Neutral methylene complexes undergo different chemical reactions depending on the pi character of the coordinate bond to the carbon centre. A weak contribution, such as in diazomethane, yields mainly substitution reactions, whereas a strong contribution, such as in ethenone, yields mainly addition reactions. Upon treatment with a standard base, complexes with a weak contribution convert to a metal methoxide. With strong acids (e.g., fluorosulfuric acid), they can be protonated to give . Oxidation of these complexes yields formaldehyde, and reduction yields methane.
Free methylene undergoes the typical chemical reactions of a carbene. Addition reactions are very fast and exothermic.
When the methylene molecule is in its state of lowest energy, the unpaired valence electrons are in separate atomic orbitals with independent spins, a configuration known as triplet state.
Methylene may gain an electron yielding a monovalent anion methanidyl (), which can be obtained as the trimethylammonium (()) salt by the reaction of phenyl sodium () with trimethylammonium bromide (()). The ion has bent geometry, with a H-C-H angle of about 103°.