Lewis Acid Properties of Tris(pentafluorophenyl)borane. Structure and Bonding in L-B(C6F5)3 Complexes

Article abstract of DOI:10.1021/om981033e

A variety of donor adducts of tris(pentafluorophenyl)borane were experimentally generated by reaction of a Lewis base with an excess of B(C₆F₅)₅ in pentane. In this way, nitrile complexes (C₆F₅)₃B·NCR(R = CH₃ 1a, p-CH₃-C₆H₄ 1b, p-NO₂-C₆H₄ 1c), isonitrile complexes (C₆F₅)₃B·CNR (R = C(CH₃)₃ 3a, C(CH₃)₂CH₂C(CH₃)₃ 3b, 2,6-(CH₃)₂-C₆H₃ 3c), and the phosphine adduct (C₆F₅)₃B·P(C₆H ₅)₃ (6) could be prepared. The compounds were characterized by IR and NMR spectroscopy and by X-ray structure analyses (1a, 1c, 3a, 3b, and 6). Coordination of the nitriles as well as the isonitriles to the neutral Lewis acid leads to a substantial increase in the C≡N bond strength. This is evident from a marked shift of the v?C≡N IR band to higher wavenumbers, and this interpretation is supported by the small but experimentally significant decrease of the C≡N bond length observed by X-ray diffraction. The experimental work is complemented by a density functional study on the model complexes (C₆F₅)₃B·L, L = CNCH₃, NCCH₃, PH₃, CO. A detailed analysis revealed that the bonding in (C₆F₅)₃B·L complexes is mainly dominated by electrostatic interaction, which in turn is responsible for the observed structural and spectroscopic changes. In the context of this work, the bonding of the neutral B(C₆F₅)₃ Lewis acid is compared to the positively charged organometallic d₀-Cp₃M⁺ system (M = Zr, Hf). It was found that electrostatic effects are more pronounced for B(C₆F_s)₃ than for the transition metal fragments. The question as to the existence of a nonclassical main group carbonyl complex, (C₆F₅)₃B·CO, is addressed.

Full text of DOI:10.1021/om981033e

1724
Organometallics 1999, 18, 1724-1735
Lew is Acid P r op er ties of Tr is(p en ta flu or op h en yl)bor a n e.
Str u ctu r e a n d Bon d in g in L-B(C6F 5)3 Com p lexes
Heiko J acobsen,† Heinz Berke,*,† Steve Do¨ring,‡ Gerald Kehr,‡ Gerhard Erker,*,‡
Roland Fro¨hlich,‡,§ and Oliver Meyer‡,§
Anorganisch-Chemisches Institut, Universita¨t Zu¨rich-Irchel, Winterthurerstrasse 190,
CH-8057 Zu¨rich, and Organisch-Chemisches Institut, Universita¨t Mu¨nster,
Corrensstrasse 40, D-48149 Mu¨nster
Received December 21, 1998
A variety of donor adducts of tris(pentafluorophenyl)borane were experimentally generated
by reaction of a Lewis base with an excess of B(C6F5)3 in pentane. In this way, nitrile
complexes (C6F5)3B‚NCR (R ) CH3 1a , p-CH3-C6H4 1b, p-NO2-C6H4 1c), isonitrile complexes
(C6F5)3B‚CNR (R ) C(CH3)3 3a , C(CH3)2CH2C(CH3)3 3b, 2,6-(CH3)2-C6H3 3c), and the
phosphine adduct (C6F5)3B‚P(C6H5)3 (6) could be prepared. The compounds were characterized
by IR and NMR spectroscopy and by X-ray structure analyses (1a , 1c, 3a , 3b, and 6).
Coordination of the nitriles as well as the isonitriles to the neutral Lewis acid leads to a
substantial increase in the CtN bond strength. This is evident from a marked shift of the
ν˜CtN IR band to higher wavenumbers, and this interpretation is supported by the small but
experimentally significant decrease of the CtN bond length observed by X-ray diffraction.
The experimental work is complemented by a density functional study on the model
complexes (C6F5)3B‚L, L ) CNCH3, NCCH3, PH3, CO. A detailed analysis revealed that the
bonding in (C6F5)3B‚L complexes is mainly dominated by electrostatic interaction, which in
turn is responsible for the observed structural and spectroscopic changes. In the context of
this work, the bonding of the neutral B(C6F5)3 Lewis acid is compared to the positively charged
organometallic d0-Cp3M+ system (M ) Zr, Hf). It was found that electrostatic effects are
more pronounced for B(C6F5)3 than for the transition metal fragments. The question as to
the existence of a nonclassical main group carbonyl complex, (C6F5)3B‚CO, is addressed.
