Polyenes open, nucleophilic attack

Davies et al. have developed a succinct set of guidelines to predict the most favorable position of nucleophilic attack on electronically saturated cationic metal complexes beating more than one unsaturated hydrocarbon ligand. In essence, the so-called DGM rules state that (i) polyenes (L -type ligands) are more reactive than polyenyls (L, -type ligands see also Scheme 11) (ii) open or acyclic ligands react before closed or cyclic ligands and (iii) addition to... 

Rule 3 For even open polyenes (e.g. fy -butadiene) nucleophilic attack at the terminal carbon atom is always preferred, for odd open polyenyls attack at the terminal carbon atom occurs only if the attached metal containing group is strongly electron-withdrawing. 

The three rules successfully correlate a large body of experimental data on the site of nucleophilic attack on cationic 18-electron complexes. They do not always work rule 3 in particular is not very well defined in relation to odd open polyenes. Rule 2 seldom fails, but several exceptions to Rule 1 are now known. Rule 1, for example breaks down in relation to the cation Sodium... 

For even, open polyenes, nucleophilic addition occurs preferentially at the terminal position for odd, open polyenes, attack occurs at the terminal positions only if the metal fragment is strongly electron withdrawing. 

---