Structure and bonding of the tris(diphenylmethyleneamino)aluminium dimer [Al(NCPh2)3]2: relative strengths of bridging and terminal Al–X bonds in systems X2Al(µ2-X)2AlX2

Abstract

The title compound, prepared by reaction of LiNCPh₂ with AlC₃ in 2:1 → 3:1 molar ratio, crystallises as the dimer (Ph₂CN)₂Al(µ₂-NCPh₂)₂Al(NCPh₂)₂, with Al–N distances (terminal 1.78, bridging 1.93 Å) and CN-Al angles (terminal 148–175, bridging 130–133°) consistent with a bonding description in which each ligand functions as a source of three electrons, making the terminal Al-N bonds twice as strong as the bridging ones, an interpretation supported by molecular orbital bond index (MOBl) calculations which show this to be a general characteristic of species Al₂X₆, whether electron-precise (XCl, NR₂, or NCR₂) or electron-deficient (XMe or H).