The chem. of diazo(1,2,3-tri-tert-butylcyclopropenyl)(trimethylsilyl)methane (6) is unexpectedly diverse. Upon irradn. it fragments exclusively into the acetylenes Me3CC.tplbond.CCMe3 and Me3CC.tplbond.CSiMe3. Flash thermolysis, however, gives - aside from the two acetylenes - tri-tert-butylazete (10) and trimethylsilyl cyanide (11). Upon heating 6 presumably isomerizes via a betaine to a Dewar-diazabenzene - which is split into 10 and 11 - and a diazabenzvalene. The CuCl-catalyzed thermal decompn. again follows a different route. Under these conditions tri-tert-butyl(trimethylsilyl)cyclobutadiene (5) is formed, probably as a weak CuCl complex. Only after addn. of ethylenebis(diphenylphosphine) free cyclobutadiene 5 can be isolated. Upon irradn. of 5 a quant. isomerization to tri-tert-butyl(trimethylsilyl)tetrahedrane occurs. This 2nd, spectroscopically unequivocally confirmed tetrahedrane melts at 179 Deg. In soln. the isomerization to cyclobutadiene 5 starts at .apprx.160 Deg. That means, it is thermally even more stable than tetra-tert-butyltetrahedrane.