Werner octahedral complex ions

It is common practice to consider the traditional Werner octahedral complex ions [MlLNle]" [M = Co(III), Rh(III), Ir(III), Cr(III), Ru(III), Pt(IV) LN = donor atom of unidentate or polydentate ammine or amine] as well as square-planar [M(LN)4p [M = Pt(II), Pd(II)] as kinetically inert compounds. Bound ammonia is generally less labile than bound water, and it has been suggested that this observation can be related to the presence of an extra and exposed electron pair in water. This may make it more sensitive to electrophilic groups in the solvation sheath, which could assist its dissociation from the metal ion (274). If we take the stance of assigning lability as a property of the ligand in such complexes, then ammonia and amines in general can be... 

In 1889 J0rgensen had prepared two isomers with the formula CoCl3.2en, where en is ethylenediamine, H2N.CH2.CH2.NH2. J0rgensen believed that these were structural isomers, but Werner maintained that they were two geometric isomers of the octahedral complex ion (Figure 12.9). In fact Werner cited the existence of two compounds with this formula as evidence for the octahedral configuration in complexes with a coordination number of 6. 

This is by for the most common coordination number. With certain ions si -coordinate complexes are predominant. For example. chromhim(lll) and cobah(MI) are almost exclusively octahedral in their complexes. It was this large series of octahedral Cr(III) and Co(III) complexes which led Werner to formulate his theories of coordination chemistry and which, with square planar plalinum(II) complexes, formed the basis for almost all of the classic work on complex compounds. Before discussing the various isomeric possibilities for octahedral complexes, it is convenient to dispose of the few nonoctahedral geometries. 

It was the optical resolution of [Co(en)2(NH3)Cl]2+ that firmly established Werner s theory and which initiated the study of the optical activity of complex ions. The realization that some octahedral complexes are chiral evidently did not occur to Werner until several years after he published his theory of coordination. He then realized that the demonstration of this property would furnish an almost irrefutable argument in favor of his theory, and he and his students devoted several years to attempts to effect such resolution. Had he but known it, the problem could have been easily solved, for cis-[Co(en)2(N02)2]X (X = Cl, Br) crystallizes in hemihedral crystals which can be separated mechanically, just as Pasteur separated the optical isomers of sodium ammonium tartrate. 

The electrophilic attack of nitric oxide on a bent nitrosyl is now realized to be the path by which hyponitrite-bridged Co species are formed. Reaction (93) was known since the time of Werner (217), but the black and red isomers of [Co(NO)(NH3)5]2+ obtained from this reaction defied definitive characterization for many years. It has now been established that the black isomer is a mononuclear, octahedral complex of Co(III) and NO- (218) while the red isomer is a hyponitrite bridged system containing two Co(III) ions (219). 

Although the existence of circular dichroism and anomalous optical rotatory dispersion for the visible d—d transitions of transition metal complexes was discovered by Cotton (7), the first resolution of an octahedral complex was achieved by Werner (2), for the chloroamminebis-(ethylenediamine)cobalt(III) ion (I, X = C1, Y = NHs). In the course of a few years he resolved (3) the trisethylenediaminecobalt(III) ion (II), a number of bis- and tris-chelated octahedral complexes of cobalt, chro-... 

Werner must have felt satisfied that the octahedral arrangement of ligands in 6-coordinate complexes was firmly established, but some critics objected because the resolved complexes contained carbon, and optically active carbon compounds were well known. Werner silenced this objection by preparing and resolving a completely inorganic complex (71), [Co (OH)2Co(NH3)4 3] , in which the chelate ligands around the central Co(III) are the complex ions c2s-[Co(NH3)4(OH)2]. With this accomplishment, the major points of Werner s coordination theory for 6-coordi-nate complexes were firmly established long before modem structural methods were available. 

The products of the substitution of a ligand G for ligand A of the species [M ABCDEF]" (in which M is a metal ion and A, B, C, D, E, and F are monodentate ligands or donor atoms of chelating ligands of an octahedral complex) are shown in Figure 1. Letters under the arrows indicate positions made adjacent by the loss of A, or the insertion position of G. The products are based on the principle that minimal atomic motion accompanies the attainment of the transition state, the reaction intermediates (if any), and the products. The results of a large number of studies support this principle, which was assumed by Werner, and which recently has been used by Pearson and Basolo (81) and by Kyuno, Boucher, and Bailar (67). 

The preparation of transition metal complexes with penten marks the last step in mononuclear octahedral complexes begun by Jorgensen and Werner last century with ethylenediamine. Stuart models indicate that the fifth and sixth amine nitrogens are progressively harder to fit into place on the octahedron. If we assume a five-membered ring strain with each tetrahedral nitrogen of 1.5°, the fourth and fifth rings would have a cumulative strain of 6 and 7.5°, respectively. The ion [Copenten] should exist in four pairs of optically-active isomers ... 

The so-called video salts (salts of the cw-[CoCl2(NH3)4] ion), which played a key role in Alfred Werner s coordination theory, were first prepared in 1907 by the Czech chemist J. V. Dubsky (1882-1946), who began his career in 1904 as one of Werner s Doktoranden in Zurich. The first attempt to resolve an octahedral complex into enantiomers is also connected with Dubsky s name. Dubsky s life and work, especially his contributions to coordination chemistry, are briefly discussed in this paper. 

The octahedral complexes of the first transition series are fairly labile except for those of Cr and Co . The inert complexes of these two cations were among those used by Werner in his historic researches. The octahedral complexes of these cations are inert when they are strong field. Cr(IIl) is a d ion and Co(III) is a d ion. Therefore it appears that when the three lower-energy d orbitals of a strong-field complex are half-filled or filled, a special stability is imparted to the complex. The electron arrangements are shown in Table 22.2. 

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). 

A process that has been studied widely in relation to phenomena such as chiral symmetry breaking, spontaneous resolution and chiral amplification is the reaction (Fig. 18) of [Co(H20)2 (OH)2Co(en)2 2](S04)2 (denoted 1) with NH4Br to give the chiral complex cA-[CoBr(NH3)(en)2]Br2 (denoted 2). This reaction is historically important, as 2 was one of the first octahedral metal complexes to be resolved into A and A stereoisomers, some years after Werner predicted that octahedral ions M(en)2XY should exist as enantiomeric pairs. 

Metal complexes have characteristic shapes, depending on the metal ion s coordination number. Two-coordinate complexes, such as [Ag(NH3)2]+, are linear. Four-coordinate complexes are either tetrahedral or square planar for example, [Zn(NH3)4]2+ is tetrahedral, and [Ni(CN)4]2 is square planar. Nearly all six-coordinate complexes are octahedral. The more common coordination geometries are illustrated in Figure 20.12. Coordination geometries were first deduced by the Swiss chemist Alfred Werner, who was awarded the 1913 Nobel Prize in chemistry for his pioneering studies. 

Werner was able to show, in spite of considerable opposition, that transition metal complexes consist of a central ion surrounded by ligands in a square-planar, tetrahedral, or octahedral arrangement. This an especially impressive accomplishment at a time long before X-ray diffraction and other methods had become available to observe structures directly. His basic method was to make inferences of the structures from a careful examination of the chemistry of these complexes and particularly the existence of structural isomers. For example, the existence of two different compounds AX4 having the same composition shows that its structure must be square-planar rather than tetrahedral. 

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