Mn CO 3 dab - A test case for the isolobal relation

Complexes [M(CO)4(dab)] (M = Cr, Mo or W), which like 18 thf contain a d ML5 fragment, do not react with dipolarophiles. This can be rationalised in the following way (i) the extra electron in the LUMO, or rather SOMO, of the dab ligand in 18 thf increases the reactivity of the 1,3-dipole sufficiently, and (ii) the carbonyl ligands in [M(CO)4(dab)] bind too strongly to the metal and cannot be lost as easily as thf from manganese in order to avoid an increase of the coordination number to seven during the cycloaddition. 

2 Changing the Additional Ligands Isocyanides for Carbon Monoxide 

The decision to replace carbonyls in 1 for isonitriles is based on two reasons (i) while CO is a pronounced 7r-acceptor, isocyanides R-NC are better c-dona-ting/less 7r-accepting, and their electronic and steric properties can be varied by the choice of R. Increased donation to the metal should raise the HOMO-energy of the 1,3-dipole and increase its reactivity (ii) like CO, isocyanides are known to undergo insertion reactions,and their introduction opened the possibility of obtaining new, isocyanide inserted compounds, and further, in mixed complexes, of studying a possible intramolecular competition between CO and isocyanide insertion in the initial cycloadducts. 

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