II Salts and Complexes

Submitted by DAVID G. HOLAH and JOHN P. FACKLER, JR. Checked by CONSTANCE M. WRlGHTf and EARL L. MUETTERTIESf 

Although it has been known since 1905 that very pure chromium metal reacts with acids, under oxygen-free conditions, to produce large quantities of chromium (II), this approach to the preparation of chromium (II) compounds has not been developed. Rather, syntheses generally involved (1) reduction of chro-mium(III), either by electrolytic means or by chemical agents (for example Zn/Hg), or (2) metathetical procedures. Both methods are inefficient and often lead to impure products. Recently extensive use of reactions between electrolytic chromium and various acids has led to the synthesis of a wide variety of chromium (II) complexes which would be considerably more difficult to prepare by other methods.  

Because of the sensitivity of chromium(II) to air oxidation, synthetic work on these systems requires the use either of vacuum-line techniques or of a nitrogen-filled box, or both. This synthesis describes the preparation of chromium(II) complexes using a combination of a closed ground-glass filter stick and a nitrogen-filled box. The filter stick enables preparations, filtrations, and recrystallizations to be carried out 

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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). 

Macrocycles with 2 nitrogen and 3 sulfur donors have been prepared (14) by a template synthesis in which the dialdehyde (XCI) is condensed with primary diamines, e.g., ethylenediamine gives XCII in boiling acetonitrile containing Fe(II) perchlorate. The reaction is typical of template condensation between carbonyl compounds and primary amines. An unusual monanionic macrocyclic ligand was produced (2) when formaldehyde was condensed with the hydrazine (XCIII) instead of a primary amine in the presence of Ni(II) salts, and complexes XCIV have been characterized. 

A detailed stu of over 45 catalysts, primarily from Group VIII metal salts and complexes, showed palladium(II) compounds to be the most effective in the dehydrogenation of a variety of aldehydes and ketones. Soluble palladium(II) salts and complexes such as dichloro(tTiphenylphosphine)palladium(II) and palladium(II) acetylacetonate have been shown to be optimal, with the salts of rhodium, osmium, iridium and platinum having reduced efficacy. Since the d ydrogenation reaction is accompanied by reduction of the palladium(II) catalyst to palladium(0), oxygen and a cooxidant are required to effect reoxidadon. Copper(II) salts are favored cooxidants, but quinones, and especially p-benzoquinone, are also effective (Scheme 24). - ... 

After Kumada and Tamao and Corriu independently reported the nickel(II) salts-and complexes-catalyzed cross-coupling reaction of Grignard reagents with aryl and alkenyl halides, the Pd-catalyzed reaction of Grignard reagents was first reported by Murahashi. ... 

Complexes of other metals are used to sensitize emulsions, and some are used to sensitize even further (supersensitize) gold-sensitized emulsions. " Among these are (NH4)2[PtCl4], (NH4)2[PdCl4], (NH4)2[PdCy, K2[IrCl6] and similar compounds with ruthenium and the other halides. Sensitization by iridium salts and complex ions has been reviewed recently. Mechanisms of action of palladium(II) salts and complex ions on gold-sensitized emulsions have been studied. Several phosphine complexes of palladium and platinum, for example (2), are reported to be effective sensitizers, as are many macrocyclic polyamine compounds and their metal complexes,for example the Cu", Ni", Fe " and Rh " chelates of the cyclen ligand, 1,4,7,10-tetraazacyclododecane (3). 

In organic synthesis, two kinds of Pd compounds, namely Pd(II) salts and Pd(0) comple.xes, are used. Pd(II) compounds are used either as stoichiometric reagents or as catalysts and Pd(0) complexes as catalysts. Pd(Il) compounds such as PdCh and Pd(OAc)2 are commercially available and widely used as... 

The standard redox potential is 1.14 volts the formal potential is 1.06 volts in 1M hydrochloric acid solution. The colour change, however, occurs at about 1.12 volts, because the colour of the reduced form (deep red) is so much more intense than that of the oxidised form (pale blue). The indicator is of great value in the titration of iron(II) salts and other substances with cerium(IV) sulphate solutions. It is prepared by dissolving 1,10-phenanthroline hydrate (relative molecular mass= 198.1) in the calculated quantity of 0.02M acid-free iron(II) sulphate, and is therefore l,10-phenanthroline-iron(II) complex sulphate (known as ferroin). One drop is usually sufficient in a titration this is equivalent to less than 0.01 mL of 0.05 M oxidising agent, and hence the indicator blank is negligible at this or higher concentrations. 