In tr od u ction
some thermodynamic stability. Initial attempts to pre-
pare this addition compound by Shepp and Bauer
proved unsuccessful;4c the authors estimated the bond-
forming reaction to be endothermic. It took more than
20 years before Klemperer and co-workers7 succeeded
in recording the microwave spectrum of F3B‚CO they
produced by supersonic expansion. Their studies con-
cluded that F3B‚CO is only weakly bound, with an
Since boranes are among the prototypes of Lewis
acids, the reaction of boron trihalides, e.g. BF3, with
strong electron donors such as NH3 provides a typical
textbook example1 of interactions between Lewis type
acids and bases. According to Pearson’s HSAB prin-
ciple,2 the two species mentioned above have to be
classified both as a hard acid and as a hard base.
Elaborating on HSAB theory, we would expect that
complexes between a soft acid and a soft base should
also lead to stable species. The classical example involv-
ing boron-based molecules is the H3B‚CO complex,
which has been known for about 60 years3 and has been
under intense scrutiny by both experimentalists4 and
theoreticians.5 The recent calculations by Bauschlicher
and Ricca6 on the energetics of H3B‚CO are expected to
reflect the most accurate results obtained to date.
The question arises whether complexes of hard boron
centers and soft bases, like F3B‚CO, might also possess
(4) (a) Bauer, S. H. J . Am. Chem. Soc. 1937, 59, 1804. (b) Gordy,
W.; Ring, H.; Burg, A. B. Phys. Rev. 1950, 78, 512. (c) Burg, A. B. J .
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Dedicated to Professor Helmut Werner on the occasion of his 65th
birthday.
† Universita¨t Zu¨rich-Irchel.
‡ Universita¨t Mu¨nster.
§ X-ray crystal structure analysis.
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10.1021/om981033e CCC: $18.00 © 1999 American Chemical Society
Publication on Web 04/09/1999

Structure and Bonding in L-B(C6F5)3 Complexes
Organometallics, Vol. 18, No. 9, 1999 1725
unusually long B-C bond distance. The experimental
findings could later be reproduced in the theoretical
calculations by Bauschlicher and Ricca,6 and others.5g,8
Very recently, Sluyts and van der Veken9 were able to
measure the enthalpy of formation for F3B‚CO, thus
confirming and quantifying the earlier results.
Those developments spawned our interest to inves-
tigate the Lewis acid properties of a related molecule,
which in the past decade “emerged from relative obscu-
rity to a position of prominence”, namely tris(pentafluo-
rophenyl)borane,10 B(C6F5)3. One of the discoveries
leading to the modern chemistry of B(C6F5)3 was cer-
tainly its effectiveness as an initiator of olefin polym-
erization, in combination with group IV metallocene
alkyls.11
cal”. Carbonyl stretches in these compounds are found
at higher wavenumbers than those of free CO, often by
more than 100 cm-1. Goldmann and Krogh-J espersen18
have analyzed the origin of this ν˜CO shift and found that
electrostatic effects, and not an enhanced σ donation,
are the responsible factors. It has been further shown
that the M-CO bond in cationic transition metal
carbonyls has a reduced π component compared to that
of neutral complexes.19 Within this context, we want to
address the question of whether (C6F5)3B‚CO might
indeed be considered a nonclassical main group carbonyl
complex and extend this problem to include other
representative ligands used in transition metal chem-
istry, such as phosphines, isonitriles, and nitriles.
To the best of our knowledge, only a few donor
adducts of B(C6F5)3 are structurally characterized in the
literature. Besides O-carbonyl adducts of C6H5C(O)R,
R ) H, Me, OEt,12 and N(iPr)2,12b and one N-imidazole
complex,13 there exist structures of two phosphine
adducts of tris(pentafluorophenyl)borane, (C6F5)3B‚
PH314 and (C6F5)3B‚PH2C(CH3)3.15 Here we will describe
the synthesis and crystal structure analysis of three
types of donor adducts of B(C6F5)3, namely isonitrile
complexes (C6F5)3B‚CNR, nitrile complexes (C6F5)3B‚
NCR, and the phosphine adduct (C6F5)3B‚P(C6H5)3. The
experimental work is complemented by theoretical
calculations on the model complexes (C6F5)3B‚L, L )
CNCH3, NCCH3, PH3, CO. The calculations are based
on density functional theory16 and are implemented to
provide a deeper understanding into the nature of the
Lewis type donor adducts.
