The first [Re-6(mu(3)-Se)(8)](2+) core-containing cluster carbonyls, [Re-6(mu(3)-Se)(8)(PEt3)(5)(CO)][SbF6](2) and trans-[Re-6(mu(3)-Se)(8)(PEt3)(4)(CO)][SbF6](2) were produced by reacting [Re-6(mu(3)-Se)(8)(PEt3)(5)l]l and trans[Re-6(mu(3)-Se)(8)(PEt3)(4)l(2)], respectively, with AgSbF6 in CO-saturated dichloromethane solutions. Spectroscopic and crystallographic studies suggest significant cluster-to-CO back-donation in these novel cluster derivatives and interesting electronic structures., Thermal and photolytic studies of the mono-carbonyl complex revealed its interesting and synthetically useful reactivity in producing new cluster derivatives.