Bonding Studies of Compounds of Boron and the Group 4 Elements

Bonding Studies

of

Compounds of Boron and

the Group 4

Elements

Part 9.-Photoelectron Spectra and Bonding Studies of Halogeno-, Dimet hylamino-, and Methyl-boranes, BX, and BX,Y

BY G . H. KING, S . S . KRISHNAMURTHY, M. F. LAPPERT AND J. B. PEDLEY

School of Molecular Sciences, University of Sussex, Brighton BN1 9QJ Received 12th June, 1972

The photoelectron (PE) spectra, excited by helium 21.22 eV radiation, of the compounds BX3 and BXzY (X and Y chosen from F, CI, Br, I, NMe,, or Me) have been measured. From these and published data, the spectra of the boron trihalides have been fully assigned ; earlier proposals have been re-assessed on the basis of (i) line shapes, (ii) calculations of spin-orbit coupling and of orbital energies as a function of distortion, and (iii) comparisons with other BX: (e.g., X = NMez, NHMe) spectra. Assignments for the other BX3 and BXzY molecules are based on (i) line shapes, and (ii) a comprehensive analysis of trends within series, e.g., for Me2NBCI2, (MeZN),BCl3-, as well as Me2NBX2 (X = F, CI, Br, I) ; calculations for these more complex molecules did not prove especially helpful, except for those relating to orbital energies as a function of twist angle (for X = NMe,).

Some earlier papers in this series have addressed themselves to problems of bonding in trigonal boron compounds BX3, BX2Y, and BXYZ (where

X,

Y, and Z are uni- valent atoms or groups, such as Hal, NMe,, OMe, SMe, or Me). In parts 2 and 3,, first ionization potentials (IP), determined by electron impact, were reported and molecular orbital (m.0.) calculations suggested that (i) in the boron halides, contrary to usual assumptions, n-back donation decreases in the series BI> BBr

=

BCl Q BF, and that a-charge drift is the dominant factor in deciding overall bond polarity ; (ii) first IP are a composite of (a) the antibonding interaction between a-orbitals on

two ligand atoms and (b) the bonding interaction between pn-orbitals on a ligand atom and boron ; and (iii) for boron halides (a) is the dominant term, while for the other compounds the reverse is the case.

In this paper we present results first, on the four boron trihalides, then on the more complex series BX, and BXzY (X and Y chosen from F, C1, Br, I, NMe,), and finally, we hope to consider extensions-eg., to PF5, PF,Cl, and PF3C12 (which have the same symmetry as BF,, BF2CI, and BFC12 respectively).

PE spectra of the boron trihalides have been discussed by Bassett and Lloyd.’ We have built on these results and made assignments, using not only line shapes but also calculations relating to spin orbit coupling and of orbital energies as a function of distortion, and comparisons with B(NRR’)3 systems. Consequently, a revised assignment of two bands is proposed.

The PE spectra of several aminoboranes, including (Me,N),BX+, (n = 1,2, or 3 ;

X = H, Me, F, C1, and Br), have been recorded by Bock and FUSS,^ and the bands attributable to the BN x m.o.’s assigned. We report data on a more comprehensive series of compounds. Accordingly, several spectra are described for the first time, and, because of the more extensive series and definitive conclusions for the boron trihalides, fairly complete assignments for the essentially nitrogen and halogen based

70

G . H. K I N G ,

s. s.

K R T S H N A M U R T H Y , M . F . L A P P E R T , J . B . P E D L E Y 71

m.o.'s are presented. These are thus based on details of line shapes and on rational- izations of trends within series. Two different series are considered for each com- pound : e.g., for Me,NBCl,, they are (Me2N),BC13-, and Me2NBX,, with trends represented graphically.

CNDO and INDO calculations have taken into account variations in twist angle in the aminoboranes and comparison with available diffraction data.'. Extended Huckel calculations giving changes in orbital energy during vibration were used to explain PE band widths and the incidence of Jahn-Teller distortions. CNDO calculations have also been used to interpret the spin orbit splitting in BBr, and B13.

E X P E R I M E N T A L

The compounds BX, (X = F, C1, Br, I, or NMe2) were purchased. Literature methods were used to prepare and purify the following compounds : B(NMe2)2X (X = CI,' Br9) ;

B(NMe2)X2 (X = C1,'O Br,'O or P o ) ; B(NMe2)2Me.11 Their purity was checked from

vapour pressures, analyses, and spectra (infra-red, 'H and IIB nuclear magnetic resonance, and mass). B(NMe2)J was prepared by adding B13 to a solution of B(NMe& in pentane at -55°C. The reaction was quantitative and after removal of the solvent at reduced pressure and room temperature, the pure B(NMe2)21 remained as a colourless liquid. The PE spectrum indicated the sample was free of starting materials and B(NMe2)12.

