Ligand field model complexes

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... 

In conclusion though, it may reasonably be maintained that the ligand field model has contributed significantly to the advances in knowledge concerning transition metal sandwich complexes over the last ten years, and that it offers a valid and convenient framework for further experimental and theoretical progress. 

A ligand field model has been used for the interpretation of the magnetic behavior of the Mo(R2Dtc)4 complexes (485). Extended Hiickel calculations compare the electron-donating characteristics of the dithiocarbamate ligand to other 1,1-dithio chelates. This calculation concluded that (1) there is more metal character in the lowest unoccupied MO in the dithiocarbamate complexes, and (2) there is more ligand character in the lowest unoccupied MO in the dithioacid complexes (485). This seems to be in agreement with the electronic spectra. 

The first coordination sphere of acido-pentamine [Cr(NH3)5X]2+ complexes has C4v symmetry and this leads to a tetragonal resolution of the 2Eg state into 2At and 1Bl components. The classical ligand field model predicts that these components will be virtually degenerate. This is based on a combination of pseudo-spherical and quasi-spin selection rules of the shell and will be discussed later on in Sect. 5.2. At present we welcome this example of a pseudo-degeneracy as another opportunity to observe fine details of the interelectronic repulsion interaction which have the proper anisotropy to induce a splitting of the 2Eg term. 

The electronic spectrum is yet another property which illustrates the similarities between the metallocenes and (7r-ollyl) metal compounds. In Table VI are listed some data for a series of Coin(absorption bands with the small extinction coefficients are probably two of the spin-allowed d-d transitions. Scott (34) has developed an approximate axial ligand field model for the carborane-transition metal complexes and has discussed the optical spectra in relation to this bonding theory. The actual assessment of bonding in the (7r-ollyl) metal compound as well as the metallocenes would be greatly aided by accurate assignments of the electronic spectra. 

The Angular Overlap Model is a ligand field model which uses parameters20 (A = a, n, 6...) for expressing the orbital energies. For d orbitals X can be only a, n and <5, but it is customary to use as parameters e = e a - ej and e = e - ej. In octahedral complexes a simple correlation exists between these parameters and the usual crystal field parameters21 ... 

Fe(II) salts cannot readily be explained probably spin-forbidden transitions at = 730 nm are also responsible in this case. The field strength parameters can be calculated using the ligand field model if the influence of the sulfate ions on the ligand field is neglected these are a measure of the stability of the aqua complexes in this series. The values of A = 10 Dq are as follows (in the sequence of the test tubes 1-7) 12,350 cm", 13,900 cm, 7,800 cm , 10,400 cm , 9,300 cm and 12,600 cm . ... 

The ligand-field model is a semi-empirical scheme of calculation that deals Avith the interpretation of the properties of complexes of metal ions with a partly filled /-shell. In this model the experiments are described in terms of empirically determined parameters whose coefficients are exactly calculated. 

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