Werner-type complex

The species discussed so far belong to the class we might label Werner-type complexes. We use this description to differentiate from carbonyl-type or other low oxidation state complexes. We stay with Werner-type complexes exclusively until Chapter 6. The radial waveforms for 3d, A-s and Ap orbitals of the metals in such... 

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. 

We can understand this powerful generalization directly from our view of the valence shell in Werner-type complexes as laid out in Chapter 2. Recall that as an extreme limit for Werner-type species, we consider the metal contribution to the valence shell for the first-row elements as 45 and 4p, with 3d orbitals excluded. So,... 

Furthermore, as discussed in Section 6.7, the ability of the elastic d orbitals to function as electron sinks contributes greatly to the rich variety of redox chemistry that is so characteristic of the cf-block elements. Here too, therefore, we recognize the bonding role of the d orbitals in Werner-type complexes as well as in carbonyl-type chemistry. 

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. 

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... 

Since we shall not obtain the comparable amount of detailed information on the mechanisms of substitution in octahedral complexes from the studies of more complicated substitutions involving chelation and macrocycle complex formation (Secs. 4.4 and 4.5) it is worthwhile summarizing the salient features of substitution in Werner-type complexes. 

The preparation, and even more the resolution, of an asymmetric tetrahedral center in Werner-type complexes has been thwarted by the configurational instability of tetrahedral complexes. However the use of ligands of the strongly a, 7t bonding type imposes stability and the forma-... 

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. 

Kinetic information on the lower oxidation states (Prob. 24) is sparse for Werner-type complexes. Co and Co(II)-bpy complexes are reduced by e to give Co(I) complexes except that the mono species yields Co"(bpy ) (H20)J. The rate constants are in the range... 

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... 

When naphtha or naphthenic gasoline fractions are catalytically reformed, they usually yield a Cx aromatics stream that is comprised of mixed xylenes and ethylbenzene. It is possible to separate the ethylbenzene and o-xylene by fractionation. It is uneconomic to separate the m- and p-xylenes in this manner because of the closeness of their boiling points. To accomplish the separation, a Werner-type complex for selective absoiption of p-xylene from the feed mixture may be used. Or, because of the widely different freezing points of the two xylene isomers, a process of fractional crystallization may be used. To boost the p-xylene yield, die filtrate from the crystallization step can be catalytically isomerized. 

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. 

In contrast to the Werner-type complexes, such as [Cr(OH2)6]3+ or [Cr(OH2)5X]2+ (X = Hal or similar anion) where broad EPR signals with g 2 are observed in the solids at 295K or in frozen aqueous solutions at 77 K, no EPR signals for [Cr(OH2)5R]2+ could be detected at r>20K. This difference, together with the analysis of low-temperature EPR spectra, points to the predominantly covalent nature of the Cr—C bonds in the [Cr(OH2)5R]2+ complexes.293... 

In the second part of this review, we have focused on systems which can be treated equally well by AI theory as Werner type complexes, but for which classical LFT breaks down or at least needs to be extended. These are systems with orbitally degenerate ground states such as tetrahedral CuCl42 (d9,2T2) and NICI42 (dx, 3Tj). 

Most studies of the mechanism of substitution in octahedral metal complexes have been concerned with Werner-type complexes organometallic complexes have entered the research field more recently. Among the former, the popular candidates for study have been Cr(III) d ) and low-spin Co(III) (fi ) species. These complexes are kineti-cally inert and their rates of reaction are relatively slow and readily followed by conventional techniques. Both Rh(III) and Ir(III) (both low-spin d ) also undergo very slow substitution reactions. There is no universal mechanism... 

I ( Aig I Lz I Tig) 1 are collectively known as the covalency or the nephe-lauxetic factor. The classical Freeman-Murray-Richards (FMR) approach emphasizes the dependence of shielding on excitation energies, whereas the independent investigations by Juranic and Bramley demonstrated the importance of the nephelauxetic effect in Co shielding variation. On the one hand, Taura convincingly showed that the solvent dependence of the Co chemical shift for K3Co(CN)6 arises mainly from the solvent shift of the Tig transition. " The Co chemical shifts of a wealth of Werner-type complexes were successfully rationalized in terms of empirical... 

Most studies of the mechanism of substitution in octahedral metal complexes have been concerned with Werner-type complexes organometallic complexes have entered the... 

As noted earlier, there is an experimental distinction between the substitution reactions of labile and inert complexes. The formation of labile complexes is virtually instantaneous upon mixing of the reactants, so that there are few practical difficulties in their preparation, but three points must be remembered. First, for classical, Werner-type, complexes it is found in practice that it is difficult to prepare such complexes with several different non-ionic ligands bonded to the same metal atom, although it is much easier to prepare complexes in which an anionic species is coordinated together with a neutral ligand. Secondly, although it may be possible to isolate and characterize a solid complex, quite a different complex may be the predominant species in solution. So, the blue complex Cs2[CoCl4] crystallizes... 

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