceed without any bond-breaking step. Calculations show that the ground and the excited state touch along the reaction coordinate (see Fig. 12 in [96]). Although conical intersections were not mentioned in these papers, the present model appears to apply to these systems. 

What particularly seemed to excite Wohler and his mentor Berzelius about this experiment had very little to do with vitalism Berzelius was interested m cases m which two clearly different materials had the same elemental composition and he invented the term isomerism to define it The fact that an inorganic compound (ammonium cyanate) of molecular formula CH4N2O could be transformed into an organic compound (urea) of the same molecular formula had an important bearing on the concept of isomerism... 

Isopentenyl diphosphate is isomerized by a shift of the double bond to form dimethylallyl diphosphate, then condensed with another molecule of isopentenyl diphosphate to form the ten-carbon intermediate ger-anyl diphosphate (Figure 26-2). A further condensation with isopentenyl diphosphate forms farnesyl diphosphate. Two molecules of farnesyl diphosphate condense at the diphosphate end to form squalene. Initially, inorganic pyrophosphate is eliminated, forming presqualene diphosphate, which is then reduced by NADPH with elimination of a further inorganic pyrophosphate molecule. 

This review will restrict itself to boron-carbon multiple bonding in carbon-rich systems, as encountered in organic chemistry, and leave the clusters of carboranes rich in boron to the proper purview of the inorganic chemist. Insofar as such three-dimensional clusters are considered at all in these review, interest will focus on the carbon-rich carboranes and the effect of ring size and substituents, both on boron and carbon, in determining the point of equilibrium between the cyclic organoborane and the isomeric carborane cluster. A typical significant example would be the potential interconversion of the l,4-dibora-2,5-cyclohexadiene system (7) and the 2,3,4,5-tetracarbahexaborane(6) system (8) as a function of substituents R (Eq. 2). 

R. Shukla, X. E. Verykios, and R. Mutharasan, Isomerization and hydrolysis reactions of important disaccharides over inorganic heterogeneous catalysts,... 

The synthesis of octylthiocyanate by reaction of n-octyl bromide with KSCN, and its subsequent isomerization to isothiocyanate, have been realized by use of TBAB under the action of MW irradiation. The effect of inorganic solid supports was studied (Si02, K10, graphite, NaCl) (Eq. 64). 

In this model the function of RasGAP was to accelerate the isomerization step and to bring Ras into a conformation competent for GTP cleavage without further support of GAP residues. A different analysis of the macroscopic readout interpreted the small change in amplitude as the hydrolysis reaction or the release of inorganic phosphate, and rejected the model of a GAP-induced conformational change in Ras [171], but this was again disputed by supporters of the conformational model [172]. 

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