General considerations and theoretical studies

Unlike the H or ER cases (E = group 13, 14 element), X moieties, when the donor atom belongs to groups 15, 16 or 17, give stable or moderately stable 

Calculated energies of the HOMO Pd-X for PdX2 fragments [203a] 

Backvall, Siegbahn et al. have addressed, using theoretical methods, the feasibility of the insertion of alkenes into M-X bonds for neutral palladium complexes 

Competitive or preferential nucleophilic (trans) addition is specially important for late transition metals since, in general, they are good a-acceptors and efficiently activate alkenes or alkynes upon coordination. Coordinated X groups that have lone pairs (X = amido, alkoxide, etc.) may give a 1,2-cis addition product by nucleophilic intramolecular attack, depending on the availability of this lone 

Early transition metals in high oxidation states can act as a- and jr-acceptors when X bears lone pairs, so a coordinated X becomes a worse nucleophile, and this two component donation makes the M-X bonds very strong [26]. The thermodynamic balance for the insertion reactions may not be favorable nor even thermoneutral, as was calculated for the insertion of alkenes into Ln-NR2 bonds (Ln = lanthanide), so if insertion reactions are to be accomplished in these systems they have to be coupled with other processes that provide extra driving force. 

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