Ammonia normal coordinates

Rg. 5 (a) Potential function rad energy levels as functions of die normal coordinate Q(Ai) (b) Structure of the ammonia molecule in its two equivalent inverted configurations. 

The absorption of ammonia by many solid halides has been known for a long time. The products vary widely in composition and stability. For example, CaCl2. 8 NH3 and CUCI2. 6 NH3, prepared from the anhydrous halides and ammonia gas, readily lose ammonia, but many other amminohalides may be prepared from ammoniacal salt solutions and some possess considerable stability. The simplest compounds of this type have the general formula MX . mNH3 (with sometimes water of crystallization), where m is the normal coordination number of M. They contain complex ions M(NH3), and X" ions. Typical ions of this sort are Ag(NH3)2,... 

In Eq. s (19 - 23), Q Is the normal coordinate and the other terms have the usual spectroscopic significance. The Important new term introduced by Wolfsberg is Gq. This correction may be as large as the 1/4 x U) correction in Eq. (20) and may be of the same or opposite ii n. Wolfsberg has extended this method to the water and ammonia molecules. In Fig. II we reproduce the comparison of Bron and Wolfsberg s (36) calculation of exchange equilibrium... 

F. Watari, S. Shimizu, K. Aida, and E. Takeyama. Vibrational spectra and normal coordinate calculations for trimethylaluminium-ammonia. Bull. Chem. Soc. Japan, 1978, 51, 1602. 

Placing a proton on the X group presumably weakens the Cr-X bond. The enhanced lability is due to a reduced enthalpy of activation, compared with that associated with the step. Normally, the removal of unidentate ammonia or amine ligands from metal complexes is not accelerated by acid, since the nitrogen is coordinately saturated. This situation changes when we consider multidentates (Sec. 4.4.2). 

The masking of the normal reaetions of simple ligands, such as the nitro, cyano, and ammonia groups, by coordination to a metal is a phenomenon encountered early by a chemist. One of the first examples of masking in a chelate complex was reported, signifieantly, in biological journals. It involves the protection by copper ion of the a-amino group in ornithine and lysine ... 

Sargeson and his coworkers have developed an area of cobalt(III) coordination chemistry which has enabled the synthesis of complicated multidentate ligands directly around the metal. The basis for all of this chemistry is the high stability of cobalt(III) ammine complexes towards dissociation. Consequently, a coordinated ammonia molecule can be deprotonated with base to produce a coordinated amine anion (or amide anion) which functions as a powerful nucleophile. Such a species can attack carbonyl groups, either in intramolecular or intermolecular processes. Similar reactions can be performed by coordinated primary or secondary amines after deprotonation. The resulting imines coordinated to cobalt(III) show unusually high stability towards hydrolysis, but are reactive towards carbon nucleophiles. While the cobalt(III) ion produces some iminium character, it occupies the normal site of protonation and is attached to the nitrogen atom by a kinetically inert bond, and thus resists hydrolysis. 

Suhrmann (62) explains the strong increase of the normal photoelectric effect of metals caused by the adsorption of water molecules and also by the molecules of ammonia, by accepting similar coordinate links to function in the chemisorption of these molecules. Dipoles are formed which point with their positive poles away from the surface, thereby decreasing the work function and, consequently, increasing the normal photoelectric effect ... 

The normal modes of ammonia, a C v molecule, are shown in Fig. 14.4. All modes for NH3 are fR active. For molecules of any symmetry, the IR active modes of vibration can be determined from the character table of its symmetry group. Modes which are infrared active belong to the same irreducible representation as one of the cartesian coordinates x, y or z, shown in one of the right-hand columns of the character table. Check this for the C2v, C v and T>ooh molecules we have considered. 

The presence of an actual water molecule coordinated to Z, when the anion contains two hydrogen atoms, was first demonstrated (55) for the SiConWn and SiComWn anions. The water in the first was shown to be replaceable by pyridine, and in the second by pyridine and ammonia. Crystalline salts of the product anions were isolated. The phenomenon seems to be a general one for XZWn anions containing Co (II) (55, 213), Co(III) (55, 203, 213), Ni(II) (55, 213), and Cr(III) (222). As predicted (55), it has also been observed for the X2C0W17 anions (176). These anions and their substitution products therefore constitute a class of hybrid complexes, intermediate between normal heteropolyanions with metal heteroatoms and conventional coordination complexes, and the significance of this has been discussed elsewhere (55). 

Complex ions are any ions that are built up by the union of simpler ions, or of an ion and one or more neutral molecules. They are usually formed around metallic ions. They may be held together entirely by coordinate hnks, as in Cu(NH3)4 and the other metal-ammonia complexes, or they may be bound by a combination of coordinate links and normal covalent links, as in Hgl4 or Fe(CN)6 . They may even be held together by purely ionic or electrostatic forces, as is the case in FeFe this complex is shown by its large magnetic moment to contain a free ferric ion, yet it is extremely stable, since the small size of the fluoride ion allows for close approach and a strong electrostatic force between Fe" and F . 

When ammonia is added to a solution of a copper(II) salt, there is a rapid reaction in which water coordinated to Cu is replaced by ammonia. Although the product of this reaction is normally represented as [Cu(NH3)4], in fact, a variety of products result, the relative amount of each species depending upon the concentrations of copper(n) ion and ammonia reactions (1) to (4). Figure 5.1 is a plot of the percentage of each copperfll) ammonia species in solution against the concentration of free ammonia. This plot indicates that [Cu(NH3)4(H20)2] is the predominant complete ion in solutions containing 0.01 to 5 M free ammonia. Outside this range, however, the other ammine complexes are more abundant. 

The explanation of endotactic heterostructures in molecular dispersion for the X-ray anomalies of ammonia iron was proved by H. Topsoe by Mossbauer spectroscopy. The Mossbauer data imply the presence of small amounts of non-metal iron components which are present, however, as large particles of structural promoter oxides. They are located in grain boundaries and at the outer surface of the catalyst. This location also explains the SIMS data on Fe-Al-O fragments which were intended to support the hypothesis of endotactic heterostructures.The EXAFS data ° ° provide clear evidence for the identical average local coordination of iron in ammonia iron and normal iron. 

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