Presented are the results of a systematic investigation of the reactions of nine-atom deltahedral clusters of germanium (Zintl ions, Ge-9(n-)) with alkynes and alkyl halides that result in alkenylation and alkylation of the clusters, respectively. The reaction pathways have been probed in depth using various, appropriately substituted alkynes and organic halides, including some typical mechanistic probes and radical clocks. The regioselectivity and stereoselectivity of the reaction with alkynes was examined by systematically varying the steric and electronic nature of the substituents. The studies showed that the Zintl clusters act as strong, anionic nucleophiles toward the alkynes and primary and secondary alkyl halides but, most likely, as electron donors in reactions with tertiary alkyl halides and halogenated olefins. The pentenyl and methylcyclopropyl functionalized clusters, [Ge-9(C5H9)](3-) and [Ge-9(CH2CH(CH2)(2))](2-), respectively, were crystallographically characterized in compounds with [K-crypt](+) countercations. All compounds were also analyzed by NMR and electrospray mass spectrometry.