Well-defined iron(0) complexes with multivinylsilicon ligands of the formula [Fe(CO)3L] and {[Fe(CO)3]2L′}, where L = dienes, trienes or vinylfunctional silicones and L′ = tetraenes, are new very active and effective catalysts for hydrosilylation and/or dehydrogenative silylation of styrene with trisubstituted silanes and hydrosiloxanes. All catalytic data and selected kinetic measurements as well as the information on stoichiometric reactions of the new precursor catalysts with model substrates (followed by NMR and GC–MS techniques), supported by the relative energies of transition states and intermediates obtained by DFT calculations, permitted proposing a rational mechanism for the catalysis in alkene hydrosilylation, proceeding according to the Chalk-Harrod and/or modified Chalk-Harrod mechanisms.