Nicholas reaction

The likely reaction intermediate in the process, [(propargylium)Co2(CO)6]+ cation 3, possesses considerable stability.

[2] Later, Richard E. Connor and Nicholas[5] were able to isolate salts of such cations 3 as stable, dark red solids by treatment of the Co2(CO)6-complexed propargyl alcohols with excess fluoroantimonic acid or tetrafluoroboric acid etherate.

The reason that these complexes are so remarkably stable is due to significant delocalization of the cationic charge onto the Co2(CO)6 moiety.

Experimental evidence for the charge delocalization includes an increase in the IR absorption frequencies of the carbon–oxygen bonds of the cobalt–carbonyl in the cationic intermediates compared with those in the parent alcohols.

This causes the atoms to exhibit a trigonal–planar arrangement and shortens the covalent bonds around the central carbon in the cation due to the increase in s character.