Abstract KR∗ 3 Si 4 , 2 , (R∗ = Si t Bu 3 ), formed by the reaction of R∗ 4 Si 4 with 2 KC 8 , is an orange red solid stable at r.t. but decomposes in solution into R∗ 4 Si 4 and a compound that reacts with excess Me 3 SiCl to form (Me 3 Si) 4 R∗ 3 ClSi 8 . Compound 2 is very sensitive to air and moisture. Its alcoholysis does not stabilize the protonated species HR∗ 3 Si 4 and ends up in R∗ 3 Si 3 H 3 . Compound 2 reacts with 1/2 equivalent ICl to form a violet solid R∗ 6 Si 8 . A 1:1 reaction of 2 with SiBr 4 runs differently to form ditetrahedranyl, R∗ 3 Si 4 –Si 4 R∗ 3 which is stable at r.t. but transforms into its violet isomer R∗ 6 Si 8 at higher temperatures. Compound 2 crystallizes as R∗ 3 Si 4 K(18-crown-6) and its crystal structure shows a Si 4 -cage with a short Si–K linkage. It opens up at higher temperatures to acquire a unique structure in which a –CH 2 –CH 2 – group detaches itself from an ether to insert into Si–Si linkage of Si 4 -unit to form a bicyclic ring. The residual chain (CH 2 ) 10 O 6 closes itself on to a Si atom to form R∗ 3 Si 3 (CH 2 –CH 2 )Si(C 10 H 20 O 6 )K(18-C-6).