The goal of the present study is two fold. As men-
tioned earlier, we want to investigate and classify the
Lewis acidity of B(C6F5)3 in comparison with prototypes
of soft and hard acids, BH3 and BF3, but we are also
interested in the donor behavior of classical π acceptor
ligands in transition metal chemistry, such as CO and
isonitriles. When these molecules coordinate to the tris-
(pentafluorophenyl)borane fragment, one might antici-
pate π contributions to be of minor importance only. In
a way, donor adducts such as (C6F5)3B‚CO might be
compared to the class of homoleptic metal carbonyl
cations,17 complexes that have been termed “nonclassi-
Resu lts a n d Discu ssion
Exp er im en ta l Stu d ies. Tris(pentafluorophenyl)bo-
rane was prepared for this study by a variation of the
procedure originally developed by Stone, Massey, and
Park.20 Pentafluorobromobenzene was converted to pen-
tafluorophenyllithium at low temperature. The poten-
tially explosive C6F5Li reagent was then reacted with
BCl3 under carefully controlled reaction conditions to
give pure B(C6F5)3, isolated in ca. 40% yield after
precipitation from pentane.
In the course of this study we have prepared and
isolated three nitrile adducts 1a -c and three isonitrile
addition products 3a -c of the organometallic Lewis acid
tris(pentafluorophenyl)borane. We had previously shown
by the NMR method of Childs et al.21 that B(C6F5)3 is a
slightly stronger Lewis acid than BF3. On the respective
scale, where BBr3 is assigned a relative Lewis acid
strength of 1.0, BCl3 (0.77) and SnCl4 (≈0.5) are framing
B(C6F5)3, to which a value of 0.72 was assigned.22
Therefore, it was not surprising that the Lewis acid
B(C6F5)3 rapidly forms an adduct upon treatment with
acetonitrile. The reaction was carried out in a way that
slightly less than 1 molar equiv of acetonitrile was
added to a solution of B(C6F5)3 in pentane at room
temperature. Under these conditions the adduct forma-
tion is quite rapid, and the H3C-CtN-B(C6F5)3 addi-
tion product 1a precipitates from the solution. The
adduct 1a was isolated in >95% yield. It shows a 11B
NMR signal at δ -10.3 ppm (in benzene-d6), which is
as expected for tetracoordinated boron in an organic
environment. Compound 1a exhibits a characteristic IR
band at ν˜ ) 2367 cm-1, which is to be compared with
the respective IR features of the free, uncoordinated
acetonitrile molecule [ν˜ ) 2292 cm-1 (CH3-CtN com-
bination) and ν˜ ) 2253 cm-1 (CtN stretch)].23
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See also: J ordan, R. F.; Dasher; W. E.; Echols, S. F. J . Am. Chem.
Soc. 1986, 108, 1718. (b) Li, L.; Marks, T. J . Organometallics 1998,
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Chem., Int. Ed. Engl. 1997, 36, 2402.

1726 Organometallics, Vol. 18, No. 9, 1999
J acobsen et al.
