PhSiH3 reacts with [Co₄(CO)₁₂] at 50 °C in hydrocarbon solvents to give [(µ₄-SiPh)₂Co₄(CO)₁₁], 2c, shown by an X-ray crystal structure determination to have a pseudo-octahedral Si₂Co₄ core. Substituted aryl-silanes behaved similarly. Mixtures of PhSiH₃, H₃SiC₆H₄SiH₃ and [Co₄(CO)₁₂] in a ca. 2 1 2 ratio gave the dimeric cluster [{Co₄(µ₄-SiPh)(CO)₁₁Si}₂C₆H₄], 3a, which has the two Si₂Co₄ cores linked by a C₆H₄ group to give a rigid molecule which an X-ray structure analysis shows to be over 23 Å long. Related dimers linked by –(CH₂)₈– groups were isolated from mixtures of PhSiH₃, α ,ω-(H₃Si)₂(CH₂)₈ and [Co₄(CO)₁₂]. Electrochemical studies show the two cluster units in 3a do not interact electronically.