J. Brian Pedley

Bonding studies of compounds of Group 3–5 elements. Part XVIII. He(I) photoelectron spectra of bivalent homoleptic alkyls and amides, especially of Group 4 elements, and of tin(II) chloride and bromide

He(I) photoelectron (p.e.) spectraof (i) themonomeric, presumed-bent, compounds MR2, M(NR′2)2, and M(NR′R″)2[M = Ge, Sn, or Pb; R = CH(SiMe3)2, R′= SiMe3, and R″= CMe3], (ii) the linear HgR2 and M′(NR′2)2(M = Zn or Hg), and (iii) the bent momomeric SnCl2 and SnBr2 have been recorded, as well as the reference compounds RH, R′2NH, and R′R″NH. For the Group 4 metal(II) alkyls the first vertical ionisation potential (i.p.) is close to that of the free metal; the metal lone-pair orbital is progressively higher in M(NR′R″), and M(NR′2)2, in which compounds it is the second band, being preceded by the antibonding b2 molecular orbital (m.o.) formed by the nitrogen lone-pair atomic orbitals (a.o.s). Confirmation of this assignment is also provided by comparison of data for the bent and linear metal(II) amides; in the latter, nitrogen lone-pair interactions are negligible. Assignments, on the basis of trends and, for MX2, of CNDO calculations, are prooosed and trends noted. There is evidence of substantial N→Si pπ–dπ interaction in the amides.

Standard heats of formation and M–C bond energy terms for some homoleptic transition metal alkyls MRn

The heats of alcoholysis in isopropyl alcohol of the d0 Ti or Zr compounds MR4(R = Me3SiCH2, Me3CCH2, or PhCH2), M(NR′2)4(R′= Me or Et), and MCl4(and some Hf analogues) have been measured, as well as the heats of solution in Pr1OH of M(OPri)4, RH, R′2NH, or HCl; from these and subsidiary data standard heats of formation and thermochemical mean bond energy terms have been derived [M–X in kcal mol–1: Ti–Cneopentyl,44; Ti–C,63; Ti–N, 81; Ti–O, 115; Zr–Cneopentyl, 54; Zr–C, 74; Zr–N, 90; Zr–O, 132; Hf–Cneopentyl, 58; Hf–N, 95; Hf–O, 137].

Bonding studies of compounds of boron and elements of groups 3–5. Part 16.—Ab initio SCFMO calculations and He(I) photoelectron spectra of halogen-bridged dimeric group 3 metal halides and methylmetal halides

He(I) photoelectron spectra are reported for the halogen-bridged dimeric species Al2Cl6, Al2Br6, Ga2Cl6 and (Me2AlX)2(X = Cl, Br, or I). The electronic structures of monomeric and dimeric AlCl3 and Me2AlCl are discussed using the results of all electron ab initio SCFMO calculations in a variety of bases of Gaussian type functions. Assignments of the spectra are made with the aid of these calculations, and by correlation with the corresponding monomers. Values for enthalpies of dimerisation imply a more significant contribution by the Al 3d-orbitals to bonding in the dimer than in the monomer, while Cl 3d-orbitals act only as polarisation functions. For all dimers, calculation and experiment show that the predominantly bridging halogen orbitals ionise at higher ionisation potential (i.p.) than in the respective monomers, in accordance with loss of electron density at the bridging halogen atom. For (AlCl3)2 the lowest i.p.s correspond to ionisation from the terminal chlorine lone pair orbitals, and the highest observed i.p.s. to the Al–Cl bridging orbitals.

Conjugative and homoconjugative effects in 2-heterobicyclo[3.2.1]octa-3,6-dienes

Abstract The 1 H-NMR, 13 C-NMR and photoelectron spectra of a variety of 2-heterobicyclo[3.2.1]octa-3,6-dienes 6 give evidence of conjugative and homoeonjugative effects in these compounds, but there is no suggestion from the data of any special contribution from the (4n+2)π neutral bishomoaromatic system. Reaction of the dienes 6 with TCNE invariably yielded the product of [2+2] addition in cases where a fully characterisable product was obtained.

Bonding studies of transition–metal complexes. Part III. He(I) photoelectron spectra of three-co-ordinate homoleptic bis(trimethylsilyl)amides of scandium, titanium, chromium, iron, gallium, and indium

The He(I) photoelectron (p.e.) spectra of a number of three-co-ordinate metal amides M[N(SiMe3)2]3(M = Sc, Ti, Cr, Fe, Ga, or In) are presented. The spectrum of the d0 scandium compound shows two bands at ionisation potential (i.p.) 9 eV, or are masked by bands due to ionisation from nitrogen lone-pair orbitals (>8.1 eV). This is attributed to a strong –I effect of the ligand which, in contrast to the situation in corresponding dialkylamides such as V(NMe2)4, is not counterbalanced by a [graphic omitted] πinteraction because of strong competing [graphic omitted](p→d)π bonding.

