TETRAALKYLHYDRIDO-DIALUMINATE(5) AND TETRAALKYLMETHYLDIALUMINATE(5) WITH ALUMINUM ALUMINUM BOND - [R2AL-ALXR2]- (R = CH(SIME3)2, X = H, ME)

Tetrakis[bis(trimethylsilyl)methyl]dialuminium(4) (1) with an aluminiu m - aluminium bond reacts with tert-butyllithium in the presence of th e chelating tetramethylethylenediamine (TMEDA) to give a hydridodialum inate(5) 2 [R2Al-AlHR2]-{R = CH(SiMe3)2} by beta-elimination and addit ion of the hydride anion to one aluminium atom. By treatment of 1 with methyllithium a similar product is formed containing a methyl group b ound to aluminium: [R2Al - AlMeR2]- 3. As shown by crystal structure d etermination the aluminium-aluminium bond of both molecules is not bri dged. Instead, the compounds contain a saturated, four-coordinated alu minium neighbouring an unsaturated, threefold coordinated aluminium at om. While in the hydrogen compound 2 the aluminium-aluminium bond leng th [266.7(3) pm] remains unaffected in comparison to the neutral start ing compound 1, it is lengthened in the methyl derivative 3 by 9 pm to 275.2(3) pm probably due to the higher sterical demand of the methyl group.