Metal f orbitals

Cobalt(lll).—Complexes. Ammine complexes. Optical activity can be induced in the complexes [Co(NH3) ] and [Cofenlj] by means of outer-sphere association with chiral anions, e.g. (- - )-tartrate. Circular dichroism is observed in the d-d bands of the cations and it is suggested that this is due to (a) direct interaction between the chiral anion and the metal f/-orbitals and (b) the preferred conformation adopted by the inner-sphere ligands in the presence of a helical outer-sphere ligand. 

Nevertheless, the existence of biscyclooctatetraenyl metal complexes, bonded via the metal f-orbitals and the aromatic 7r-systems of the rings, was first predicted on the basis of Wolfsberg-Helmholz MO calculations by Fischer (7) in 1963, who treated the then hypothetical U(Cot)2 molecule, and demonstrated experimentally by Streit-wieser and Miiller-Westerhoff (8) in 1968. These authors succeeded in synthesising the uranium derivative, U(Cot)2, and gave to it the trivial name, uranocene, in order to underline its resemblance to the d-series metallocenes both structurally and, as they proposed, in its mode of metal-ring bonding. 

For the calculation of the bonding or anti-bonding energies of the metal f-orbitals it is now however necessary to consider how the one-electron H terms may be estimated, both for the metal and for the ligand symmetry orbitals. In the latter case the Hl terms for the levels of a(o), e1(7r), e2(5), and e3(0) symmetry have been estimated using a simple Hiickel technique by Fischer (7), and the values - 15, - 13, - 9, and... 

For both the complexes considered the metal f-orbitals lie, on a one-electron picture, below the ligand orbitals of e3(0) symmetry and above those belonging to the a(a), ex(rr), and e2(5) representations. Consequently, as indicated in Section 1, the systems may be pictured as accommodating the 20 electrons of the two cyclo-octatetraenyl dianion rings in the a2u, alg, elu, e2u, and e2g, mainly ligand levels, with the one or two f-electrons of Cem or UIV respectively located in the mainly metal f, e3u level. Moreover, the a2u, eiu, and e2u mainly metal f-levels will all be anti-bonding in character, whilst the e3u f-orbital will be bonding. 

Bonding in cyclopentadienyl complexes of block metals (see Chapter 23) can be described in a similar manner but must allow for the participation of metal [Pg.509]

On the other hand, for certain ligands such as CO and the CN ion it may be considered that the donor atom n-orbitals which overlap with the metal donor atom electrons, and identification of would be with the t2gW separation, the... 

The electron transfer ) character of the visible absorption spectra of iron complexes of conjugating imine ligands was first recognized by WiUiams (77). From the opposite effect of methyl substituents on max of ferrous (or cuprous), and of ferric tris-phen complexes, it was concluded that the excitation involves a partial electron transfer from a filled metal electron transfer (ET) transitions in spin-paired iron(II) tris-diimine complexes, was further elaborated by Jorgensen (78). 

By analogy, a ring-metal interaction is symmetry permitted between the highest occupied MOs of two [8]annulene dianions and two lanthanide or actinide metal f-orbitals (1 = 2). An empirical model depicting this interaction is shown in Figure 2. In this example, the central metal ion shown, uranium(IV), has two electrons remaining in metal f-orbitals thus, the HOMO of uranocene is located on the metal. 

Early MO calculations (LCAO) by Hayes and Edelstein on bis([8]annulene)actinide(IV) complexes did indeed indicate that there was significant interaction between the ligand 62 orbitals and the metal 5f 2 orbitals. In these calculations, only the ligand tt orbitals and the metal f orbitals were used, since it was considered that the metal d-orbitals of the actinides were too high in energy to have significant interaction with the ligand MOs. 

A more complete bonding model from the Xa scattered-wave (SW) treatment of uranocene and thorocene was proposed by RDsch and Streitwieser.Agreement with the PES was remarkably good, considering that relativistic effects had not been included in the calculations. These calculations supported the assignment of ionization potentials by Clark and Green and verified that the contribution of the metal d orbitals to the ring metal bond in these complexes is at least as important as the contribution of the metal f orbitals. 

A linear free-energy relationship between the composites = kikjk-i and the metal electrode potentials had a slope of 0.51 0.06, close to the theoretical Marcus value. Values for both iron(m) and ruthenium(m) fall on the same line, indicating that the outer-sphere reaction is little affected by the size of the metal f/-orbitals. It is suggested that the electron transfer takes place via the periphery of the polypyridyl ring. An isotope effect, kiiD oyk XHiO) = 2.7, can be accounted for by the (ki/A i) term indicative of electron transfer before loss of proton from the radical cation (8). 

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