1.724 1.740( q(M) 2.127 2.114 2.095 1.904 q(N) -1.919 - For all complexes examined, the HOMO is a p-symmetry orbital centred primarily on the imide nitrogen, and orientated perpendicular to the Al-N-X plane; the corresponding HOMO-1 in each case defines a second N-centred p-symmetry orbital lying perpendicular to the HOMO, and stabilised by the mixing in of N s-orbital character within the bent Al-N-X unit. The LUMO in all cases is xanthene ligand-based. Within the aluminium silylimide series [1a] -, [1b] -and [1c] -, sequential stabilization of both the HOMO and HOMO-1 orbitals is observed on going from N(Si i Pr3) derivative [1c] -to N(Si t BuPh2) system [1b] -and finally N(SiPh3) complex [1a] -(Table ). This trend reflects the possibility for pdelocalization involving the Si-C s* orbitals, and the relative pacceptor capabilities of the silyl substituents, Si i Pr3 < Si t BuPh2 < SiPh3. In addition, in the case of the HOMO-1 orbitals, the narrowing of the Al-N-Si angle (Si i Pr3 > Si t BuPh2 > SiPh3), presumably on steric grounds, would be expected to lead to greater N 2s character (less 2p character) in the N-centred lone pair and consequent energetic stabilization. The dominant overall effect appears to be the stabilization of the HOMO-1, such that the energetic separation between the two N-lone pairs increases in the order Si i Pr3 < Si t BuPh2 < SiPh3.