Lewis Structures of Boron Compounds Involving Multiple Bonding Darel K. Straub University of Pittsburgh, Pittsburgh, PA 15260 I n most general chemistry textbooks the discussion of Lewis structures i s unnecessarily complicated by the misuse made of the octet rule. (For textbooks consulted see ref. I.)Several pages are devoted to apparent "exceptions" to this rule, mainly involving boron compounds such as BF3 or BCl8 (which is shown with a n "incomplete octet" of six electrons around the boron) and oxygen-containing compounds of the nonmetals (oxides, oxoacids, oxoanions such as SO3, HzS04,Clod-, with a multitude of mesomeric and resonance forms involving anywhere from eight to 16 electrons around the central atom). Many of these discussions are misleading and some are incorrect. I t is the purpose of this paper to consider evidence for multiple bonding i n boron compounds. A snhsequent paper will consider oxygen bonds to other nonmetals. Discussion Much of t h e confusion regarding the supposed exceptions to the octet rule can be circumvented by considering i t to apply only to compounds of the 2p elements B, C, N, 0, and F, and then very strictly. That is, there are (almost) always eight electrons around the B, C, N, 0, and F atoms in their compounds, never more than eight, and less than eight only i n cases where eight is mathematically impossible, i.e., when there is a n odd number of valence electrons, as i n NO and NOz, or when there are not enough valence electrons. a s i n boron hvdrides. or com~oundslike (CH&B. . (Consideration of hyperconjugation is inappropriate i n a eeneral chemistrv context.) In this view. the octet rule takes precedence over electronegativity a n d formal charge concepts (2) for these elements but does not apply to compounds of the 3p, 4p, and 5p nonmetals. This modified intemretation i m ~ l i e sthe maximum amount of n-bondine .. (the! so-celled dative pn-pn bonding, in boron compounds. Thus. ~t mav overem~hasizethe extent of rnulti~lehondint( between b&on and the heavier 4p, 5p elements. despite its simplicity, i t i s remarkably effective i n predicting relative bond lengths (assuming that triple bonds are shortest, followed by double bonds, with single bonds longest). Data on measured bond lengths for representative boron-nitrogen, boron-oxygen, a n d boron-fluorine compounds are given i n Tables 1, 2, and 3. Included also are bond orders predicted by the Lewis structures that have octets of electrons around the boron atoms. There i s very good agreement i n all cases between the predicted bond order and the measured bond length. (Use of the bond notation BZX
-
ow eve;
indicates partial double-bond character due to two or more resonance forms.) There is also little overlap i n the ranges of bond len@hs for single, double, or triple bonds: BEN, B-N, 1.65-1.70A; BeO, 1.22-1.26 A; B=N, 1.34;1.41 l.18-l.20.&; B=O, -~1.35A;B-O, 1.47 A; B=F, 1.21-1.26 A; B-F, 1.37-1.40 A. There are good structural data for only one compound predicted to involve a B=F bond (HzBF); a n early electron diffraction study of (CH&BF
A;
-
494
Journal of Chemical Education
