Molecules with low-valent Ge atoms are generally synthesized from organohalogen germanes as precursors. The Zintl phase K4Ge9 provides reactive building blocks for a targeted synthesis of germanium-rich molecules. The silylation of Ge9 clusters with chlorosilanes ClSiR2R', that carry unsaturated olefin groups R' leads to the introduction of olefinic side chains of variable lengths allowing for further reactions. The compounds K[Ge9{Si(SiMe3)3}2(SiPh2R')] (R' = -CH[double bond, length as m-dash]CH2 (1); -(CH2)3CH[double bond, length as m-dash]CH2 (2)) carry one such functionality, whereas K[Ge9(SiPh2R')3] (3 and 4) offer the possibility for an interconnection of clusters due to three functional groups on the Ge9 core. XPS measurements show that the silylated clusters are much more air-stable than the unsubstituted, bare cluster units.