Single crystals of 1a suitable for an X-ray crystal
structure analysis were obtained by the diffusion method,
letting pentane slowly condense into a solution of 1a in
benzene-d6. The X-ray crystal structure analysis of 1a
confirms the formation of a 1:1 adduct between aceto-
nitrile and B(C6F5)3. The two components are connected
by means of a strong B-N bond (1.616(3) Å). The boron
atom is tetracoordinated. The average length of the
B-C(aryl) linkages amounts to 1.629(4) Å. The boron
coordination geometry is distorted tetrahedral with
average N-B-C(aryl) bonding angles of 104.0(2)° and
average C(aryl)-B-C(aryl) angles of 114.3(2)°. In the
crystal, the L-B(aryl)3 unit attains a chiral conforma-
tion, as is often observed for such borane adducts and
also for related triarylborate systems,25 but this relative
orientation of the aryl planes of 1a at boron is not
persistent in solution. In the crystal, the H3C2-C1tN-
[B] axis is close to linear, with bond angles C2-C1-N
(178.9(3)°) and C1-N-B (177.1(2)°) both being close to
180°. The C2-C1 bond length is 1.452(4) Å, and thus
in the expected C(sp3)-C(sp) single bond range. Most
noteworthy is the C1-N bond length of 1a , for which a
value of 1.124(3) Å was determined. We had recently
carried out a rather accurate crystal structure analysis
of the uncoordinated acetonitrile molecule.26 Crystals
for that specific study were obtained on a Laser zone
melting apparatus, as developed by Boese et al.,27 which
was connected to a diffractometer. For acetonitrile (R-
phase at 208 K) a value of 1.141(2) Å was obtained for
the -CtN triple bond. Thus we note that coordination
of acetonitrile to the neutral strong organometallic
Lewis acid tris(pentafluorophenyl)borane leads to a
measurable strengthening of the -CtN bond. Structur-
ally this effect is similar in magnitude as observed upon
complexation of acetonitrile to the positively charged d0-
configurated organometallic Lewis acid Cp3Zr+ (dCtN in
the complex [Cp3Zr-NtC-CH3]+ 2a : 1.126(5) Å).26 We
conclude that in both systems, the neutral acetonitrile-
B(C6F5)3 1a and the positively charged acetonitrile-
F igu r e 1. View of the molecular structure of the aceto-
nitrile/B(C6F5)3 adduct 1a in the crystal. Selected bond
lengths (Å) and angles (deg): B-N 1.616(3), B-C11 1.639-
(4), B-C21 1.625(4), B-C31 1.622(4), N-C1 1.124(3), C1-
C2 1.452(4); N-B-C11 102.5(2), N-B-C21 104.0(2), N-B-
C31 105.5(2), C11-B-C21 114.9(2), C11-B-C31 115.7(2),
C21-B-C31 112.4(2), B-N-C1 177.1(2), N-C1-C2 178.9-
(3).
Sch em e 1
+
ZrCp3 2a , the resulting nitrile -CtN triple bond
becomes stronger upon Lewis acid/Lewis base adduct
formation. Upon adduct formation in both systems, the
crystallographically determined dCtN value decreases
and the ν˜CtN band shifts considerably to higher wave-
numbers [∆ν˜ ) +114 cm-1 1a ; +59 cm-1 2a ].
Next we prepared the p-methylbenzonitrile/B(C6F5)3
adduct 1b. The compound was isolated in 77% yield and
characterized by elemental analysis and NMR and IR
spectroscopy. In this case, the IR ν˜CtN band is again
substantially shifted to higher wavenumbers on going
from the free nitrile [ν˜CtN ) 2227 cm-1 (neat)] to the
adduct [1b: ν˜CtN ) 2322 cm-1, in KBr, ∆ν˜ ) +95 cm-1].
We then examined p-O2N-C6H4-CtN-B(C6F5)3 1c
and found an increase of the ν˜CtN IR band by ∆ν˜ ) +99
cm-1 on going from the free nitrile to the tris(pentafluo-
rophenyl)borane adduct (1c: 2334 cm-1). The adduct 1c
was also characterized by X-ray diffraction. The bond
lengths and angles of 1c (see Figure 2) around boron
are very similar to those found for 1a (see above) (1c:
N6-B7 1.595(3) Å, average B-C(aryl) 1.632(3) Å, aver-
age N-B-C(aryl) angle 104.5(2)°, average C(aryl)-B-
C(aryl) angle 114.0(2)°). The B7-N6-C5-C43 moiety
is linear (angles B7-N6-C5 178.9(2)°, N6-C5-C43
178.3(2)°), and the C5tN6 bond is again very short at
1.135(3) Å.
(24) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson,
D. G.; Taylor, R. J . Chem. Soc., Dalton Trans. 1989, S1.
(25) Ahlers, W.; Temme, B.; Erker, G.; Fro¨hlich, R.; Fox, T. J .
Organomet. Chem. 1997, 527, 191.
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U. Chem. Ber. Recl. 1997, 130, 899.
(27) Brodalla, D.; Mootz, D.; Boese, R.; Osswald, W. J . Appl.
Crystallogr. 1985, 18, 316. Boese, R.; Nussbaumer, M. In situ Crystal-
lization Techniques. In Correlations, Transformations and Interactions
in Organic Crystal Chemistry; IUCr Crystallographic Symposia, Vol.
7; J ones, D. W., Katrusiak, A., Eds.; Oxford Science Publications:
Oxford, 1994; pp 20-38.