Table 1. Boron-Nitrogen Bond Lengths Compounda
Predicted Bond Order
tBUBNtBU tBu3CsHzBNtBu (CH+SkBNtBu NBN* HzBNHz ClzBNCsHs (mes)nBNHCsHs (mes)nBNHmes
B=N B=N B=N B=N B=N
HB(NHz)z
F
k N k N k N
N
Measured E N Bond Length. Aa 1.258(s) 1.232(s) 1.221(s) 1.336(s) 1.391(g) 1.379(s) 1.407(s) 1.406(s) 1.418(g)
Ref. b
c d
e f
b
9 9 h
HsBNH3 €LN 1.672(g) I HBN(CH3)3 E N 1.638(g) k F3BN(CHsh E N 1.674(g) I C13BN(CH3)3 E N 1.652(g) I Br3BN(CH3)3 E N 1.663(g) I (CH3)3BN(CH3)3 E N 1.698(g) in 'Abbreviations: tBu, 1-butyl:mes, mesityl; g, gaseous state;s, solid state. b~aeizald, P. A d v lnorg. Chem. 1987, 31, 123-170. 'Elter. G.; Neuhaus, M.;Meller, A,; Schmidt-Base,D. J. Organomst. Chem. 1990. 381.29%313. '~aase,M.; Klingebiel, V.; Boese, R.; Polk, M. Chem. Ber 1985, 119, 11171126. vamana. H.; Kikkawa, S.; Koimmi, M. J SolidSfate Chem. 1987, 71, 1-11. 'sugie, M.: Takea, H.: Malsurnura, C. J. Mol. Spectmc.1987, :23,288-292; Oniz. J. V Chem. Phys Len. 1989, 156489493. %hen, H.; Banlen, R.A.;Oimstead,M. M.: Power, P. P.: Sh0ner.S. C. J A m . Chem. Soc 1990, 112,1048-1055. ?home, L. R.; Gwinn, W. D.: J. Am. Chem. Soc 1982, 104,3822-3827. 'Harshbarger, W.; Lee. G.; Poner, R. F ; Bauer, S. H. lnorg. Chem. 1969, 8, 1683-1689. Thome, L. R.; Suenrum, R. D.; Lavas, F. J. J Chem. Phys. 1983, 78,167170. *~assoux,P.: Kuczkowski, R. L.; Bryan, P. S.; Taylor, R. C. loo= Chem. 1975, 14,128-129. 'lijirna, K.: Shibata, S. Bull. Chem. Soc. Japan 1980, 53, 1908-1911. "'Kuznesot, P. M.: Kuczkawski, R. L. Inorg. Chem. 1978, 17,2308-2311.
gave a B-F bond length of l.29A (3).For fractional n-bonding cases: B-X, X=N, 0 , F the BX bond lengths are close to B=X, rather than showing intermediate values between B-X and B=X; thus: BZN, 1.42-1.44 4 B-0, 1.36-1.37 & B-F, 1.31-1.33 A.
Table 3. Boron-Fluorine Bond Lengths
Table 2. Boron-Oxygen Bond Lengths Compound HBO
Predicted Bond Measured B-0 Order Bond Length. Aa BE0
FBO
1.201(9)
b
EEO
1.185(g)
C
ClBO
B S
1.I 79(9)
d
H2BOCH3
B=O
1.352(g)
e
(CH3)zBOB(CH3)2
B=O
1.359(g)
f
HB(OH$
wo
1.360(g)
9
FB(0H)z
-0
1.362(g)
9
B(OHk
B-0
1.368(s)
h
B(OCHd3
~-0
1.367(g)
i
(HBOzk
BZO
1.361(s)
i
(CsHsB0)3
W O
1.386(s)
k
CsHsB(OSi(CsHs)z)20
W
CsHsB(OSi(CsH5)2)30 F O (605)"
B-0
BfOHlZ
&O
1.370(s)
Compound
Ref.
k
1.359(s)
k
1.372(s)
I
1.470(s)
rn
'Abbreviations: g. gaseous state:5 , solid state. '~awashima.Y.; Endo,Y ; Himta. E. J M d Speamsc. 1989. 133,115127:3. S o . S. P J. Mol. Struct. (Theochem.)1985. 122,311-316. K.:Endo, Y.: Hirota. E. J. Mol Spectrosc.1982,93.381388. d~awaguchi, eKawashima.Y;Takw, H.; Matsurnura,C.J Mol. Speamsc. 1986. 116.2M2: Carpenter. J. D.; Ault. 6. S. J. Chem. Phys 1992,9642854294. underse sen, G.; Vahrenkarnp,H. J Mol. Struct. 1976. 33.97-105. 'Boggs. J. E.; Cordeli. F. R. J. Mol Struct. 1981, 76,329-347. kqhjhede, M.; Larsen. S.: Renrup. S. Ada. Cyst 1986,842,545552. 'Gundersen, G. J. Mol. Stmct. 1976,33, 7949. 'peters, C. R.: Milberg, M. E. Acta Cyst. 1964, 17,229-234. k~wcher, 0. A,; L0ugh.A.J.;Manners I. lnog Chem 1992.31.30343043;12. '~irtel. A. Acta Cyst. 1987,843, 333-343. mAverageof seven cases: ERenberger. H. Acta Cryst. 1982,838.8245.
