Electronic Structure and Bonding in Four-Coordinate Organometallic Complexes of Aluminum. Valence Photoelectron Spectra of BHT-H, Me₃Al(PMe₃), and Me₂(BHT)Al(PMe₃)

The He I valence photoelectron spectra of the Lewis acid-base adducts Me₃Al(PMe₃) and Me₂(BHT)Al(PMe₃) (BHT-H = 2,6-di-tert-butyl-4-methylphenoI) have been obtained to characterize the electronic structure and bonding in four-coordinate organometallic complexes of aluminum. To aid in the assignment of the spectrum of Me₂(BHT)Al(PMe₃), the spectrum of the free alcohol, BHT-H, was also obtained. The first and second ionizations of the free BHT-H alcohol show vibrational progressions associated with the symmetric C-C phenyl ring stretching modes, consistent with the b₁ and a₂ Π ionizations, respectively, of monosubstituted phenyl rings. In the photoelectron spectrum of BHT coordinated to aluminum in Me₂(BHT)Al(PMe₃), the corresponding phenoxide a₂ ionization retains the vibrational structure, but the individual vibrational components are lost in the ionization that corresponds most closely with the b₁. The loss of vibrational fine structure associated with ionization from the phenyl Π b₁ orbital in the coordinated phenoxide shows that the phenoxide is involved in a Π interaction with the Me₂Al(PMe₃) portion of the molecule. In addition, the aluminum center in Me₂(BHT)Al(PMe₃) feels a more negative charge potential than the aluminum center in Me₃Al(PMe₃), as shown by the Al-P σ ionization occurring at lower binding energy in Me₂(BHT)Al(PMe₃). This is counter to the σ inductive effects of an alkoxide compared to an alkyl and shows that the BHT is acting as a π electron donor. The change in band shape of the Al-P σ ionization between Me3Al(PMe3) and Me₂(BHT)Al(PMe₃) indicates that the oxygen p σ orbital of the phenoxide ligand is interacting directly with the Al-P bonding orbital. The relationship between experimental ionization potentials and bond strengths of the Al-P σ bond in Me₃Al(PMe₃) and Me₂(BHT)Al(PMe₃) is developed, and the results show that the Al-P σ bond is stronger in Me₃Al(PMe₃) than in Me₂(BHT)Al(PMe₃), consistent with σ donation from the phenoxide into the predominantly Al-P σ^* orbital.