F orbital covalency

Kerridge A. f-orbital covalency in the actinocenes (An=Th-Cm) multiconfigurational studies and topological analysis. RSC Adv [Internet]. 2014 [cited 2014 Feb 26] 4(24) 12078-86. Available from http //pubs.rsc.org/en/Content/ArticleLanding/2014/RA/c3ra47088a. 

R5sch N, Haberlen OD, Dunlap BI. Bonding in Endohedral Metal-FuUerene Complexes f-Orbital Covalency in Ce C2g. Angew Chem, hit Ed Engl. 1993 32(1) 108-110. 

Quadrupole splittings are often interpreted from ligand field models with simple rules for the contributions from each occupied f-orbital (see discussion above). However, these models fail even qualitatively in the case of more covalent metal-ligand bonds. An example concerns the quadrupole spUttings of Fe(IV)-oxo sites in their 5 = 1 or 5 = 2 spin states. Here, ligand field considerations do not even provide the correct sign of the quadrupole splitting [60]. 

Relativistic effects are more pronounced for the actinides because of their higher nuclear charge. As a result, the s and p orbitals screen the charge of the nucleus better and the d and f orbitals expand, and are destabilized 2,3). The shielding of the 5/ orbitals by filled outer s and p orbitals is thus not as effective, and actinide ions form more covalent bonds and are found in higher oxidation states, at least at the beginning of the 5/ series. 

A pioneering work for their qualitative understanding was the covalent treatment of the 5f-6d hybridization due to FriedeP Like p orbitals, f orbital are antisymmetric (il)(— R) = — tl)(R)) thus by linear combination with symmetric d orbitals, they give rise to highly directive hybrid orbitals (like s-p hybrids). Thus in Pa, U, Np and Pu metals each atom has four nearest neighbours more or less coplanar and at 90°. Let s take the example of a-U and put the nearest neighbours along the x and z axes. Necessary hybrid orbitals for such positions are ... 

Strong metal-ligand orbital interaction Strang preference in bond direction Bond strengths determined by orbital interaction, normally decreasing in following order CN-, NHj. R,0. OH-, F" Often covalent covalent complexes may exchange slowly... 

The obvious deduction from these observations is that the orbital energy splitting is not primarily of a simple electrostatic nature, but reflects rather the much shorter range effects to be expected of covalence in chemical bonding to the immediate donor atoms. The conclusion is reinforced by the fact that when the known interionic separations are used together with free ion 3d-orbital wave functions to evaluate Dq for first transition series ions in an MF2 lattice, values too small by an order of magnitude are obtained.6 20-22... 

The coordination catalysts usually applied for the polymerisation and copolymerisation of heterocyclic and heterounsaturated monomers involve a wide range of metal derivatives that are characterised by moderate nucleophi-licity and relatively high Lewis acidity. Metal derivatives possessing free p, d or f orbitals of favourable energy are used as catalysts for epoxide polymerisation. In particular, compounds of group 2 and 3 metals, such as zinc, cadmium and aluminum, and transition metals, such as iron, as well as lanthanum and yttrium, are representative coordination catalysts. The appropriate Lewis acidity of the metal and the appropriate nucleophilicity of the metal substituent in these catalysts make the monomer coordination favourable prior to the nucleophilic attack. The nucleophilic attack of the covalently bound metal substituent on the monomer molecule coordinated at the metal atom at the catalyst active... 

In general, the FEUDAL model appears to answer many of the fundamental questions regarding the bonding within f-electron systems however, certain discrepancies exist within the physical data of the actinide systems and the theoretical understanding of the radial distribution functions of the actinides. Evidence that the f orbitals are accessible for covalent bonding continues to be found. 

For the lanthanide complexes, even if the amount of covalent interaction is very small, we might have a chance to get the evidence of f-orbital participation in bonding (Figure 4). 

The electrons in the outer shell, or valence shell, of an atom are the electrons involved in bonding. In most of our discussion of covalent bonding, we will focus attention on these electrons. Valence shell electrons are those that were not present in the preceding noble gas, ignoring filled sets of d and f orbitals. Lewis formulas show the number of valence shell electrons in a polyatomic molecule or ion (Sections 7-4 through 7-7). We will write Lewis formulas for each molecule or polyatomic ion we discuss. The theories introduced in this chapter apply equally well to polyatomic molecules and to ions. 

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