Werner ligand

The carbonate ion, CO3-, by contrast, is a classic Werner ligand whieh forms innumerable complexes as a monohapto, dihapto or bridging donor. Examples of this latter mode... 

According to Werner, ligands are spatially distributed around the metallic ion. If, for example, we are considering the complexes of coordinence 6, they are distributed at the summits of a regular octahedra whose center is occupied by the coordinator metal. This is the case with the hexamminecobalt(III) cation [Co(NH3)6] + (Werner s luteocobaltic complex) (Fig. 23.2). In general, a coordinence number is associated with one or several particular geometries of the complex. For example, the complexes of Cu(I), Ag(I), Au(I), and Hg(II) are... 

Negatively coordinated groups are given before neutral coordinated groups in the examples of Werner scheme names above. Ewens and Bassett presented good reasons why that order should be reversed. The lUPAC rules (20) recommend that ligands be cited in alphabetical order regardless of their... 

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). 

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. 

Bray, M. R., Deeth, R. J., Paget, V. J., Sheen, P. D., 1996, The Relative Performance of the Local Density Approximation and Gradient Corrected Density Functional Theory for Computing Metal-Ligand Distances in Werner-Type and Organometallic Complexes , Int. J. Quant. Chem., 61, 85. 

A striking feature of the formulas (4.68a)-(4.68c) is that the coordination number (CN) of ligands bonded to Co is six (CN = 6) in each case. Werner conjectured that such six-fold coordination corresponds to idealized octahedral geometry about the central Co ion, which leads to unique structures for [Co(NH3)6]3+ and [Co(NH3)5C1]2+ but distinct cis and trans isomers for [Co(NH3)4Cl2]+, as observed. X-ray studies subsequently confirmed the accuracy of Werner s brilliant structural inferences. 

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,... 

Fischer, E. O., Werner, H. Metall jt- complexes. Vol. 1. Complexes with di- and oligo-olefinic ligands. Amsterdam Elsevier 1966 ... 

When a metal atom donates electron density to a bound ligand, usually by means of Ji-back bonding, electrophilic substitution reactions may be promoted. This is observed then usually with metals in low oxidation states and is therefore prevalent with organometallic complexes - and less with those of the Werner-type, where the metals are usually in higher oxidation states. Nevertheless there have been detailed studies of electrophilic substitution in metal complexes of P-diketones, 8-hydroxyquinolines and porphyrins. Usually the detailed course of the reaction is unaffected. It is often slower in the metal complexes than in the free ligand but more rapid than in the protonated form. 

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-... 

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 green and violet tetraammines have the same chemical composition, that is, they are isomers and are the only two isomers with this composition. Werner realized that this was possible only if the six ligands were deployed about the cobalt(III) center in an octahedral arrangement (cf. octahedral coordination in solids, Sections 4.3 and 4.4) for example, a flat hexagonal complex Co(NH3)4Cl2+ would have three isomers, like ortho-, meta-, and para-disubstituted benzenes. Werner correctly identified the green compound as the trans isomer (chloro ligands on opposite sides of the octahedron) and the violet as cis (same side), as in Fig. 13.1. 

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