[en] We report the detection of molecular gas which appears in absorption at large negative velocities near the galaxtic center for a number of millimeter-wavelength molecular transitions previously observed only in emisson. We have made millimeter-continuum emission measurements which show that the absorbing line is optically thick in HCO⁺ and has an excitation temperature consistent with no collisional excitation form --117 to --83 km s^-1 toward Sgr B2. At these velocities CO is seen in emission at a level of 3 K. We obtain molecular column densities for five species and estimate a molecular hydrogen column density of approx.5 x 10²³ cm^-2 to Sgr B2. We associate this absorbing gas with the nuclear disk and esimate that the total mass involved is approx.8 x 10⁹ M/sub sun/. It is shown that spiral arms in the galactic disk also appear as absorption features in HCO⁺ and HCN