Ligands, 877, strong-field

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). 

Nevertheless, despite the discouraging predictions of the early calculations, some attempts were made to apply the ligand field model to sandwich complexes. The first approach on these lines, due to Matsen (28), assumed a strong ligand field of D h symmetry, which was shown to split the d-orbital manifold into three levels characterised by the value of m. Unfortunately, the calculated energetic order for the metallocenes was found to be... 

No Ligand Field Weak Ligand Field Strong Ligand Field... 

A = 1 in the strong ligand field limit, 1.5 in the weak field case. For T2 ) terms, L remains at unity. 

In the case of an octahedral cfi complex, the ground state is lAlg (strong ligand field case), and the first absorption band usually corresponds to the transition to the 1T g state. There are, in addition, triplet and quintet states which may be low-lying, especially in less symmetric complexes. 

Trimethyldipyrromethane (dpm) yields the low-spin six-co-ordinate tris-complex with iron(m). Strong ligand-field bands in this complex are consistent with the presence of a trigonal distortion and highly covalent metal ligand bonds.217 The complex [Fe(pbz), ]Cl, [pbz = 2-(2 -pyridyl)benzimidazole] has also been reported.214... 

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