UPS study of compounds with metal-silicon bonds: M(CO)_nSiCl₃ (M = Co, Mn; n = 4, 5) and Fe(CO)₄(SiCl₃)₂

The (trichlorosilyl) (and -germyl)metal carbonyls Co(CO)₄SiCl₃, Co(CO)₄GeCl₃, Mn(CO)₅-SiCl₃, and cis-Fe(CO)₄(SiCl₃)₂ have been synthesized in high yields. He I photoelectron spectra (UPS) of the complexes have been measured and interpreted on the basis of empirical arguments and density functional theory (DFT) calculations. For cis/trans-Fe(CO)₄(SiCl₃)₂ isomers the calculations predict the trans-isomer to be more stable than cis by 15.2 kJ mol^-1. The cis-isomer is exclusively present in the solid state and in the gas phase. UPS results suggest the importance of charge polarization effects and the lack of π-back-donation along TM-Si bonds. The calculated M-Si bond distances amounted to 2.34 Å (Co-Si), 2.43 Å (Co-Ge), 2.47 Å (Mn-Si), 2.41 Å (trans-Fe-Si), and 2.45 Å (cis-Fe-Si). For Co(CO)₄SiCl₃, the DFT calculated Co-Si bond dissociation energy was 317.7 kJ mol^-1, compared to 222.2 kJ mol^-1 for Co-C energy in Co(CO)₄CCl₃.