All the compounds studied were hydrolytically sensitive, therefore samples were distilled into glass tubes that were sealable with Rotaflow taps. In this manner, the materials were stored without decomposition, at room temperature [except for B(NMe2)C12, which was stored at

-

78°C to inhibit dimerization].

The spectra were recorded on a PS 16 instrument to which was attached a vacuum line and manifold. Rotaflow tubes containing samples were connected at the manifold and material distilled out at room temperature. Trap-to-trap distillation (at various temperatures depend- ing on the compound) prior to introduction ensured purity. Holding the bulk material at O"C, sampIe vapour was bled directly into the instrument target chamber.

RESULTS

The experimental PE data are shown in fig. 1-3 and tables 1 and 2 ; new results refer to (Me2N),B13,, (n = 1 or 2). The fourth and fifth bands in the spectrum of BF3 are shown in fig. 4 at high resolution and expanded scale.

TABLE 1 .-IONIZATION POTENTIAL (eV) DATA d FOR THE BORON TRIHALIDES assignment BF3 a BCl3 a BBr3

4

15.95 11.62 10.65 3e' 16.65 12.28 11.36 e" 17.10 12.53 11.71 a2 19.15 14.35 13.18 2e' 20.10 15.49 14.20 2 4 - 17.70 16.74 14.46) BI, * 9.36 9.92 10.10 10.36 10.48 11.74 12.96 15.39

0 Figures agree to within & 0.05 eV with data in ref. (5) ; b figures are those stated in ref. (5) ; C spin-orbit interaction causes mixing of these levels ; d vertical IP.

Calculations were performed for B(NMe2)3, B(NMe,),Cl, B(NMe2)CI2, and BCl3, using standard CNDO, INDO, and extended Huckel procedures. Bond lengths taken were the averaged values from the electron diffraction data for

B(NMe2)CI2 and B(NMe2)3,7 and X-ray data for BCI3,l2 i.e., B-Cl = 1.75&

72 B O N D I N G STUDIES OF B O R O N C O M P O U N D S

B-N = 1.4-OA, C-H = 1.1 1

A,

N-C = 1.45

A.

Bond angles were chosen as NBN = 120" = NBCl = C1BC1 = CNC, and HCH = 109'8'. For molecules con- taining the moiety -B(NMe,),, the question of twist of the -NMe2 groups out of the NBN plane arises. Although there are no diffraction data for B(NMe,),Cl, the electron diffraction structure of B(NMe2)3 indicates that the angle of twist is 32".

To illustrate the importance of twist, an extended Huckel calculation was per- formed on B(NMe,), where the angle was varied from 0 to 90". The minimum total energy was obtained when the angle was 44", which agrees well with the observed value.

A A A A A

fi

4-r

16 !8

eV

FIG. 1.-The PE spectra of the boron triha1ides.a

a For BBr3 and B13, spectra are those reported in ref. (5)

However, the orbital energies obtained in all the other calculations on dimethyl- amino-compounds did not correspond either in absolute value or as trends with the observed bands. In particular, if the angle of twist of the NMe, group was not taken into account, the highest levels were not predominantly nitrogen in character.

Using an extended Huckel procedure, we have calculated the changes in orbital

G . H . K I N G , S . S. K R I S H N A M U R T H Y , M. F. L A P P E R T , J . B . PEDLEY 73 energy for the a;, en, 3e’, a:, 2e’, and 2a; orbitals of the boron trihalides during a symmetric vibration v1 and during a bending motion where one of the XBX angles changes. The latter motion may be related to the pair of non-totally symmetric,

A

8 I0 12 14 16 18

eV

FIG. 2.-The PE spectra of (Me2N)2BX, (X = C1, Br, and I) and (Me2N)3 B.

doubly degenerate E‘ type fundamental modes v3 and v4 in the molecules. Illustrative results are in fig. 5(i), (ii), and (iii); further details are discussed elsewhere l 3 and

are used in the interpretation of the boron halide spectra. The magnitude of the rate of change of orbital energy during distortion will be related to the width of the vibrational envelope.

For the heavier trihalides, BBr, and B13, the significant spin orbit coupling constants for Br (( = 0.30 eV) and I

(c

= 0.62 eV) give rise to a splitting in certain of the bands. We use Dixon’s l 3 CNDO calculation, which considers the matrix

elements of the angular momentum operator between the one centre basis a.0. The splittings were found to be 0.91

cBr

and 0.88 [I for E’(3e’) and 0.73

CBr

and 0.70

TI

for

E’(2e’). Second order spin-orbit coupling is considered by analysis of off-diagonal

elements between states having the same representation in the double point group.

