D Orbital ligands

Silicon, germanium, tin and lead can make use of unfilled d orbitals to expand their covalency beyond four and each of these elements is able (but only with a few ligands) to increase its covalency to six. Hence silicon in oxidation state -f-4 forms the octahedral hexafluorosilicate complex ion [SiFg] (but not [SiCl] ). Tin and lead in oxidation state -1-4 form the hexahydroxo complex ions, hexahydroxostannate(IV). [Sn(OH) ] and hexahydroxoplum-bate(IV) respectively when excess alkali is added to an aqueous solution containing hydrated tin(IV) and lead(IV) ions. 

The d orbital splitting depends on the oxidation state of a given ion hence twb complex ions with the same shape, ligands and coordination number can differ in colour, for example... 

The splitting of the d orbital energy levels when ligands are bonded to a central transition atom or ion has already been mentioned (p. 60). Consider the two ions [Co(NH3)g] and [Co(NH3)g] we have just discussed. The splitting of the d orbital energy levels for these two ions is shown in Figure 13.2. 

For Iran sition metals th c splittin g of th c d orbitals in a ligand field is most readily done using HHT. In all other sem i-ctn pirical meth -ods, the orbital energies depend on the electron occupation. HyperCh em s m oiccii lar orbital calcii latiori s give orbital cri ergy spacings that differ from simple crystal field theory prediction s. The total molecular wavcfunction is an antisymmetrized product of the occupied molecular orbitals. The virtual set of orbitals arc the residue of SCT calculations, in that they are deemed least suitable to describe the molecular wavefunction, ... 

Table 7.9 Symmetry species of orbitals resulting from the splitting of d orbitals by various ligand arrangements...

Class-b acceptors on the other hand are less electropositive, have relatively full d orbitals, and form their most stable complexes with ligands which, in addition to possessing lone-pairs of electrons, have empty n orbitals available to accommodate some charge from the d orbitals of the metal. The order of stability will now be the reverse of that for class-a acceptors, the increasing accessibility of empty d orbitals in the heavier halide ions for instance, favouring an increase in stability of the complexes in the sequence... 

The difference between the two extremes is essentially that, in the former, the Re retains its valence electrons in its d orbitals whereas in the latter it loses 6 of them to delocalized ligand orbitals. In either case paramagnetism is anticipated since rhenium has an odd number of valence electrons. The magnetic moment of 1.79 BM corresponding to 1 unpaired electron, and esr evidence showing that this electron is situated predominantly on the ligands, indicates that an intermediate oxidation state is involved... 

Complexes of and The effect of complexation on the splitting of d orbitals is much greater in the case of second- and third-than for first-row transition elements, and the associated effects already noted for Ni are even more marked for Pd and Pi as a result, their complexes are, with rare exceptions, diamagnetic and the vast majority are planar also. Not many complexes are formed with O-donor ligands but, of the few that arc, [M(H20)4] ions, and the polymeric anhydrous acetates [Pd(02CMe)2l3 and [Pt(02CMc)2]4 (Fig. 27.10), are the most... 

The effect of configurational mixing of higher-lying s orbitals into the ligand field d-orbital basis set is also likely to favour elongation rather than contraction. ... 

Frontier Molecular Orbital theory is closely related to various schemes of qualitative orbital theory where interactions between fragment MOs are considered. Ligand field theory, as commonly used in systems involving coordination to metal atoms, can be considered as a special case where only the d-orbitals on the metal and selected orbitals of the ligands are considered. 

As six ligands approach a central metal ion to form an octahedral complex, they change the energies of electrons in the d orbitals. The effect (Figure 15.10, p. 419) is to split the five d orbitals into two groups of different energy. 

To see why this splitting occurs, consider what happens when six ligands (e.g., HzO, CN-, NH3) approach a central metal cation along the x-, y-, and z-axes (Figure 15.9). The unshared electron pairs on these ligands repel the electrons in the d orbitals of the cation. 

Since n bonding is believed to be more important in low oxidation states, as d orbitals contract with increasing oxidation state leading to poorer dw-pw overlap, this would not be expected on the basis of a 7r-bonding mechanism. Similarly, one can compare /(Pt-P) for pairs of isomers in the +2 and +4 states in a planar platinum(II) complex, the platinum 6s orbital is shared by four ligands whereas in an octahedral platinum(IV) complex it is shared by six ligands. Therefore, the 6s character is expected to be only 2/3 as much in the platinum(IV) complexes, correlating well with the 7(Pt-P) values, which can be taken to be a measure of the a-character in the bond. 

Like palladium(II) and platinum(II), gold(III) has the d8 electronic configuration and is, therefore, expected to form square planar complexes. The d-orbital sequence for complexes like AuC14 is dx2 yi dxy > dvz, dxz > dzi in practice in a complex, most of these will have some ligand character. 

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