Coordination compounds reactions

Reactions of coordination compounds share some characteristics with reactions of other molecules, both organic and inorganic, so an understanding of coordination compound reactions can draw on some familiar concepts. However, the chemistry of coordination compounds has some additional features because the molecules have more complex geometries and more possibilities for rearrangement, the metal atoms exhibit more variability in their reactions, and different factors influence the course of reactions. 

Studies of solid state thermal transformatioiis of metal complexes have led to the development of new and often simple methods for the synthesis of some classes of coordination compounds. Reactions that proceed in the absence of solvent frequently lead to formation of products that differ from those formed in solution. In solution there exist possibilities for solvent molecules, or their deprotonated forms, to function as ligands or reagents. This possibility is excluded when a reaction occurs in the solid state. 

Boron trifluoride catalyst may be recovered by distillation, chemical reactions, or a combination of these methods. Ammonia or amines are frequently added to the spent catalyst to form stable coordination compounds that can be separated from the reaction products. Subsequent treatment with sulfuric acid releases boron trifluoride. An organic compound may be added that forms an adduct more stable than that formed by the desired product and boron trifluoride. In another procedure, a fluoride is added to the reaction products to precipitate the boron trifluoride which is then released by heating. Selective solvents may also be employed in recovery procedures (see Catalysts,regeneration). 

The physical and chemical properties are less well known for transition metals than for the alkaU metal fluoroborates (Table 4). Most transition-metal fluoroborates are strongly hydrated coordination compounds and are difficult to dry without decomposition. Decomposition frequently occurs during the concentration of solutions for crysta11i2ation. The stabiUty of the metal fluorides accentuates this problem. Loss of HF because of hydrolysis makes the reaction proceed even more rapidly. Even with low temperature vacuum drying to partially solve the decomposition, the dry salt readily absorbs water. The crystalline soflds are generally soluble in water, alcohols, and ketones but only poorly soluble in hydrocarbons and halocarbons. 

In addition to the processes mentioned above, there are also ongoing efforts to synthesize formamide direcdy from carbon dioxide [124-38-9J, hydrogen [1333-74-0] and ammonia [7664-41-7] (29—32). Catalysts that have been proposed are Group VIII transition-metal coordination compounds. Under moderate reaction conditions, ie, 100—180°C, 1—10 MPa (10—100 bar), turnovers of up to 1000 mole formamide per mole catalyst have been achieved. However, since expensive noble metal catalysts are needed, further work is required prior to the technical realization of an industrial process for formamide synthesis based on carbon dioxide. 

Reactions of the Hydroxyl Group. The hydroxyl proton of hydroxybenzaldehydes is acidic and reacts with alkahes to form salts. The lithium, sodium, potassium, and copper salts of sahcylaldehyde exist as chelates. The cobalt salt is the most simple oxygen-carrying synthetic chelate compound (33). The stabiUty constants of numerous sahcylaldehyde—metal ion coordination compounds have been measured (34). Both sahcylaldehyde and 4-hydroxybenzaldehyde are readily converted to the corresponding anisaldehyde by reaction with a methyl hahde, methyl sulfate (35—37), or methyl carbonate (38). The reaction shown produces -anisaldehyde [123-11-5] in 93.3% yield. Other ethers can also be made by the use of the appropriate reagent. 

Chromium(III) Chemistry. The most characteristic reactions of Cr(III) in aqueous solution at >4 pH, eg, in the intestine and blood, and hydrolysis and olation (147). As a consequence, inorganic polymeric molecules form that probably are not able to diffuse through membranes. This may be prevented by ligands capable of competing for coordination sites on Cr(III) (see Coordination compounds) (147). Thus any large fraction of ingested Cr(III) should be absorbed. Chromium (ITT) in the form of GTF may be more efficiendy absorbed. 

Ammonia forms a great variety of addition or coordination compounds (qv), also called ammoniates, ia analogy with hydrates. Thus CaCl2 bNH and CuSO TNH are comparable to CaCl2 6H20 and CuSO 4H20, respectively, and, when regarded as coordination compounds, are called ammines and written as complexes, eg, [Cu(NH2)4]S04. The solubiHty ia water of such compounds is often quite different from the solubiHty of the parent salts. For example, silver chloride, AgQ., is almost iasoluble ia water, whereas [Ag(NH2)2]Cl is readily soluble. Thus silver chloride dissolves ia aqueous ammonia. Similar reactions take place with other water iasoluble silver and copper salts. Many ammines can be obtained ia a crystalline form, particularly those of cobalt, chromium, and platinum. 