74

8

BONDING STUDIES OF B O R O N C O M P O U N D S

eV

FIG. 3.-The PE spectra of (Me,N)BX,, (X = C1, Br, and I).

I9 2 0

eV

21

FIG. 4.-The fourth and fifth bands in the PE spectrum of BF3. Resolution 28 meV for Ar doublet, time constant 60 s.

G . H . K I N G , S . S . K R I S H N A M U R T H Y , M . F . L A P P E R T , J . B . P E D L E Y 75 The finite off-diagonal elements were found to be 0.5 (, but interaction will be signi- ficant only for states which are close in energy, namely, 2E’, E+ with 2E”, E+.

The significance of this second order perturbation is discussed during the assign- ment of the 3e’ and e” levels of BBr, and BIB.

- 16.5- -17.0- - 17.5-

5

3

8

8

- 18.0-

-

18.5- - I9.0- 3e:\

4

e > 2 e

7

I 2 0 1 I * * I / 1 . 0 9 5 1.295 1.495 IRO 1 2 0 6 0 1.85 2 ) 5 2.25 (0 (ii) (iii)

FIG. L-Extended Huckel calculations of the orbital energy changes in BF3 and B13 during vibration : (i) symmetric stretch in BF3 : (ii) bending in BF3 : (iii) symmetric stretch in B13. The ordinate on the left-hand side of the figure refers to (i) and (ii), that on the right to (iii). In (i) and (iii) the abscissa scales are for B-X bond lengths in A ; in (ii) for the FBF angle in degrees, all molecular dimensions

being taken from Sutton.22 DISCUSSION

A S S I G N M E N T O F T H E S P E C T R A O F THE T R I H A L I D E S

The symmetry orbitals in the boron trihalides which are accessible to 21.21

eV

radiation are a;, 3e’, e”, a;, 2e’, and 2a; (not 2a; for BF3). We agree with the earlier assignments (see table 1) except for the 3e’ and e” levels and are able to place the formerly tentative proposals for the remainder on a firmer basis.

76 BONDING STUDIES OF B O R O N COMPOUNDS

Extended Hiickel calculations for BCl, indicate that the 2a; orbital has substantial B 2s character and very similar proportions af C13s and 3p orbitals anti-bonded and bonded respectively to boron 2s ; thus, the net effect is that the 2 4 orbital is essentially non-bonding. Further, the effect of a symmetric stretch is to alter the orbital energy linearly, with a shallow gradient (i.e., narrow band) intermediate between those for BF3 [fig. 5(i)] and B13 [fig. 5(iii)]. This situation predicts that the frequency of sym- metric vibration in the BCl? ion is very close to that in the molecule and that the vibrational progression is very short. Thus, we assign the sixth band in the BC1, spectrum to 2a;, as it shows a short progression of three components, having an interval 443 cm-’+40 cm-l (cf., 11BC13, v = 471 cm-’) l 4 and has adiabatic IP very

close to the vertical IP. Turning to BBr, and BI,, the calculations indicate increased B 2s and halogen 2s character in this orbital so that its anti-bonding nature is increased. This is consistent both with the increased gradient of orbital energy change under the symmetric stretch for the heavier halides [cf., fig. 5(i) and (iii)] and with the broadening envelope for the observed PE band.

The next lowest orbitals above 2a; will be either the a: (n) or the 2e’ (partially bonding 0). We assign the fourth and fifth bands in BF, and BCl, (and the corres-

ponding bands in BBr, and BI,) to a; and Ze’, respectively, because of the following calculations. The non-degenerate a; orbital may show an excitation in v1 only [fig. 5(i) and (iii)], whereas the 2e’ orbital is also susceptible to non-totally symmetric bending vibrations. The fourth band in BF3 and BC13 clearly shows (especially BF,, fig. 4) only one progression, for which the observed intervals are 725 cm-’ & 50 cm-l (cf.,14 v 1 = 888 cm-l) and 406 cm-l_+50 cm-l (cf.,14 v 1 = 471 cm-I), respectively. The loss of fine structure in the observed bands for the heavier halides is due to the low

v 1 for these molecules. Additionally, there is a considerable narrowing of the envelope

in going from F through to I ; as also indicated by calculation [fig. 5(i) and (iii)], in that the gradient of the orbital energy change for the a; orbital under a symmetric stretch becomes much less positive down the series and would lead to a less extensive progression. This situation contrasts with that for the 2a; orbital.

