Treatment of hexamethylphosphoramide (HMPA) solutions of the trisodium tetracarbonylmetalates(3-) of manganese and rhenium with excess liquid ammonia causes precipitation of very thermally stable materials (dec ≥300 °C) that are isolated in high yields (85-95%) and represent the first available pure samples of unsolvated Na3[M(CO)4]. Infrared and NMR spectral studies of the protonation of Na3[M(CO)4] in HMPA show initial formation of the new monohydrides, HM(CO)42-, which are then converted quantitatively to the dihydrides, H2M(CO)4-. The new manganese dihydride, which is isoelectronic with the first known carbonyl hydride, H2Fe(CO)4, and the previously established rhenium complex are isolated in high yields (75-80%) as the pure salts, [Ph4As] [cis-H2M(CO)4]. The monohydride dianions, HM(CO)42-, also react with Ph3SnCl to provide cis-H(Ph3Sn)M(CO)4-, which can be isolated as the Et4N+ salt for rhenium. The initial successful alkylation reactions of carbonylmetalates(3-) are also reported. Treatment of Na3[Re(CO)4] with methyl p-toluenesulfonate (MeOTs) in HMPA first provides MeRe(CO)42- (identified by IR), which can be protonated or further methylated to give cis-H(Me)Re(CO)4- or cis-(Me)2Re(CO)4-, both of which are isolated in good yields (70-80%) as Ph4E+ (E = P, As) salts. These represent the first isolated alkyl derivatives of metal carbonyl trianions. IR, 1H, and 13C NMR spectra also confirm that Na3[Re(CO)4] reacts with 1,4-butaneditosylate to provide solutions of the somewhat thermally unstable rhenacyclopentane CH2CH2C-H2CH2Re(CO)4-, the initial example of an anionic 18-electron metallacycloalkane. Infrared, 1H, and 13C NMR spectral data have been obtained for many of these compounds and are compared with those of analogous previously known iron and osmium species.