This situation is reminiscent of cases involving multiple bonding betwee; carbon, nitrogen, and oxygen, e.g., NzO ( 4 )(N-N, 1.127 A; N-O,1.185 A), CNOJ50 (C-N, 1.153A; N-0, 1.228 or N j (6) (N-N, 1.176 A), where t h e bond lengths correspond to
A),
N=N, N=O, CEN, N=O, and NEN, respectively, but whose resonance hybrid structures a r e written
because of the octet limitation. The reluctance shown by college general chemistry textbook authors to consider n-bonding between boron a n d fluorine arises from the fact t h a t such bonding would put a positive formal charge on the fluorine, contrary to electronegativity considerations. (However, carbon monoxide is always shown a s : C a : , with a +1formal charge on the oxygen consistent with experimental data.) Pi bonding in BF3 is well-established, both experimentally and theoreti-
Predicted Bond Order
Measured B-F Bond Length, (A) (in gas)
Ref.
BF
B=F
1.263
a
HBV
BEF
1.210
b
FBN(CH3h
B-F
1.374
9
FBHzN(CHd3
B-F
1.402
h
BFi(solid)
F E -!
1.388
i
"Cazroli, G.; Ciudi, L.; Degli Esposti. C.; Dore. L. J Mol Spectmsc. 1989. 134. 159-167. aCazroli. G. Degli Esposti. C.; Dore. L.; Favero, P G. J. Mol. Spectrosc 1987. 121,275282. CTakeo,H.: Sugie. M.: Matsumura. C. J. Mol Spectrosc 1993. 158.201-207. d ~ a xA., P.;Hubbard.S.0.;WaterMd,S. J. Mol. Spectrosc.1986, 118,459670 .. .. %keo. H.; Curl, R. F. J. Chem. Phys. 1972,56,4314-4317. '~ovas.F J.: Johnson, D. R. J Chem. Phys. 1973,59,2347-2353. glijima,K.; Shibala,S. Bull. Chem Soc Japan 1980. 53. 1908-1913. ~ a s s o u xP.; , Kuczkowski. R. L.: Fong. G. D.: Geanangei. R. A. J. Mol. Sfrun 1978 ~.48 25-32~ ~Averaged wee ra Is haBF4 KBF, a n 0 Ca,BF, :. Brmton. G A m C~ysl. 1969 825 2161-2162 Jordan. T n . D crens B Schrosoer L W B r w n , W E Acta Cqsr 3975,831 664-672 ~~
~
cally, and is known widely to inorganic chemists (7).It is the extent of n-bonding in the boron trihalides (and not the "incomplete octet") t h a t accounts for the Lewis acidity ordering B13 > BBr3 > BC13 > BFa (8). Pi bonding also is present in H3B03 (9).The best Lewis structures for BF3 and H3B03a r e (FhB=F (3 resonance forms) and (H-OkB=O-H (three resonance forms). Both are isoelectronic with C03" and NO3-, with similar resonance forms. An especially instructive example involving multiple bonding between boron a n d fluorine i s provided by monomeric boron monofluoride, BF (10). This well-investigated diatomic molecule, isoelectronic with N2, CO, CN-, NO', has a dipole moment of 0.9 D with the fluorine a t the positiue end of the dipole. The very short B-F bond length indicates t h a t the single best structure is :BEE , with a +2 formal charge on the fluorine. When boron is bonded to two different potentially nbonding atoms (nitrogen, oxygen, or fluorine in t h e simplest cases), there arises ambiguity in selecting its single best mesomeric form. In such cases it is reasonable to assume, from both size and electronegativity considerations, that n-bonding to nitrogen is strongest, followed by n-bonding to oxygen, then n-bonding to fluorine. Thus, in a compound such a s FBO, the single form F-B=O contributes more heavily to the "real" structure t h a n the single form FIB-0 or the single form F=B=O. However, all n-bonded forms do contribute to the overall structure.