Vanadium(V) Oxytrichloride. Vanadium(V) oxytrichloride (VOCl ) is readily hydrolyzed and forms coordination compounds with simple donor molecules, eg, ethers, but is reduced by reaction with sulflir-containing ligands and molecules. It is completely miscible with many hydrocarbons and nonpolar metal hahdes, eg, TiCl, and it dissolves sulfur. 

Trialkyl- and triarylarsine sulfides have been prepared by several different methods. The reaction of sulfur with a tertiary arsine, with or without a solvent, gives the sulfides in almost quantitative yields. Another method involves the reaction of hydrogen sulfide with a tertiary arsine oxide, hydroxyhahde, or dihaloarsorane. X-ray diffraction studies of triphenylarsine sulfide [3937-40-4], C gH AsS, show the arsenic to be tetrahedral the arsenic—sulfur bond is a tme double bond (137). Triphenylarsine sulfide and trimethylarsine sulfide [38859-90-4], C H AsS, form a number of coordination compounds with salts of transition elements (138,139). Both trialkyl- and triarylarsine selenides have been reported. The trialkyl compounds have been prepared by refluxing trialkylarsines with selenium powder (140). The preparation of triphenylarsine selenide [65374-39-2], C gH AsSe, from dichlorotriphenylarsorane and hydrogen selenide has been reported (141), but other workers could not dupHcate this work (140). 

G. Wilkinson, R. D. GUlard, and J. A. McCleverty, Comprehensive Coordination Chemistry, The Synthesis, Reactions, Properties od dpplications of Coordination Compounds, Vols. 1—7, Pergamon Press, Oxford, New York, Beijing, Frankfurt, Sro Paulo, Sydney, Tokyo, Toronto, 1987. 

Irradiation of coordination compounds in the charge-transfer spectral region can often enhance redox reactions. The quantum yields are variable. 

Whereas this reaction was used to oxidize ethylene (qv) to acetaldehyde (qv), which in turn was oxidized to acetic acid, the direct carbonylation of methanol (qv) to acetic acid has largely replaced the Wacker process industrially (see Acetic acid and derivatives). A large number of other oxidation reactions of hydrocarbons by oxygen involve coordination compounds as detailed elsewhere (25). 

Metal coordination compounds may also provide alternatives to the heterogeneous catalysts used for the water gas shift reaction. In fact, Ru, Rh, Ir, and Pt coordination compounds have all shown some promise (27). 

Beryllium, calcium, boron, and aluminum act in a similar manner. Malonic acid is made from monochloroacetic acid by reaction with potassium cyanide followed by hydrolysis. The acid and the intermediate cyanoacetic acid are used for the synthesis of polymethine dyes, synthetic caffeine, and for the manufacture of diethyl malonate, which is used in the synthesis of barbiturates. Most metals dissolve in aqueous potassium cyanide solutions in the presence of oxygen to form complex cyanides (see Coordination compounds). 

Furazano[3,4-/]quinoxaline, 7,8-diphenyl-synthesis, 6, 412 Furazanothiophene synthesis, 6, 417 Furazans, 6, 393-426 biological activity, 6, 425 bond angles, 6, 396 bond lengths, 6, 396 coordination compounds, 6, 403 diamagnetic susceptibilities, 6, 395 dipole moments, 6, 395, 400 heats of combustion, 6, 400 heterocyclic ring reactions, 6, 400-403 IR spectra, 6, 398 isoxazoles from, 6, 81 mass spectra, 6, 399 microwave spectroscopy, 6, 395, 396 MO calculations, 6, 395 monosubstituted... 

The N-coordinated compound [AuCljL] (L= l-ethyl-2-phenylimidazole) with silver tetrafluoroborate gives the cycloaurated species 102, which on further reaction with triphenylphosphine and sodium tetrafluoroborate or ammonium hexafluorophosphate forms the cationic complexes 103 (X = BF., PF,) (00JCS(D)271). 

