Upon u.v. irradiation of [Fe(CO)4(PR 3 ′ )] with HSiR3 (HSiR3 = HSiMePh2, PR′3 = PPh3; HSiR3 = HSiMe2Cl, PR′3 = PPh3 or PMe2Ph; HSiR3 = HSiMeCl2, PR′3 = PPh3, PMePh2, PMe2Ph or PMe3; HSiR3 = HSiCl3, PR′3 = PPh3, PMePh2, PMe2Ph, PMe3 or PBu 3 n ) the corresponding hydridosilyl complexes [Fe(CO)3H(PR′3)SiR3] are formed. The complexes have themer configuration with acis disposition of the hydride and the silyl ligands. Prolonged irradiation with an excess of silane results in the formation of bis-silyl complexes [Fe(CO)3(PR3)(SiR3)2], if electron density at the metal is not too high. Thus, [Fe(CO)3H(PPh3)SiMePh2] and [Fe(CO)3-H(PMe2Ph)SiMe2Cl] can be obtained but not the corresponding bis-silyl complexes. Most bis-silyl complexes are obtained asmer-isomers with acis-arrangement of the silyl ligands. Only for [Fe(CO)3(PR′3)(SiCl3)2] with small phosphine ligands (PR′3 = PMe3 or PMe2Ph) is thefac-isomer formed.