Volume 72 Number 6 June 1995
495
Table 4. Boron Bond Lengths with 3 p 5 p Nonmetals
Compounda PEP* (mes)zBP(CsHs)z (mes)SP(mes)n (mes)zBP(mes)H3PBH3 H3BP(CH3)3 H3BPHzCH3
Predicted 8-X Bond Order
Measured B-X Ref. Bond Lenqth. - Aa
B 3 B 3 B=S F S
1.767(s) 1.859(s) 1.839(s) 1.823(s) 1.937(g) 1.901(g) 1.906(g) 1.868(s) 1.931(s) 2.035(g) 1.599(g) 1.603(g) 1.779(g) 1.805(g)
B(SeCHd3
WSe
1.936(g)
BCI HzBCI HBCIz
BlCl
B=CI BxCI
1.715(g) 1.735(g) 1.735(g)
CHsBCIz
FCI
CsHsBCIz
FCI
BCI3
FCI
1.739(g)
(CH3)3NBC13 CH3BBrz
&CI BZBr
1.836(g) 1.908(g)
BBr3
BzBr
(CHd3NBBr3
&Br
ASBAS>
(mes)zBAsCsHsH3BAs(CH3)3 HBS CH3BS (CH3)zBSCH3 B(SCH3)3
B=P B=P B=P B=P &P B-P B--P B=AS B=As &As
B ~ I
B13
1.753(9)
1.893(g) 2.001 (g) 2.118(g)
6819.
b u r i g . J. R.: Li. Y. S.: Carreira. L. A,: Odom. J. 0. J Am. Chem. Soc. 1973, 95.2491-2496. 'Bryan, P S.: Kuczkowski,R. L. lnorg. Chem. 1972. 11.553-559. gPetrie,M. A : Olmstead. M. M.: Hope, H.: Baden. R. A,: Power, P. P. J Am. Chem Soc 1993. 115,3221-3226. 'DU", J. R.; Hudgens, B. A,: Odom, J. D. Inorg. Chem. 1974. 13. 2 3 0 6 2312. 'Grein. F. J Mol. Spectrosc. 1986. 115,47-57. 'Kirby C.: Krato, H. W J Mol. Speclrosc.1980, 83, 1-14. "Brendhaugen,K.: Nilssen. E. W.; Seip, H. M. Acla Chem Scand. 1973.27. 79651977~ ....
o oh an sen. R.: Nilssan, E. W.: Seip, H. M.; Siebert,W. Acta Chem. Scand.
1973.27.3015-3020. mLindey,S.; Seip. H. M.: Siebert,W. Acta Chem. Scand 1975, M9, 265272. - -
"Maki.A. G.: Lovas, F. J.; Suenram.R. D. J. Mol. Speclrosc. 198291,424429. 'Kawashima, Y : Takea. H.: Sugie, M.: Matsumura, C.: Hirota. E. J. Chem. Phys. 1993,9 9 , 8 2 8 2 6 . %ox, A. P.; Hubbard. S. D.; Waterfield. S. J Mol Specfmc. 1986. 118, 459-470. 'Iijima. K.; Shibata,S. Bull. Chem. Soc Japan 1980, 53, 1908--1913. 'Kakubari. H.; Konaka. S.: K i m u a , M. Bull. Chem. Soc Japan 1974. 47, 2337-2338. Journal of Chemical Education
B=P in PBP" (11), B=As in ASBA& (Il),(me~ityl)~BAsC~H, (121, B=S in CH3SB(CH3)2 (131, B2Se in (CH3Se),B (141, and
Pi interaction i n some BP (161, BS (171,and BCI (18) bonds can be as strong (perhaps in some cases even stronger) than x-interaction in BN, BO, and BF bonds, respectively. I t recently has been experimentally verified that the B-C1 double bond character in HzBCl is greater than i n HBCL although the B-C1 bond lengths are comparable (19). Even in bonds between boron and iodine, despite the great size disparity between the 2p and 5p orbitals, x-bonding has been postulated, based upon experimental evidence: e.g. in BIB(20) and i n boron cage compounds such a s BgIg (21). Triple bonds between boron and 3 p 5 p elements are predicted i n the present context, but their existence is more problematical. The Lewis structure H-B=S: predicts, correctly, a linear molecule with a very short B-S bond; calculations (22) indicate that the B-S bond has order 2.5, i.e.:
1.761(9)
'Abbreviations: mes, mesityl: g, gaseous state:s. solid state. bvon Schnering. H a . ; Somer, M.: Hameg, M.: Peten, K. Angew Chem., Int Ed. Engl 1990.29, 6547. Pestma. D. C.: Power, P P J. Am. Chem. Soc 1991. 113.84268437. "Bartlen,R. A,; Feng, X.: Power, P P. J. Am. Chem. Soc 1986, 108, 6817-
496
There is disagreement i n the literature over the strength of x-bonding between boron and the heavier ( 3 p 5 p ) nonmetals, although not i n the possibility or actuality that such bonding does occur. Some bond length data are given i n Table 4. Specific examples for which there is strong experimental and theoretical evidence for multiple bonding include
H-B=S: very similar to HBO. The structure :B=CI: for BC1 also predicts correctly a very short B-Cl bond and a dipole 'BCli+'(15).
