He(1) resonance photoelectron spectra are presented for the borane complexes of ammonia, mono-, di-and tri-methylamine, carbon monoxide and phosphorus trifluoride. The spectra are assigned by correlation with each other, with the spectra of the free bases, and with calculations. Agreement with calculations is good, and evidence is presented in favour of a simple Lewis donoracceptor bond in the compounds, with some additional " back donation '' in the carbon monoxide and phosphorus trifluoride complexes.In part 1 of this series we examined the He(1) resonance photoelectron spectra of the simplest stable boron hydride, diborane B2H6. Monomeric borane, BH,, has a very short lifetime, but it is a strong Lewis acid and may be stabilized by complexing with Lewis bases; many such complexes have been isolated. Part 2 reported a study of the boron trihalides,' which are also strong Lewis acids. Borane resembles the boron trihalides in forming strong 1 : 1 complexes with protonic bases such as amines, but it also forms complexes with carbon monoxide and phosphorus trifluoride, which have negligible proton basicity. In this BH, resembles transition metal atoms in low oxidation states, and like them has been classified as a " soft " i3Cid, in contrast to the " hard '' acids such as the boron trihalide~.~ It has been suggested that the complexing of BH, by CO or PF, may be associated with a n-type delocalization of the BH, e orbitals into formally unoccupied orbitals of PF3 or CO. In an attempt to investigate this further we have examined the He(1) photoelectron spectra of BH, complexes of ammonia, the methylamines, CO and PF3 ; the spectrum of trimethylamine is also reported. Preliminary reports on some of this work have been
The He I and He I1 photoelectron (p.e.) spectra of the title compounds are reported. Ab initio SCF-MO calculations have been carried out on diborane, pentaborane(9) and tetraborane(l0) using a double zeta basis of Slater-type orbitals. Assignment of the spectra is made with the aid of these calculations and by correlation with the spectrum of diborane. The p.e. spectrum of pentaborane(9) shows marked differences from one previously reported.In Part 1 of this series the He I p.e. spectrum of diborane was reported. Two independent reports have also a~peared,~. one of which also presented the He I1 spectrum and showed that the spectra could be related to ab initio self-consistent field molecular orbital (SCF-MO) calculations. 5-7 While near-Hartree-Fock calculations, involving extensive Slater and gaussian orbital basis sets, have been performed on diborane, ab initio studies of the higher boron hydrides, tetraborane( pentaborane(9) and decaborane(l4) l o * l 1 have been carried out with minimum basis set wavefunctions. The present paper reports the He I and He I1 p.e. spectra of the latter three compounds, together with calculations of the spectra of diborane, tetraborane( 10) and pentaborane(9), using Koopmans' theorem, carried out in a double zeta (DZ) basis set of Slater-type orbitals (STO). During the course of this work a report of the He I spectrum of pentaborane(9) has appeared l 2 ; our spectra are only in partial agreement with this report. The minimum basis set calculations of ref.(1 1) are used in discussing the spectra of decaborane(l4) ; the experimental ionization potentials (i.p.) have been presented previously. EXPERIMENTAL Tetraborane(lO), B4H10, was prepared from diborane by room temperature storage under pressure and removing the B,H,, at intervals. Two samples of pentaborane(9), B5H9, were examined: one prepared by high temperature pyrolysis of diborane, and a further commercial sample provided by Prof. M. G. H. Wallbridge. Both compounds were purified by vacuum distillation and their purity was checked by infra-red spectros c ~p y . ' ~* l 4 Decaborane(l4), B10H14, was prepared by Dr. A. G. Massey. Photoelectron spectra were obtained on a Perkin-Elmer PSI 6 spectrometer using techniques described previously.
COMPUTATIONAL DETAILSAll electron ab initio SCF-MO calculations of diborane, tetraborane(l0) and pentaborane(9) were performed using a double zeta (DZ) basis of STO.
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