Tetrakis(trimethylsilylethynyl)stannane reacts with non-cyclic trialkylboranes BR3 and 9-alkyl-9-borabicyclo[3.3.1]nonanes (alkyl = Et, neopentyl) to give the stannaspiro[4.4]nona-1,3,6,8-tetraenes I [R = Me, Et, Pr, CHMe2; R (at C) = Et, neopentyl (II), and BR2 = borabicyclo[3.3.1]nonyl] in high yield. The reaction proceeds stepwise, and intermediates as well as final products were characterized by 1H, 11B, 13C, 29Si, 119Sn NMR spectroscopy. According to an x-ray structure anal. of II.0.5 CH2Cl2 (space group P2/c) the endocyclic CSnC bond angles are small [84.9(2) and 84.2(2) Deg] and the two five-membered rings are not exactly planar. Steric crowding causes a strong twist of the skeleton of the other ring systems. The agreement between solid-state and liq.-phase NMR data shows that main structural features are retained in soln.