'-
Conclusions The very common statement i n texts that the Lewis acidity of BF3 (and increasingly BC13 i s due to a n incomplete octet of six electrons around the boron, which electron deficiency is relieved by adduct formation with Lewis bases like F- or NH3, is incorrect. I n some texts, the three FzB=F resonance forms are shown, only to be discounted as "not preferred" or "not making sense"; whereas, they are the very forms corresponding closest to the experimentally determined structure. I n writing Lewis structures of compounds containing boron bonded to a p-block nonmetal, forms involving a n octet of electrons around the boron (if a t all possible), regardless of any resulting formal charges, are greatly preferable to incomplete octet forms, whose existence is suspect a t very least and misleading i n descriptions involving bond length, bond strength and restricted rotation about the bond to boron. Literature - Cited 1. Atkins. P W ; Berm, J. A. Oemml Chemistry. 2nd ed.; ScientificAmerican: New York, 1992:Bailar, Jr, J. C.:Moeller T.; Kleinberg. N.; Cuss. C. 0.; Castellion, M. ~~
E.; Metz, C. Chmnisfry, 3rd ed: HBJ:New York. 1989; Brady, J. E. Genaml Chem. istry, 5th ed.: Wiley: New York. 1990; B~ady,J. E.: Holum, J. R. Chemistry; Wiley: New York, 1993: wmwn. T. I.;LeMZ.5 Jr, H. E.; Bursten, 8. E. Chemistry, The Cmlml Sclencp, 5th ed.; Prentie+Hall: Englewod Cliffs. NJ, 1991; Chsng, R. Chemistry 4th ed.: McGraw-Hill: New York, 1991; Ebbing, D. D. Opnsmi Chamistry. 4th ed.;Houghtan Millin: Boston, 1992:Fine, L. W.; Beall. H. Chamistry ,for Engineers and Scientists; Saunders: Philadelphia, 1990; Gillespie. R. J.:Hum. phreys, D. A,: Baird, N. c.;Robinson, E. A. Chemistry, 2nd ed.:Ally3 and Bacon: Boston, 1989 Holteelaw Jr, H. F.: Robinson, W. R. Gonemi Chemistry. 8th ed.: Heath: Lexinpon, MA, 1989; Kask. U; b u n , J. D. Oemml Chomidry: Wm. C. Bm-: Dubuque, IA, 1993:Masterson.W. L.; Hurley C. N. Chrrnslry; Saunders: Philadelphia. 1989; Oxtoby. D. W: Naehtreib. N. H. Principier o f M d e m Chomistry,2nd ed.;Ssunders: Philadelphia, 1990; Pefrueei. R.H.; Hamood. W. S. Oenerol Chemistry, 6th ed.: Macmillsn: New York, 1993;Radel, S. R.; Nsvidi, M. H.Chrmistry, West Publishing Co.: NewYork. 199(1:Umland.J. B. Genemi Chsmislry, West Publishing Co: New York, 1993:Whitten, K W.; Gailey, K. D.; Davis. R. E. Oemml Cheminiry, 4th ed.; Saunders: Philadelphia, 1992.
