Silicon-carbon unsaturated compounds. 19. Nickel-catalyzed reactions of silacyclopropenes and (phenylethynyl)polysilanes wih phenyl(trimethylsilyl)acetylene and molecular structure of 1,4,4-trimethyl-3,6-diphenyl-1-1,2,5-tris(trimethylsilyl)-1,4-disilacyclohexa-2,5-diene