Mercuration and chloromethylation reactions as well as halogena-tion seem to proceed with preliminary coordination followed by substitution in the coordination compound. Such reactions as nitration and sulfonation in concentrated acids appear to proceed differently as evidenced by the substitution of the phenyl nucleus on nitration and by the sulfonation of phenylisoxazolcs. 

Unsubstituted phthalocyanines are accessible by the reaction of phthalonitrile with metals, alloys, metal salts or metal coordination compounds. Often a mixture of these compounds and phthalonitrile are heated without solvent beyond the melting point of phthalonitrile. 

Investigations of silicon-metal systems are of fundamental interest, since stable coordination compounds with low valent silicon are still rare [64], and furthermore, silicon transition-metal complexes have a high potential for technical applications. For instance, coordination compounds of Ti, Zr, and Hf are effective catalysts for the polymerization of silanes to oligomeric chain-silanes. The mechanism of this polymerization reaction has not yet been fully elucidated, but silylene complexes as intermediates have been the subject of discussion. Polysilanes find wide use in important applications, e.g., as preceramics [65-67] or as photoresists [68-83],... 

A particularly elegant pathway to stable coordination compounds of disilenes has been found with the reaction of the Pt-phosphine complex 80 with 81 ab, which... 

Fig. 7-2. Potential energy E as a function of the reaction coordinate for reactions of the P-nitrogen of arenediazonium ions with nucleophiles yielding (Z)- and (is)-azo compounds, a) Reactant-like transition states (e. g., reaction with OH) b) product-like transition states (e. g., diazo coupling reaction with phenoxide ions product = cyclohexadienone-type o-complex (see Sec. 12.8).

This section is almost entirely concerned with the kinetics of solid phase decompositions of classical coordination compounds, since most of the information available refers to these substances. The hydrates, in which the ligands are water only, are correctly classified under the present heading, but as their dehydrations have been so intensively studied, a separate section (Sect. 1) has been devoted to the removal of water from crystalline hydrates. A separate water elimination step also preceeds many decomposition reactions. 

Many investigations of the decompositions of coordination compounds have been concerned with the qualitative identification of the steps involved, characterization of any intermediates formed and comparisons of reactivities of related salts containing systematically varied constituents. Observations and conclusions from such work [1113,1114] are outside the scope of this review, though the results can serve to identify systems worthy of more detailed investigation. The content of this section, reflecting the content of the relevant literature, is restricted to accounts of the behaviour of a number of representative substances. Features distinguishing these reactions from those of simple salts are emphasized. 

Comparative studies [1127] of the kinetics of decomposition of similar salts containing related pyridine ligands have been used to investigate the strength of M—N bonds in coordination compounds. Non-isothermal DSC measurements were used to determine values of E for the reactions... 

It is apparent, from the above short survey, that kinetic studies have been restricted to the decomposition of a relatively few coordination compounds and some are largely qualitative or semi-quantitative in character. Estimations of thermal stabilities, or sometimes the relative stabilities within sequences of related salts, are often made for consideration within a wider context of the structures and/or properties of coordination compounds. However, it cannot be expected that the uncritical acceptance of such parameters as the decomposition temperature, the activation energy, and/or the reaction enthalpy will necessarily give information of fundamental significance. There is always uncertainty in the reliability of kinetic information obtained from non-isothermal measurements. Concepts derived from studies of homogeneous reactions of coordination compounds have often been transferred, sometimes without examination of possible implications, to the interpretation of heterogeneous behaviour. Important characteristic features of heterogeneous rate processes, such as the influence of defects and other types of imperfection, have not been accorded sufficient attention. 

Formulation of the detailed mechanisms of decomposition of coordination compounds are likely to remain difficult. Reliable kinetic and supporting observations are not easily obtained where several initiating reactions are possible and subsequent chemical changes may occur, before the first-formed product has left the crystallite of reactant. 

Solid state photochemical reactions of transition metal coordination compounds. E. L. Simmons and W. W. Wendlandt, Coord. Chem. Rev., 1971, 7.11-27 (88). 

The application of reaction mechanisms to the synthesis of coordination compounds. J. L. Burmeis-ter, Prep. Inorg. React., 1968, S, 1-43 (211). 

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