Outer-orbital complex theory

Taube s classification designates complexes as labile if they react completely at room temperatures within the time of mixing 0.1 If solutions, and as inert if under these conditions the rate of reaction can be followed by conventional kinetic techniques. For six-coordinated complexes on the basis of the valence bond theory it is possible to classify labile systems as being either outer-orbital complexes ([CoFe] , [A1(C204)3] , [Fe(H20)6] +, etc.) or inner-orbital complexes having at least one vacant d orbital (CTiFe] , CV(H20)6] ", [Sc(H20)6] , etc.). It follows then that only inner-orbital complexes containing no vacant d orbitals are inert, e.g., [Co(NH3)6] +, [Cr(H20), +, [Fe(CN) ] -, etc. 

Extended Hiickel Theory (EHT) uses the highest degree of approximation of any of the approaches we have already considered. The Hamiltonian operator is the least complex and the basis set of orbitals includes only pure outer atomic orbitals for each atom in the molecule. Many of the interactions that would be considered in semi-empirical MO theory are ignored in EHT. EHT calculations are the least computationally expensive of all, which means that the method is often used as a quick and dirty means of obtaining electronic information about a molecule. EHT is suitable for all elements in the periodic table, so it may be applied to organometallic chemistry. Although molecular orbital energy values and thermodynamic information about a molecule are not accessible from EHT calculations, the method does provide useful information about the shape and contour of molecular orbitals. 

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