The nature of the fifth band in BC1, and BF,, which we have assigned as 2e’, is complex. In BF,, the dominant progression has eight components at an interval of 750 cm-1 & 10 cm-l (fig. 4), caused by excitation in vl. The calculated orbital energy changes for 2e’ and a,” are similar under a symmetric vibration [fig. 5(i)], as is consistent with both the similarity in envelope shape and the observed interval (750 cm-’ for 2e’, 725 cm-l for a;). A bending vibration [fig. 5(ii)] removes the degeneracy of the 2e‘ orbital giving rise to a Jahn-Teller state of the ion, distorted to

C2,

symmetry. The predicted excitation of an E’ vibration appears as a secondary progression, having a slightly smaller interval than the more dominant progression. The E’ fundamental modes of 11BF3 are v3,14 1446 cm-I, and v4,14 480 cm-l, a drastically

reduced v, being the more plausible assignment. This is to be compared with other Jahn-Teller distorted states, 2E” for BF, and 2E for B(NR2),, (see later in this section), where excitation of the high-frequency asymmetric mode is unequivocal. The symmetrical displacement of the components of the

2E’

(2e’) state predicts a

fairly simple vibrational envelope [cf., 2E’ (3e’) state].

From the orbital energy calculations for BCI, we would expect the same complexity of fine structure as in BF3, but with both symmetric and bending vibration less strongly excited. By analogy with BF,, we might expect a dominant progression having an interval about 85

%

of v l , i.e., x400 cm-l. However, no fine structure is observed, the band consisting of two unresolved overlapping envelopes which closely resemble Jahn-Teller split bands in some other molecules (e.g., ethane,l and pro- pane 16). It is also possible that the loss of fine structure is partially caused by the

G. H . K I N G , S. S. K R I S H N A M U R T H Y , M. F. L A P P E R T , J . B . P E D L E Y

77

first order spin-orbit splitting of the 2E’ state. In BBr3 and B13, the dominant fine structure in the band corresponding to the 2e’ orbital is now the spin-orbit splitting, and vibrational structure is not visible. The calculated spin-orbit splittings agree well with experiment [BBr,, 0.22 (0.26) ; BI,, 0.44 (0.50) eV, experimental values in parentheses]. For the 2e’ and a; orbitals there are similar decreases in the gradient of orbital energy change under symmetric stretch in going down the series F-I, thus correlating with the narrowing of the PE bands. The observed areas of the a,” and 2e’ bands in BC13 are approximately equal, and, even when allowing for the inttrumental decrease in sensitivity at lower electron kinetic energy, the ratio is far from the expected 1 : 2 predicted on occupancy criteria. It appears to be a feature of the PE spectra of the lighter halides that the cross-section for doubly degenerate 0 orbitals

is very low, and that for doubly degenerate n: orbitals very high. For B13, the total area of the spin-orbit split components for 2e‘ orbitals is almost double that for al; and so appears not to be anomalous.

The bands corresponding to the three highest occupied orbitals, e”, 3e’, and a; show no clear-cut vibrational fine structure in any of the molecules, so assignment is not straightforward. Calculations 17* are in disagreement concerning the sequence

of the orbitals, but show them to be very close together in energy (within 0.04eV). However, from the PE spectra it is possible to deduce a fairly definite order. Our assignment is that the orbital sequence in BF3 and BC13 is a;

<

3e’ <en in order of increasing binding energy.

For BF3 and BC13, the third band is the one which possesses the features character- istic of a non-bonding n: orbital and in particular a doubly degenerate n orbital. The band has at least twice the intensity of the first two bands and has a very sharp onset of ionization so that the vibrational envelope is very asymmetric. The presence of incipient, or just resolvable, vibrational fine structure is also a feature of a band arising from such orbitals. A similar sort of envelope is observed in the correspond- ing bands for B(NMe,), (fig. 2), B(NMe2),Me,6 benzene,’

’

B(NMe2),HY6 B(NR2)3

(R,

= H and Me, Et,, and M ~ ( Z - C ~ H ~ ) ~ . ~ ~ Further, the ratio of the areas of the e and a2 bands molecules of symmetry C3 is always approximately 3 and the separation of these pairs of orbitals is 1.5j2.0 eV.

