Werner-type coordination complexes

Although the properties which can be computed are limited, LFT has provided for over half a century a reasonably useful, semi-quantitative picture of metal-ligand bonding in Werner-type coordination complexes (3,25-27). In the present context, the advantage of LFT is its computational efficiency. Therefore, we added LFT to MM to give the ligand field molecular mechanics (LFMM) method (28). 

The three-center metal-H2 interaction complements classical Werner-type coordination complexes where a ligand donates electron density through its nonbonding electron pair(s) and jt-complexes such as olefin complexes in which electrons are donated from bonding jt-electrons (Scheme 5.5). 

Many pigments and dyes of historical interest exhibit the capability of forming Werner-type coordination complexes. The relationships between the structures and color characteristics of these colorants are reviewed in this paper, with particular attention to the work of Paul Pfeiffer, a student and long-time assistant of Alfred Werner s who did extensive work in the characterization and application of Werner s coordination theory to alizarin-related dyes. 

Linear Werner-type coordination polymers are defined as linear polymers, the main chains of which contain coordinate covalent bonds of transition metals. In this article, we restrict the discussion to the linear coordination polymers which can exist as a stable form in solution and thus our purpose is somewhat complementary, for example, to review dimensional inorganic complexes such as those of Miller and Epsteinld). 

The class of complexes considered in the present review, represents species which can be considered to lie at the borderline between classical Werner-type coordination compounds and organometallic molecules. In the former, the distinction of four types of excitation in the electronic spectra has been very useful [1] ... 

Coordinated structures similar to Werner s coordinated complexes were also found in crystals in the very early days of X-ray crystallography. W. H. and W. L. Bragg reported in 1913 that in the rock salt crystd each Na is surrounded octahedrally by 6 Cl (also each Cl by 6 Na) (13) and that in the mineral sphalerite each Zn is surrounded tetrahedrally by 4 S (also each S by 4 Zn) (14). Cubic coordination of 8 Cl around Cs and 8 Cs around Cl was soon discovered. Also, X-ray diffraction studies have revealed other types of coordination complexes, such as 12 A1 icosahedrally coordinated about Mo in the compound M0AI12 (15). 

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... 

Although there is a tendency to associate coordinated water with Werner-type complexes, where it is extensively established, organometallic aqua ions are known.945 The simple [(Cp )Co(OH2)3]2+ has been established, and is prepared via Equation (8). The lower pATa is 5.9, similar to values in aminecobalt(III) compounds, and reversible deprotonation and dimerization has been identified as part of the reactions of the aqua ion.946... 

Werner-type complexes with these coordination numbers have been characterized. However a large majority of the complexes showing these coordination numbers are organometallic in nature and generally outside the scope of this book. Examples are shown in Structures 9-11, and discussion of the associated rearrangements will be necessarily brief. 

Photocalorimetry offers a convenient alternative to other methods of AH determination and, in some instances, may be the only practical method. The ligand substitution reactions of robust Werner-type complexes are a case in point. Conventional thermochemical measurements are complicated by the slowness of the substitution process and/or by competing reactions. Some of these same complexes, however, undergo clean photosubstitutions with high quantum yields and thus are excellent candidates for photocalorimetry. Examples include [Cr(NH3)6]3+, [Cr(CN)6]3-and [Co(CN)6]3-.192 Photocalorimetric measurements of AH have also been obtained for isomerization and redox reactions of coordination compounds.193194... 

Octahedral Six Coordinate In addition to the tris-bidentate complexes and the unidentate complexes discussed above, there are many other examples of chiral metal complexes that are based on octahedral geometry. For example, there are a number of reports of edge-sharing binuclear structures containing four bidentate chelates. The individual metal centers in these binuclear species can exist as either A or A enantiomers, and, therefore, the overall structure can be chiral AA and AA, or the achiral (meso) AA pair as illustrated in Figure 5.12a and b. Studies of complexes of this type were important in the proof of Werner s coordination theory. In these pioneering studies, Werner was able to prove by experiment the existence of the three isomers (AA, AA, and AA) of [Co2(p-NH2)( l-NC>2)(en)2]4 +. 23 For Cr(III) complexes, Werner showed that the / >.vo-AA- Cr2(p-OH)2(en)414 1 may be prepared by... 

Tetrahedral Four-Coordinate T-4 In contrast to the chemistry of carbon compounds, and some main group compounds such as the structure shown in Figure 5.4, Werner-type T-4 coordination complexes are too labile to isolate in solution as individual enantiomers. The presence of chiral ligands can influence the racemic (diastereomeric) equilibrium and make the preparation and isolation of... 

This method, also named immediate (direct) interaction of ligands and sources of metal center [2], is the oldest and most widespread, in most cases the safest, preparative method. It is comparatively accessible with, in general, high yields of final products - metal complexes. Precisely as a result of its use, the majority of Werner coordination compounds with the simplest ligands water (aqua complexes), ammonia (amino complexes), halides, (5-di ketones, etc. - have been obtained. At present, the above method allows us to obtain practically all types of complexes (see Sec. 1.2) with all types of ligands (see Chap. 2). 

Pseudorotaxanes represent a particular class of complexes—a term first used to describe Werner-type [75] metal complexes, also called coordination compounds. Complexes formed by organic molecules began to attract the attention of chemists around 1950, Mulliken in particular, in a landmark series of papers [76], formulated a theoretical treatment to explain the bonding characteristics and spectral properties of organic complexes based on charge-transfer interactions. During the past 20... 

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