Bonding studies of compounds of boron and the Group 3–5 elements. Part XIII. He(I) photoelectron spectra of phosphines RnPX3–n(R = Me or But; X = H, Cl, or F; n= 1–3), (Me2N)nPCl3–n(n= 1–3), and (R2N)PF2(R = Me or Et)

The He(I) photoelectron spectra of the title compounds have been measured. An empirical assignment of most of the bands is proposed and the effects of various substituents on the phosphorus lone-pair ionisation potentials are assessed. Correlations are made with various other parameters relating to phosphine basicity, and it is concluded that the phosphorus lone-pair i.p. provides a reasonable measure of relative basicity or σ-donor ability within related series of compounds.

Bonding studies of boron and the Group 3–5 elements. Part XV. He(I) photoelectron spectra of monomeric Group 3 trihalide, trimethyl, and mixed halogenomethyl species

The He(I) photoelectron (p.e.) spectra of monomeric Group 3 metal trihalide, trimethyl. and mixed halogeno-methyl species, MX3, MeMX2, Me2MX, and MMe3(X = Cl, Br, or I; M = B, Al, Ga, or ln), are analysed. There is close similarity between the p.e. spectra of the heavier Group 3 metal trihalides and those of their boron congeners and this forms the basis for assignments. Aspects of earlier interpretations of the spectra are reassessed in the light of the data now available for the various series. For BMe3 the BC e′ orbital is split by a Jahn–Teller effect in the excited state; C–H non-bonding interactions decrease as B is replaced by a heavier metal in MMe3 and the splitting is obscured in these molecules. For MX3 the highest observed ionisation potential (i.p.) is the substantially central-atom a1 which decreases with increasing mass of M. The next in energy is the e which shows an essentially constant and high spin–orbit splitting and thus has little central-atom M character. This is followed by the a2″ which may give some measure of X→M π bonding (greatest for B). It is proposed that the baricentre of the first two i.p.s for each of these molecules provides a measure of their relative Lewis acidity: Al > B ∼ Ga > ln.

Bonding studies of transition-metal complexes. Part II. Helium-I photoelectron spectra of homoleptic d0,d1, and d10 tetrakis(dialkylamides) of transition and Group 4B metals and tungsten hexakis(dimethylamide)

He(I) Photoelectron (p.e.) spectra of the compounds M(NR2)4(M = C, Si, Ge, Sn, Ti, Zr, Hf, or V; R = Me). M′(NR′2)4(M′= Ti, Zr, or Hf; R = Et), and W(NMe2)6 have been measured. The first band(s) in all spectra [except that of V(NMe2)4 which has a lone d electron] have been assigned to ionisation from molecular orbitals (m.o.s) regarded as linear combinations of the nitrogen ‘lone-pair’ atomic orbitals (a.o.s). The next band corresponds to the main σ(M–N) bonding orbital, while there is an unresolved collection of peaks to higher ionisation potential (i.p.) corresponding to ionisation from the –NR2 bonding m.o.s. From the form of the nitrogen ‘lone-pair’ section of the spectra it is possible to infer structural information about the molecules.

Bonding studies of compounds of boron and the Group 3–5 elements. Part XIV. Redistribution equilibria in phenylboron dihalide and boron trihalide systems

Redistribution reactions for two sets of trigonal boron systems, (i) PhBX2–PhBY2 and (ii)PhBX2–BY3, have been investigated, principally by 11B n.m.r. spectroscopy for samples in sealed tubes. For (i) when X = F and Y = Cl or Br, equilibrium constants at 316·5 K have been obtained, and small ΔH⊖ values and close to statistical ΔS⊖ for the former have been derived from second-law plots. For (ii), PhBF2 and BCl3 or BBr3 give quantitative transformation into PhBCl2 or PhBBr2 and BF3 at 316·5 K. From these and literature data, standard heats of reactions are derived for various systems related to (i) and (ii).

The He I photoelectron spectra of mixed boron trihalides and the microwave spectrum of BClF2

Although the mixed boron halides BXnX′3 – n cannot be isolated pure, but exist in essentially statistical equilibrium with BX3 and BX′3, it is shown that their He I photoelectron (p.e.) spectra, as well as their microwave spectra, can be measured; the p.e. spectra of BC1F2 and BC12F, and the microwave spectrum, structure, and quadrupole interaction data for BC1F2 are described and are supported by ab initio SCF MO calculations.