The calculations predict for BF3, that the symmetric vibration is excited to a certain extent [fig. 5(i)], but that E’ type vibrations (involving a bending motion) are strongly excited [fig. S(ii)]. Thus, we assign the first pair of lines in the third band in BF3, which have an interval of 636 & 30 cm-l to a progression in the E’ type vibration, v3 (11BF3 molecule, l 4

1446

cm-I). The percentage reduction in v3 is similar to

that observed in the band corresponding to the 2e’ orbital. Numerically, an interval of 770 cm-’ could be a progression in a reduced v1(BF3,I4 v1 = 888 cm-I), but this is unlikely since the corresponding bands in the spectra of B(NMe2)3 and

B(NHMe)3

have intervals of 1254+50 cm-l (cf.,,l 1449 cm-l in molecule) and

1232f80

cm-l (cf.,21 1499 cm-l in molecule), respectively. These intervals can correspond only to the B-N asymmetric, E’ type vibrations. We assign the shoulder at 17.27eV to unresolved structure which is contributing to the broadening of the envelope, not to a third member of the vibrational progression. We observe no fine structure at all in the corresponding band for BC13.

The calculation of the energy changes in the 3e’ orbital under the vibrations [iig. 5(i) and (ii)] predict complex vibrational structure. It is expected that both the symmetric stretch and bending vibrations will be strongly excited. The components of the 3e’ level are split by the bending motion [fig. 5(ii)] to give a highly asymmetric potential energy surface [cf. (2e’ level)] involving vibrations of differing frequency. Thus, we assign the second band in the spectra of BF3 and BC13 to ionization from

78 B O N D I N G STUDIES OF B O R O N C O M P O U N D S

this orbital,

on

account of its broadness and lack of resolution. For the 3e’ level, the relative area of the band is very small, particularly in BF3, which was also noted for the 2e” level.

For BF, and BCl,, the first band is assigned as a;, the most interesting feature of which is the lack of any fine structure even though the extended Huckel calculations predict that only the totally symmetric vibration v1 should be excited.

Unperturbed First order Second order

€512 I 1

T

E5’2 7--- / \ B B r 3 E3/* E ’12 E3/2 I f I I /

-

‘

calculated 0.20 0.30 0.30 0.35 observed 0.18 0.26 0.12 0.36

FIG. 6.-First and second-order spin-orbit splitting patterns (eV) for BBrJ and B13. For BBr, and B13, the a;, 3e’, and e” levels are assigned chiefly by consideration of spin-orbit interaction. The first band in the spectra of both these compounds is immediately assignable as a;, since the separation of this band from the remainder is too great for it to be part of a multiplet pattern.

Bands two and three in BBr, and bands two to five in B13 are grouped together in table 1 since the

2E’

state (3e’ level) and

2E”

state (e” level) are mixed by multiplet splitting.

The effect of first order spin-orbit coupling is to split both the E’ states into the doublets, E+ and E+. The other states remain unperturbed unless the second-order effect is taken into account (fig. 6). Under asecond-order effect, in principle there will be an interaction of 0.5c between all states of the same species, but the only doublets close enough in energy to interact effectively are the E4 species arising from the E” (8 level) and E‘ (3e‘ level) states. For B13, if we assume the unperturbed E” species lies at a similar level to the E+ (E’) species, then after second-order interaction we obtain a splitting pattern similar to that observed experimentally. Similarly for BBr,, if the E species lies near the E3 (E’) species, then three of the components overlap in the

G . H . K I N G , S. S. KRISHNAMURTHY, M . I?. LAPPERT, J . B . PEDLEY 79 second PE band, whilst the fourth component is sufficiently lower in energy to appear as a separate band.

A S S I G N M E N T O F THE S P E C T R A OF BN C O M P O U N D S

The PE spectra are shown in fig. 2 and 3 and ionization energies are listed in table 2.

The following approximation will be made for the aminoboranes (Me2N),BX3-,

(n = 0- 3, X = F, Cl, Br, or I) : there are two separate sets of orbitals, one associated primarily with the Me,N substituent and the other with X. The validity of this approximation has been discussed in part 3,3 mainly on the basis of first IP data.

Fig. 7(i) and 7(ii) show the approximate forms of the halogen symmetry orbitals, for Q and n system, respectively; symmetry labels appropriate to DSh (n = 0 or 3)

or

C,,

(n = 1 or 2) point groups are indicated. These take into account halogen

TABLE 2.-IONIZATION POTENTIAL (eV) DATA FOR B(NMe2)3 AND B(NMe2)JG-n

(X = Cl, Br, I) assignment B(NMe2)3 Cl a Br a I CI a Br a I B(NMe2kX - B(NMedX2 7.60 9.25 - (11.6) (12.3) (12.7) (1 4.5) 16.0 (1 6.7)

-

8.15 9.45 10.90

-

11.64 (12.4) (13.1) (13.7) - 8.16 9.35 10.09 - 10.88 (1 2.0) (12.8) (13.6) - 8.11 8.81 9.32 - 10.21 11.20 (1 2.9) (13.7) -

