We have studied with theoretical and mechanistic models the intramolecular 1,2-hydrogen shift in trans-hydroxycarbene-formaldehyde and trans-methylhydroxycarbene-acetaldehyde/vinyl alcohol, and the corresponding intermolecular complexation between hydroxycarbene (cis and trans) with formaldehyde. The purpose of our work is making an analysis of the proposed intramolecular hydrogen shifts mainly based on thermodynamic principles and the product selectivity on monochromatic irradiation of trans-hydroxycarbene in carbon monoxide and molecular hydrogen, and formaldehyde. A geometric visualization of the various intermolecular complexes between hydroxycarbene and formaldehyde, based on ab initio results, is demonstrated and discussed with a concept abstracted from the van't Hoff dynamics for a regular tetrahedron in which the interstitial carbon changes its position from tetrahedral into a trigonal-bipyramidal configuration. This concept has been also used for interactions via proton transfer, concretized as hydrogen-bonded complexation in hydroxycarbene-formaldehyde. With the introduced definitions for various van't Hoff ratio numbers, it was possible to judge the ab initio results of the intermolecular complexes between the hydroxycarbenes and formaldehyde. To make an eye-opener, we discuss the role of the carbene lone pair in hydroxycarbene extended to the formation of mono- and dications demonstrating intermediates or transition states with an exclusive mechanistic behavior. Especially, substituted diaryl methylene dications are of interest for nucleophilic substitution reactions, which occur via an in-plane tetracoordinate carbon intermediate, showing coherence with the orbital organization of methylene dications. A reaction of this type differs fundamentally from the classical S reaction. © 2012 Wiley Periodicals, Inc.