The unsaturated homoleptic manganese carbonyls Mn-2(CO)(n) (n = 7, 8, 9) are characterized by their equilibrium geometries, thermochemistry, and vibrational frequencies using methods from density functional theory (DFT). The computed metal-metal distances for global minima range from 3.01 Angstrom for the unbridged Mn-2(CO)(10) with a Mn-Mn single bond to 2.14 Angstrom for a monobridged Mn-2(CO)(7) formulated with a metal-metal quadruple bond. The global minimum for Mn-2(CO)(9) has a four-electron bridging mu-eta(2)-CO group and a 2.96 Angstrom Mn-Mn distance suggestive of the single bond required for 18-electron configurations for both metal atoms. This structure is closely related to an experimentally realized structure for the isolated and structurally characterized stable phosphine complex [R2PCH2-PR2](2)Mn-2(CO)(4) (mu-eta(2)-CO). An unbridged (OC)(4)Mn-Mn(CO)(5) structure for Mn-2(CO)(9) has only slightly (<6 kcal/mol) higher energy with a somewhat shorter metal-metal distance of 2.77 Angstrom. For Mn-2(CO)(8) the lowest energy structure is a D-2d unbridged structure with a 2.36 Angstrom metal-metal distance suggesting the triple bond required for the favored 18-electron configuration for both metal atoms. However, the unbridged unsymmetrical (CO)(3)Mn-Mn(CO)5 structure with a metal-metal bond distance of 2.40 Angstrom lies only 1 to 3 kcal/mol above this global minimum. The lowest energy structure of Mn2(CO)7 is an unbridged C-s structure with a short metal-metal distance of 2.26 Angstrom. This is followed energetically by another C-s unbridged Mn2(CO)7 structure with a somewhat longer metal-metal distance of 2.38 Angstrom.