-

15.7 17.7

-

15.4 17.1

-

(16.4) (17.1) - 9.68 11.14 11.62 12.01 12.96 b (13.5) -

-

15.7 (1 6.0) (17.3) - 9.55 10.19 10.73 11.13 12.04 12.52 13.58 - - - 15.2 15.9 (17.5) - 8.95 9.27 9.69 10.04 1 1.03 11.50 12.97 (13.5)

-

14.2 (15.6) (17.3) a agree with values given in ref. (6) to within k 0.05 eV for well-defined band maxima ; b not observed ; C bands corresponding to orbitals associated predominantly with the - NMe2 system ; d vertical IP. Figures in parentheses indicate ill-defined maxima.

I

z

(0 (6)

FIG. 7.--Schematic energy level diagram for BX3 systems: (i) orbitals in plane (u); (ii) orbitals perpendicular to the molecular plane (n). Positive lobes of p orbitals are indicated by directions of arrows for U, and by

+

signs for n orbitals. For the a orbitals, only for a: and a; are the directions

of the arrows precisely defined by symmetry.

80 BONDING STUDIES OF BORON COMPOUNDS

p-orbital interactions and neglect s-orbitals. (Angular deformations, discussed

earlier, somewhat complicate the picture : but it is still possible to define 0 and n type

orbitals associated predominantly with the NMe, groups). Although symmetry arguments do not define the relative positions of the a; and the upper e’ orbitals for BX3, it is evident that for the -BX2 systems (n = 1) these correlate with the two bl orbitals of lower energy, which converge in the BX system (n = 2). Likewise, the lower e’ and a; orbitals of BX3 correlate with a, orbitals of the -BX2 and BX systems. M.o.’s involving Npn orbitals are expected to lie at higher energy than those relating to N-C or C-H orbitals (cf., HNMe, 23), and have the form shown in fig. 7(ii).

\

/

\

/

O R B I T A L S ASSOCIATED W I T H NMe, GROUPS

The first two bands in the PE spectrum of (Me2N)3B (fig. 2 and table 2) are assigned respectively to the doubly-degenerate and non-degenerate n-type orbitals [fig. 7(ii)] derived from appropriate symmetry combinations of predominantly NMe, n-type orbitals. The large, structureless envelope, starting at 11.2 eV, is assigned to strongly bonded orbitals associated mainly with the NMe, groups. The form and position of this envelope appears to be almost independent of substituent effects when one or two of the NMe, groups are replaced by Cl, Br, or I (fig. 2 and 3).

In

(Me2N),BX3-, (X = C1, Br, and I ; n = 1 or 2), the predominantly halogen orbitals are immediately identifiable by their regular trend towards lower IP on

changing from C1 to Br to I, whilst the bands associated with the NMe, groups remain approximately constant in energy (fig. 2 and 3).

The first two bands in (Me2N),BX are therefore assigned to n(aJ and n(b,) orbitals of the NMe, groups, as in (Me2N),B, and the change An in the interval between these levels is dependent on the following considerations (i)-(iv).

(i) Changing from (Me2N)3B to (Me,N),BX lowers steric hindrance; hence there is a reduction in the angle of twist of the NMe, group, and a larger value for An. Similarly an increase in the NBN angle leads to a decrease in Ax.

(ii) Inductive effects of the halogen atoms are expected to stabilize the n(a,) and

n(b2) levels to the same extent.

(iii) In the absence of other factors, removing one of the NMe, groups should reduce An by one third [fig. 7(ii)].

(iv) Interaction with the n(b2) orbital of the BX moiety causes destabilization of

the n(b2), but has no effect on the n(a2) NMe, orbital; An is therefore decreased.

This effect is greatest when X = I where the two n(a,) and n(b2) orbitals are closest together. The n(a2) and n(b2) orbitals of (Me,N),BMe l 3 and the average values for

(Me,N),BB(NMe,), 24 have energies within 0.1 eV of those in (Me2N)3B. The Me

group has very low lying n orbitals and the B-B bond in the latter compound is particularly long, so that the n orbitals should differ from those in (Me2W3B only by virtue of considerations (i) and (iii). It appears, therefore, that in these molecules these factors are almost mutually compensating. This suggests that for (Me2N),BX the effect of (i) and (iii) is small and only (iv) causes significant changes in the value of

An.

