Square-based pyramidal ML5 complexes

This aspect of their electronic structure means that these complexes are likely to undergo addition reactions, in which their 18-electron shell is completed by the arrival of new ligands. For example, the addition of a molecule of dihydrogen H2 to Vaska s complex leads to the formation of an octahedral complex (2-40). In this addition product, the oxidation state of the metal is - -3 (a d complex), which, when the 12 electrons associated with the bonds are included, does indeed correspond to an 18-electron complex. We also note that this addition reaction is accompanied by a change in the metal s oxidation state, from - -1 in the reactant (d ) to - -3 in the product (d ). This is therefore called an oxidative addition reaction, since the oxidation state of the metal has increased. The reverse reaction is called reductive elimination (Ir(Ill) lr(l)). 

In an ML5 complex which adopts a square-based pyramidal (SBP) geometry, four ligands (L1-L4) are located at the corners of a square which 

In the complex shown in 2-41a, all the angles between the apical bond M-L5 and the basal bonds M-Li 4 are equal to 90°. This strucmre may formally be obtained by removing one of the ligands from an octahedral complex (Lg, 2-42). We may therefore establish the MO of the d block for the SBP by starting from those that we already know for an octahedron, following the method previously used for square-planar complexes ( 2.2.1). 

In most SBP complexes, the metal is located above the base of the pyramid (0 90°, 2-41b). This geometrical deformation of the preceding structure leads to a displacement, of the same amplitude, of the ligands Li and L3 in the yz plane, and of the ligands and L4 in the xz plane (2-45). These movements change the shape and energy of some of the 

The xy orbital has its nodes in the xz and yz planes, the two planes in which the movements of the ligands occur. As the ligands remain located in the nodal planes, no interaction is possible with the xy orbital (S = 0), whose shape and energy are unchanged (a pure d orbital that is strictly nonhonding, 2-46). 

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Consider a pseudo-octahedral d complex of the type [ML5( j -C2H4)] (3-38), where the five L ligands are supposed to have only a interactions with the metal centre. If this complex is decomposed into a 4 fragment MLs, with a square-base pyramidal (SBP) geometry, and an ethylene fragment, the interaction between the orbitals on the two fragments enables us to analyse the electronic factors that are at the origin of the ethylene-metal bond. 

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