This is an alternative method of introducing copper into an o-hydroxyazo dye structure. The azo compound is treated with a copper(II) salt and an oxidant in an aqueous medium at 40-70 °C and pH 4.5-7.0. Sodium peroxide, sodium perborate, hydrogen peroxide or other salts of peroxy acids may be used as oxidants, the function of which is to introduce a second hydroxy group in the o -position [25]. This process is reminiscent of earlier work on Cl Acid Red 14 (5.51 X = H), an o-hydroxyazo dye that will not react with a chromium (III) salt to form a 1 1 complex but will do so by oxidation with an acidified dichromate solution. This oxidation product was later found to be identical with that obtained by conventional reaction of Cl Mordant Black 3 (5.51 X = OH) with a chromium(III) salt [7]. 

In its chemistry, cadmium exhibits exclusively the oxidation state 4- 2 in both ionic and covalent compounds. The hydroxide is soluble in acids to give cadmium(II) salts, and slightly soluble in concentrated alkali where hydroxocadmiates are probably formed it is therefore slightly amphoteric. It is also soluble in ammonia to give ammines, for example [Cd(NH3)4]2+. Of the halides, cadmium-(II) chloride is soluble in water, but besides [Cd(H20)J2+ ions, complex species [CdCl]+, [CdCl3] and the undissociated chloride [CdCl2] exist in the solution, and addition of chloride ion increases the concentrations of these chloro-complexes at the expense of Cd2+(aq) ions. 

Because of their convenient preparation from palladium(II) salts and stable NHC-precursors (vide supra), paUadium(ll) complexes were first examined as potential catalysts for Heck-type reactions. Due to the high thermal stability, temperatures up to 150°C can be used to activate even less reactive substrates, like, e.g., aryl chlorides. Inunobilization of such catalysts has been shown recently (vide infra) ... 

A second type of organopalladium intermediates are 7r-allyl complexes. These complexes can be obtained from Pd(II) salts and allylic acetates and other compounds with potential leaving groups in an allylic position.79 The same type of 7i-allyl complexes can be prepared from alkenes by reaction with PdCl2 or Pd(02CCF3)2.80 The reaction occurs by electrophilic attack on the n electrons followed by loss of a proton. The proton loss probably proceeds via an unstable species in which the hydrogen is bound to... 

Quadricovalent complexes of silver(II) should have the same planar configuration as those of copper(II). This has been verified33 for the silver(II) salt of picolinic acid, which is isomorphous with the cop-per(II) salt and which shows moreover the high birefringence expected for a parallel arrangement of planar molecules with the structure... 

The general method of preparation of neutral complexes of the types Ni(N—N)2 and Ni(N4) (N—N and N4 stand for monoanionic bidentate and dianionic tetradentate ligands, respectively, having nitrogen donors) is the direct reaction of a nickel(II) salt and the appropriate... 

Cobalt complexes of tridentate metallizable azo compounds do not occupy such an important position in the dyestuffs field as chromium complexes. Without doubt the principal reason for this is the fact that 1 1 cobalt complexes cannot be prepared by methods analogous to those employed in the preparation of 1 1 chromium complexes (Section 58.2.3.1 (i)). It is, therefore, impossible to prepare unsymmetrical 2 1 cobalt(III) complex dyestuffs by methods comparable to those used in the preparation of unsymmetrical 2 1 chromium complexes. So-called unsymmetrical 2 1 cobalt complexes have been prepared48 by the interaction of cobalt(II) salts and equimolecular mixtures of two different metallizable azo dyestuffs but these are, in fact, statistical mixtures of the three possible 2 1 complexes (40a-c). 

Cu2(02CMe)4(H20)2, which has the binuclear tetraacetate bridged structure typical of a number of metal(II) acetates, is a stabilizer for various polymers. Copper(II) gluconate, Cu[02C CH(0H) 4CH20H]2, is used as a deodorant. Copper(II) soaps, mainly the oleate and stearate, find application in antifouling paints, and as fungicides for textiles. A summary in tabular form of all applications of simple salts and complexes of copper is available.96... 

In contrast, cyclizations of /V-alkoxy derivatives with iodine are much less selective (Table 30, entry 3), and similar results are observed with systems containing internal double bonds of either ( )- or (Z)-configuration.238 The stereoselectivity of the aminomercuration reaction of /V-methyl derivatives has been reported to be controlled by the choice of mercury(II) salt and solvent (entries 6 and 7).244 Cyclization with platinum salts showed little selectivity (entry 8).239 An iodocyclization of a complex N-alkyl system has been used in a recent synthesis of (+)-croomine.238... 

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