The cyclopentadienylrhenium carbonyls Cp2Re2(CO)(n) (Cp = eta(5)-C5H5; n = 5, 4, 3, 2) have been studied by density functional theory. The global minima for the Cp2Re2(CO)(n) (n = 5, 4, 3, 2) derivatives are predicted to be the singly bridged structure CP2Re2(CO)(4)(mu-CO) with a formal Re-Re single bond; the doubly semibridged structure CP2Re2(CO)4 with a formal Re=Re double bond; the triply bridged structure CP2Re2(U-CO)3 with a formal Re Re triple bond; and the doubly bridged structure CP2Re2(U-CO)2, respectively. The first three of these predicted structures have been realized experimentally in the stable compounds (eta(5)-C5H5)(2)Re-2(CO)(4)(mu-CO), (eta(5)-Me5C5)(2)Re-2(CO)(4) and (eta(5)-Me5C5)(2)Re-2(mu-CO)(3). In addition, structures of the type Cp2Re-Re(CO)(n) with both rings bonded only to one metal and unknown in manganese chemistry are also found for rhenium but at energies significantly above the global minima. The unsaturated Cp2Re-Re(CO)(n) structures (n = 4, 3, 2) have agostic Cp hydrogen atoms forming C-H-Re bridges to the unsaturated Re(CO), group with a Re-H distance as short as 2.04 angstrom.