In research, Jeannin shows an interest in the chemistry of transition metals, in the synthesis and structure of the species they form. First in solid state chemistry with the study of the non-stoichiometry of binary and ternary chalcogenides of titanium and zirconium, then he studies the iron complexes formed by solvation in non-aqueous media, the synthesis and X-ray study of organometallic polymetallic species, and finally the chemistry of polyoxotungstates. In the latter case, it is essentially the compounds containing the XW9 brick that have attracted his attention. The laboratory has made a major contribution to the development of their synthesis and structural study by X-ray and NMR of tungsten, holding the record for the largest known polytungstate. In organometallic chemistry, study of the action of aminoalkynes and thioalkynes on iron carbonyl and ruthenium carbonyl; cluster compounds of up to five iron atoms have been isolated. He has also been interested in the coordination chemistry of copper and molybdenum. In order to carry out this research on the creation of a centre for structure determination by X-ray diffraction, he was made available to French and foreign chemists and to external laboratories.
This research has resulted in more than 300 publications.