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The group 14 Zintl clusters [E₉]⁴⁻ (E = Ge, Sn, Pb) have been reacted with organometallic reagents in solution in the presence of alkali metal cation sequestering agents. The synthesis, characterisation and reactivity studies of the resultant complexes are reported herein. These negatively charged clusters reductively cleave one of the M–C bonds in the group 12 homoleptic organometallic reagents MR₂ (M = Zn, Cd; R = Ph, mes, ⁱPr) to yield η⁴-coordinated functionalised clusters closo-[E₉MR]³⁻. They can also activate both of the M–C bonds in Cd(mes)₂ to form metal-bridged dimers [Ge₉CdGe₉]⁶⁻ and [Pb₉CdCdPb₉]⁶⁻. Investigating the reactivity of the functionalised cluster [E₉CdPh]³⁻ (E = Sn, Pb) towards HSn(ⁿBu)₃ results in the synthesis of the novel closo-clusters [E₉CdSn(ⁿBu)₃]³⁻. The reaction of K₄Ge₉ with the heteroleptic organometallic reagent Fe(COT)(CO)₃ yields the metalated cluster anion [Ge₈Fe(CO)₃]³⁻, in which the nuclearity of the Zintl anion is altered upon coordination of the [Fe(CO)₃] moiety. Two side products have also been isolated as [K(2,2,2-crypt)]⁺ salts of [Fe(η³-C₈H₈)(CO)₃]⁻ and [Fe₂(η³, η′³-C16)(CO)₆]²⁻. In the presence of highly reduced Zintl anions, all the M–C bonds in homoleptic mid-row transition metal organometallic reagents can also be cleaved. These ‘naked’ metal atoms have templated the formation of the endohedral clusters [Fe@Ge10]³⁻, [Fe@Sn10]³⁻ and [Mn@Pb12]³⁻. These clusters adopt very different geometries and the electronic origin of their structures has been investigated in-depth by DFT calculations. Structural characterisation of some side products is also reported for: [E₉(mes)]³⁻ (E = Ge, Sn) and [Ge₉Mn(mes)]³⁻.