Bonding heavier ligand atoms

The second important factor is the E—R bond strength, which intuitively should be weaker in ER " species with the heavier E atom. For example. Me loss from dimethyl-arsinometalcarbonyl ions occurs only after the loss of the carbonyl ligands, whereas it competes with CO elimination from the molecular ions of its Sb-containing analogue. Loss of all organic radicals connected to the E atom together with the elimination of... 

Compared to the representative elements, the transition metals are remarkable in that little variation in atomic sizes occurs in going from the first to the second and third series. Orbital sizes do not change greatly, and the strength of covalent bonds to ligands remains much more constant (28). This statement means that the atom size factor, which separates out the heavier donor atoms from C, N, O, and F, will not be present for the transition metals. 

TT-orbital interactions comes from the orbitals which are the out-of-plane orbitals where the plane is defined by the ligand atoms. As the carbon homologues become heavier, the covalent character of the TM-E bond increases and the contribution of the u interaction becomes more important. 

As we have seen, the Lewis theory of acid-base interactions based on electron pair donation and acceptance applies to many types of species. As a result, the electronic theory of acids and bases pervades the whole of chemistry. Because the formation of metal complexes represents one type of Lewis acid-base interaction, it was in that area that evidence of the principle that species of similar electronic character interact best was first noted. As early as the 1950s, Ahrland, Chatt, and Davies had classified metals as belonging to class A if they formed more stable complexes with the first element in the periodic group or to class B if they formed more stable complexes with the heavier elements in that group. This means that metals are classified as A or B based on the electronic character of the donor atom they prefer to bond to. The donor strength of the ligands is determined by the stability of the complexes they form with metals. This behavior is summarized in the following table. 

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