The inductive effect appears to be independent of the halogen atom involved, the

first IP [n(a2)] being approximately constant (table 2); the value for (Me2N),BF is 8.04 eV.6 The second IP [n(b2)] of (Me2N),BF is 9.53 eV and we do not expect this

A

G. H . K I N G , S. S . K R I S H N A M U R T H Y , M. F . L A P P E R T , J . B . P E D L E Y 81 value to be significantly affected by interaction with the F 4 b 2 ) orbital, which lies below 15 eV. The change in the IP of the NMe, n(b,) orbital for the other halogen compounds provides a measure of the interaction energy with the halogen n(b2) orbitals [C1(0.08), Br (0.18), and I (0.72 eV)]. This increasing mesomeric effect down the series corresponds with the narrowing of the NMe2 n(b2) band [C1(0.45), Br (0.41), and 1 (0.24eV) full widths at half-height], whereas the width of the nitrogen z(u2) band is constant at 0.32 eV.

For (Me,N)BX,, there is only one predominantly NMe, orbital, n(b2) [fig. 7(ii), BX moiety]. The first band is assigned to this orbital because it shows the least change in energy down the series X = C1, Br, 1 (fig. 3). This orbital is expected to be strongly bonded to boron because of the coplanarity of the CNC and BX2 fragments.8 The first IP for a series of halides and mixed halides (Me,N)BXX' (X, X' = F, C1, Br,

and I) measured by electron impact, lie in the narrow range 9.6-9.7 eV (except for

X = X' = I). This indicates an approximately constant inductive effect of the

halogens [as in B(NMe,),X] with a significantly different mesomeric effect [interaction with halogen ~ ( b , ) orbital] only for MezNB12 (due to the close proximity of I and NMe2 orbitals). This conclusion is supported by the significant sharpening of the NMe2 ~ ( b , ) orbital (fig. 3), and the relative broadness of the I ~ ( b , ) band assigned in the next section.

The pattern of NMe, '~t orbitals is shown in fig. 8(i), (ii), and (iii) which show the

trends in the experimental IP across the series (Me2N),BX3-, (n = 0-3), [cf., fig. 7(i) and (ii) for the predicted trends].

O R B I T A L S OF P R E D O M I N A N T L Y H A L O G E N C H A R A C T E R

The precise symmetries of the halogen orbitals in (Me,N),BX,-, (n = 1 or 2) are identified by comparison with the firmly assigned levels in the corresponding BX3 compounds. For the 3e' and e" states of BBr, and B13 where there is spin-orbit interaction, a common average value was taken. Fig. 7(i) and (ii) show the trends in

b3

L

- - -

10

J

I I I I 1 t b 1 0 i 2 3 0 1 2 3 0 1 2 3 (9 (ir? (Wl

FIG. 8.Xorrelation of experimental IP data : (i) (Me2N)nBC13-n ; (ii) (Me2N)nBBr3-n ; and (iii) (Me2N),B13-,, for n = 0-3, (- - -) indicating ?r orbitals, (-) u orbitals.

82 BONDING STUDIES OF B O R O N COMPOUNDS

orbital energies due to decreasing the number of halogen atoms excluding an inductive effect due to the NMe, group. The a(b,) orbitals are stabilized, the n(a2) orbital is unaffected, and the a(aJ orbitals are destabilized. An inductive effect causes destabilization of the n(a,) orbital, an even greater destabilization of the a(al) orbitals, whereas in the a(bJ orbitals the effect of decreasing the number of halogen atoms and the inductive effect are in opposition. The only possible assignment of the halogen orbitals in Me,NBX, on this basis is that shown in fig. 8(i), (ii), and (iii). In the absence of interaction between the n(b2) orbital of the BX2 moiety and the NMe, n(b2) orbital, the interval between the n(a,) and

n(b,)

halogen levels is in the ratio of 3 : 2 for BX3 and (Me2N)BX2, respectively. For X = C1, this is a good approximation [fig. 8(i)], but when X changes successively to Br and to I [fig. 8(iii) and (iii)], the ratio tends towards 1 : 1 ; this supports an earlier conclusion that only in (Me2N),BI is the n type interaction between the NMe, and X orbitals substantial.

In the monohalo compounds, the halogen a(al), a&), and n(b,) orbitals are degenerate if there is no interaction between the halogen orbitals and the rest of the molecule, [fig. 7(i) and ($1. The a(al) orbital is assigned to the fifth IP because it should be the most strongly bonded to B, as are the corresponding orbitals in BX3 and Me2NBX2 (fig. 8). This is confirmed for (Me2N),BI by the broadness of the band (cf., 2a; band in B13, fig. l), but for the corresponding chloride and bromide the exact location of the fifth band is not clear. For the ~ ( b , ) and n(b,) halogen orbitals, any interaction with the rest of the molecule raises the energy of the former by anti- bonding interaction with the low lying Q orbitals, and depresses the energy of the latter

by interaction with the NMe, n(b,) orbital. The third and fourth IP are therefore assigned to a(bJ and n(b2), respectively. The n interaction energy increases in the order C1 <Br<I, whereas the separation of the a@,) and n(b2) levels [C1(0.74), Br (0.79), I (0.88 eV)J changes very little. This indicates that the a@,) destabilization decreases concomitantly as the halogen a&) orbital rises in energy.

