Ligands spectrochemical series

Spin-pairing in manganese(II) requires a good deal of energy and is achieved only by ligands such as CN and CNR which are high in the spectrochemical series. The low-spin complexes. [MnfCN) ] and [Mn(CNR)6] + are presumed... 

Further similarity with Mn may be seen in the fact that the vast majority of the compounds of Fe are high-spin. Only ligands such as bipy and phen (already mentioned) and CN , which are high in the spectrochemical series, can induce spin-pairing. The low-spin [Fe(CN)g] , which is best known as its red, crystalline potassium salt, is usually prepared by oxidation of [Fe(CN)6]" with, for instance, CI2. It should be noted that in [Fe(CN)6] the CN ligands are sufficiently labile to render it poisonous, in apparent contrast to [Fe(CN)6]" , which is kinetically more inert. Dilute acids produce [Fe(CN)5(H20)] , and other pentacyano complexes are known. 

Use the spectrochemical series to predict the effect of a ligand on the color, electron configuration, and magnetic properties of a d-metal complex (Examples 16.4 and 16.5). 

J) -CHj, 318 (74) NH3, 345 (II) HjO, 380 ( 75). This shows that both organo-ligands exert a very strong ligand field comparable with that of CN and establishes their position in the spectrochemical series. It is a pity that the second d-d transitions ( T2g-(- A,g), which would establish the position of these ligands in the nephelauxetic series, have not yet been reported. Two bands have, however, been reported for the binuclear complex [(NC)5Co(CFjCFj)Co(CN)5] - at 320 and 276 nm (I2I). 

Studies of many coordination complexes reveal a common pattern in the energetic effects of the various ligands. This pattern is described by the spectrochemical series, in which ligands are listed in order of increasing energy level splitting ... 

The spectrochemical series was established from experimental measurements. The ranking of ligands cannot be fully rationalized using crystal field theory, and more advanced bonding theories are beyond the scope of general chemistry. 

Ligand Cl" is lower in the spectrochemical series than H2 O. Thus, [FeClg P has low splitting... 

The colors of Cr coordination complexes depend on the positions of the ligands in the spectrochemical series. 

C20-0093. As ligands, chloride and cyanide are at opposite ends of the spectrochemical series. Nevertheless, experiments show that [CrClg] and [Cr(CN)g] have about the same amount of magnetism. Explain how this can be so. 

Now, consider leaving group orders in general. The order of inertness at Pt(II) (which corresponds to the theoretically comprehensible trans effect series) is largely opposite to the order of ligands in the well-known spectrochemical series. ... 

If there are different kinds of ligands, those which have the smaller influence according to the spectrochemical series prefer the positions with the stretched bonds. For example, in the [CuC14(OH2)2]2 ion two of the Cl atoms take the positions in the vertices of the elongated axis of the coordination polyhedron. 

Notice that, with the current data, one cannot draw immediate parallels with the spectrochemical series (nor with the nephelauxetic series). At any rate, these comparisons are not trivial depending on the symmetry, CF parameters of orders 4 and 6 are more or less important compared with those of order 2. Thus, the ratio charge/distance cannot generally quantify the strength of the LF exerted by a kind of ligand, or at least not in a way that is totally independent of the geometry of the complex. 

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