Structure and bonding Organoaluminium chemistry

1 structure , bonding

1.1 aluminium(iii) compounds

1.1.1 ligand exchange in trialkylaluminium compounds


1.2 low valent organoaluminium compounds

structure , bonding
aluminium(iii) compounds

organoaluminium compounds feature three- , four-coordinate al centers, although higher coordination numbers observed inorganic ligands such fluoride. in accord usual trends, four-coordinate al prefers tetrahedral. in contrast boron, aluminium larger atom , accommodates 4 carbon ligands. triorganoaluminium compounds dimeric pair of bridging alkyl ligands, e.g., al2(c2h5)4(μ-c2h5)2. thus, despite common name of triethylaluminium, compound contains 2 aluminium centres, , 6 ethyl groups. when organoaluminium compound contain hydride or halide, these smaller ligands tend occupy bridging sites. 3 coordination occurs when r groups bulky, e.g. al(mes)3 (mes = 2,4,6-me3c6h2 or mesityl) or isobutyl.

structure of trimethylaluminium, compound features five-coordinate carbon.


ligand exchange in trialkylaluminium compounds

the trialkylaluminium dimers participate in dynamic equilibria, resulting in interchange of bridging , terminal ligands ligand exchange between dimers. in noncoordinating solvents, al-me exchange fast, confirmed proton nmr spectroscopy. example, @ −25 °c h nmr spectrum of me6al2 comprises 2 signals in 1:2 ratio, expected solid state structure. @ 20 °c, 1 signal observed because exchange of terminal , bridging methyl groups fast resolved nmr. high lewis acidity of monomeric species related size of al(iii) center , tendency achieve octet configuration.

low valent organoaluminium compounds

the first organoaluminium compound al-al bond reported in 1988 (((me3si)2ch)2al)2 (a dialane). typically prepared reduction of dialkylaluminium chlorides metallic potassium:

(r2alcl)2 + 2 k → r2al-alr2 + 2 kcl

another notable group of alanes tetraalanes containing 4 al(i) centres. these compounds adopt tetrahedrane core, illustrated (cp*al)4 , ((me3si3c)al)4. cluster [al12(i-bu)12] obtained related investigations on reduction of organoaluminium compounds. dianion adopts icosahedral structure reminiscent of dodecaborate ([b12h12]). formal oxidation state less one.