In general, for compounds BX,Y3-, (n = 1 or 2) there will be significant

X/Y

n-type interaction only if the p(n) type 1P from X and Y are close (within ca. 2 eV). Thus, for the present series, only for X = NMe, and Y = I was there considerable n-type interaction. For X = Me and Y = halogen, the reverse may well be true. It is interesting that B-N bond lengths

Z

decrease from (Me2N)3B (1.43

A)

to Me,- NBC12 (1.379

A).’*

We predict that Me2NB12 has Zcloser to the former compound, and Me2NBF2 has

Z

similar or shorter than the latter. Similar conclusions may apply to force constants or bond energy terms in these molecules.

We thank Prof. R. N. Dixon and Dr. W. Schmidt for calculations, Prof.

S.

G . Gibbins for a gift of (Me,N),BI, the U.S. Air Force Office of Scientific Research for their support, and the S.R.C. for purchase of the spectrometer.

part 8, J. C. Baldwin, M. F. Lappert, J. B. Pedley and J. S. Poland, J.C.S. Dalton, 1972, in press.

M. F. Lappert, M. R. Litzow, J. B. Pedley, P. N. K. Riley and A. Tweedale, J. Chem. SOC. A ,

1968,3105.

M. F. Lappert, M. R. Litzow, J. B. Pedley, P. N. K. Riley, T. R. Spalding and J. A. Treverton,

J. Chem. SOC. A, 1970,2320.

M. F. Lappert, M. R. Litzow, J. B. Pedley, T. R. Spalding and H. NBth, J. Chem. SOC. A , 1971, 383.

P. J. Bassett and D. R. Lloyd, J. Chem. SOC. A , 1971, 1551.

H. Bock and W. Fuss, Chem. Ber., 1971,104, 1687.

A. H. Clark and G. A. Anderson, Chem. Comm., 1969,1082.

*

F. B. Clippard and L. S. Bartell, Inorg. Chem., 1970, 9, 2439.

G . H . K I N G ,

s.

s.

K R I S H N A M U R T H Y , M . F . L A P P E R T , J . B . PEDLEY 83 R. J. Brotherton, A. L. McCloskey, J. L. Boone and H. M. Manasevtit, J. Amer. Chem. SOC., 1960,82,6242.

l o A. J. Banister, N. N. Greenwood, B. P. Straugham and J. Walker, J. Chem. SOC., 1964, 995.

l 1 H. Noth, 2. anorg. Chem., 1963,322,297.

l 3 G. H. King, D. Phil. Thesis (University of Sussex, 1972).

l4 G. Herzberg, Molecular Spectra and Molecular Structure, vol. 11, Infiared and R a m Spectra

of Polyutomic Molecules (Van Nostrand, London, 1964), chap. 2, p. 178.

D. W, Turner, C. Baker, A. D. Baker and C. R. Brundle, Molecular Photoelectron Spectroscopy (Wiley-Interscience, London, 1970), chap. 6, p. 175.

D. R. Annstrong and P. G. Perkins, Theor. Chim. Acta, 1969,15,413. M. Atoji and W. N. Lipscomb. J. Chem. Phys., 1957,27, 195.

l6 W. Schmidt, p e r ~ 0 ~ 1 communication.

l 8 M. F. Guest, J. B. Pedley and W. Schmidt, unpublished results.

l9 ref. (15) chap. 10, p. 271.

'O G. H. King and A. F. Orchard, to be published.

''

D. W. Aubrey, M. F. Lappert and H. Pyszora, J. Chem. SOC., 1960, 5239.

''

Tables of Interatomic Distances and Configuration in Molecules and Ions (Chem. SOC. Spec.

Publ. No. 11, ed. L. E. Sutton) (The Chemical Society, London, 1958).

23 A. B. Cornford, D. C. Frost, F. G. Herring and C. A. McDowell, Canad. J. Chem., 1971,49, 1135.

24 B. Cetinkaya, G. H. King, S. S. Krishnamurthy, M. F. Lappert and J. B. Pedley, Chem. Comm.,

1971, 1370.