Werner complexes, transition metal

While the structural manifestations of coordination bonds find deep roots as discussed in the the work of Werner involving transition—metal—ion complexes, coordination bonds have found found widespread application in supramolecular chemistry.Coordination bonds are of... 

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). 

Figure 2-2. Schematic representation of the radial waveforms for 3d, 45 and 4p orbitals in first row transition-metal ions of intermediate oxidation state (Werner-type complexes).

Two other, closely related, consequences flow from our central proposition. If the d orbitals are little mixed into the bonding orbitals, then, by the same token, the bond orbitals are little mixed into the d. The d electrons are to be seen as being housed in an essentially discrete - we say uncoupled - subset of d orbitals. We shall see in Chapter 4 how this correlates directly with the weakness of the spectral d-d bands. It also follows that, regardless of coordination number or geometry, the separation of the d electrons implies that the configuration is a significant property of Werner-type complexes. Contrast this emphasis on the d" configuration in transition-metal chemistry to the usual position adopted in, say, carbon chemistry where sp, sp and sp hybrids form more useful bases. Put another way, while the 2s... 

Experimentally, spin-allowed d-d bands (we use the quotation marks again) are observed with intensities perhaps 100 times larger than spin-forbidden ones but still a few orders of magnitude (say, two) less intense than fully allowed transitions. This weakness of the d-d bands, alluded to in Chapter 2, is a most important pointer to the character of the d orbitals in transition-metal complexes. It directly implies that the admixture between d and p metal functions is small. Now a ligand function can be expressed as a sum of metal-centred orbitals also (see Box 4-1). The weakness of the d-d bands also implies that that portion of any ligand function which looks like a p orbital when expanded onto the metal is small also. Overall, therefore, the great extent to which d-d bands do satisfy Laporte s rule entirely supports our proposition in Chapter 2 that the d orbitals in Werner-type complexes are relatively well isolated (or decoupled or unmixed) from the valence shell of s and/or p functions. 

A central theme in our approach, which we believe to be different from those of others, is to focus on the changing chemistry associated with higher, middle and lower oxidation state compounds. The chemical stability of radical species and open-shell Werner-type complexes, on the one hand, and the governance of the 18-electron rule, on the other, are presented as consequences of the changing nature of the valence shell in transition-metal species of different oxidation state. 

Coordination compounds have been produced by a variety of techniques for at least two centuries. Zeise s salt, K[Pt(C2H4)Cl3], dates from the early 1800s, and Werner s classic syntheses of cobalt complexes were described over a century ago. Synthetic techniques used to prepare coordination compounds range from simply mixing the reactants to employing nonaqueous solvent chemistry. In this section, a brief overview of some types of general synthetic procedures will be presented. In Chapter 21, a survey of the organometallic chemistry of transition metals will be presented, and additional preparative methods for complexes of that type will be described there. 

Already in the early twentieth century it was realized that definitions such as (D1) do not adequately cover all units of interest in chemistry. Thus, by 1902 Werner had demonstrated (Section 4.5.1) that numerous covalently saturated ligand (L) species (L = CO, NH3, H20, etc.) could exist both as free molecular species and in coordinated form as components of transition-metal complexes ML with open-shell metals M,... 

Seventeen years is a long time between editions of a book. In order to add some of the vast amount of new material which has been published in that time, I have needed to abridge the older edition and in so doing apologise to oldtimers (myself included ) whose work may have been removed or modified. Nevertheless, the approach used is unchanged. In the first three chapters I have dealt with the acquisition of experimental data and discussed use for building up the rate law and in the deduction of mechanism. In the second part of the book, the mechanistic behavior of transition metal complexes of the Werner type is detailed, using extensively the principles and concepts developed in the first part. 

In the present paper we demonstrated the feasibility of a semiempirical description of electronic structure and properties of the Werner TMCs on a series of examples. The main feature of the proposed approach was the careful following to the structural aspects of the theory in order to preclude the loss of its elements responsible for description of qualitative physical behavior of the objects under study, in our case of TMCs. If it is done the subsequent parameterization becomes sensible and successful solutions of two long lasting problems semi-empirical parameterization of transition metals complexes and of extending the MM description to these objects can be suggested. 

It is well known that transition-metal salts and metal complexes, unlike non-Werner-type ferrocene compounds, act as inhibitors in the polymerization of vinyl monomers. For example, the radical polymerization of vinylpyridine is strongly inhibited in the presence of Cu(II) or Fe(III)32 However, vinylpyridine with Cu(I)... 

The complexing capacity of hydroxamic acids was predicted by Werner in 1908, who also indicated the metals most likely to form stable complexes.288 Since then, the formation of poorly soluble and intensely coloured hydroxamates has been used for analytical determinations for a number of metal ions, such as Fe3+, Mos+, Vs+ etc. A recent general review of transition metal complexes of hydroxamic adds included the few known examples of silver(I) complexes.289... 

Keywords TDDFT Excitation energies Excited states Transition metal complexes Electronic spectra Metallotetrapyrroles Metallocarbonyls a-diimine complexes Porphyrins Porphyrazines Phthalocyanines Werner complexes Sandwich complexes... 

Several types of Werner complexes have been investigated over the last few years by TDDFT methods. They include metal oxide, metal halide, metal oxyhalide compounds, and transition metal complexes with bidentate ligands such as ethylenediamine and acetylacetonato. 

To end this section, it is worthwhile mentioning the recent TDDFT study by Autschbach et al [107] on the electronic and circular dichroism (CD) spectra of several chiral Werner complexes, since it represents the first application of TDDFT to the computation of the circular dichroism (CD) spectra of transition metal complexes. The absorption and CD spectra of the... 

In this review article, we discuss the fundamental basis of the bimolecular electron-transfer reactions of electronically excited transition metal complexes and then collect and examine the data so far obtained in this field. Although a wide range of systems are discussed, we focus primarily on quantitative studies, the majority of which involves Werner-type complexes in fluid solution. 

Transition metal complexes are cationic, neutral or anionic species in which a transition metal is coordinated by ligands. A classical or Werner complex is one formed by 2 metal in a positive oxidation state with donor ligands such as H20, NH3, or halide ions. 

Tphe bright colors of the coordination complexes of transition metal elements, including the platinum group metals, were of great assistance to pioneer workers with these materials. Thus, chemical changes could be followed visually it was frequently very easy from their colors to demonstrate the existence of isomers upon which Alfred Werner was able to base his monumental theory of coordination. Such early studies were limited to a simple qualitative visual evaluation of the color. 

R. W. Lass, P. Steinert, J. Wolf, and H. Werner, Synthesis and Molecular Structure of the First Neutral Transition-Metal Complex Containing a Linear... 

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. 

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-... 

There are, however, several aspects of contemporary transition metal chemistry whose existence could not have been extrapolated from the Wernerian principles. Among these one could mention considerable areas of metal carbonyl type chemistry, much of the current field of organometallic chemistry and, most unambiguously, the chemistry of compounds containing metal-metal bonds. Although Werner dealt extensively with polynuclear complexes, these were conceived simply as two or more mononuclear complexes united only by the ligands they shared. 

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