The tert-butylisocyanide/B(C6F5)3 adduct 3a was analo-
gously prepared by treatment of tris(pentafluorophenyl)-
borane with Me3C-NtC in pentane. The adduct was
isolated in 64% yield. Single crystals were obtained by
allowing pentane to very slowly condense into a benzene
solution of 3a . The typical 11B NMR chemical shift of
the addition product at δ ) -21.8 ppm indicates the
formation of a tetracoordinate boron species. The coor-
dination of the isonitrile again has caused a distorted

Structure and Bonding in L-B(C6F5)3 Complexes
Organometallics, Vol. 18, No. 9, 1999 1727
F igu r e 2. Molecular geometry of 1c in the crystal.
Selected bond lengths (Å) and angles (deg): B7-N6 1.595-
(3), B7-C80 1.634(3), B7-C90 1.626(3), B7-C100 1.640-
(3), N6-C5 1.135(3), C5-C43 1.431(3), C40-N2 1.477(3),
N2-O1 1.219(3), N2-O3 1.224(3); N6-B7-C80 104.6(2),
N6-B7-C90 104.6(2), N6-B7-C100 104.2(2), C80-B7-
C90 113.5(2), C80-B7-C100 112.5(2), C90-B7-C100
116.0(2), B7-N6-C5 178.9(2), N6-C5-C43 178.3(2), C40-
N2-O1 117.7(2), C40-N2-O3 116.8(2), O1-N2-O3 125.4-
(2).
F igu r e 4. Molecular structure of 3b. Selected bond lengths
(Å) and angles (deg): B-C1 1.624(4), B-C61 1.637(4),
B-C71 1.639(4), B-C81 1.624(4), C1-N2 1.136(3), N2-
C3 1.473(4), C3-C4 1.536(4), C4-C5 1.534(4); C1-B-C61
106.2 (2), C1-B-C71 105.1(2), C1-B-C81 105.1(2), C61-
B-C71 111.6(2), C61-B-C81 113.9(2), C71-B-C81 114.0-
(2), B-C1-N2 177.9(3), C1-N2-C3 172.7(3), N2-C3-C4
107.0(2), C3-C4-C5 123.5(3).
charged d0-Lewis acid Cp3Zr+: free Me3C-NtC and the
[Me3C-NtC-ZrCp3]+ complex 4a exhibit identical dCt
values within the experimental accuracy.29 Coordina-
N
tion of Me3C-NtC to the charged Cp3M+ systems leads
to a noticeable shift of the IR ν˜CtN band to higher
wavenumbers (free Me3C-NtC ν˜ ) 2140 cm-1; Zr-
complex 4a 29 2209 cm-1; Hf-complex 4b30 2211 cm-1
;
Me3C-NtC-BMe3 5 2247 cm-1). The corresponding
IR effect upon binding Me3C-NtC to B(C6F5)3 is much
larger, as was to be expected from the observed crystal-
lographic bond-shortening effect. The corresponding
isonitrile band in the Me3C-NtC-B(C6F5)3 addition
31
F igu r e 3. Molecular structure of the Me3C-NtC/addition
product 3a . Selected bond lengths (Å) and angles (deg):
B-C1 1.624(4), B-C11 1.632(4), B-C21 1.644(4), B-C31
1.623(4), C1-N2 1.133(3), N2-C3 1.470(4), C3-C4 1.517-
(4), C3-C5 1.512(4), C3-C6 1.507(5); C1-B-C11 104.5-
(2), C1-B-C21 103.1(2), C1-B-C31 107.6(2), C11-B-
C21 114.6(2), C11-B-C31 114.4(2), C21-B-C31 111.5(2),
B-C1-N2 174.7(3), C1-N2-C3 173.1(3), N2-C3-C4
105.1(3), N2-C3-C5 107.3(3), N2-C3-C6 107.4(3), C4-
C3-C5 112.4(3), C4-C3-C6 112.4(3), C5-C3-C6 11.7(3).
product 3a was found shifted to ν˜CtN ) 2310 cm-1
.
We have prepared two additional R-NtC/B(C6F5)3
adducts. The 2,6-dimethylphenylisocyanide/tris(pen-
tafluorophenyl)borane adduct 3c was characterized by
elemental analysis and spectroscopically. Again, a sub-
stantial IR ν˜CtN increase was observed (∆ ) +53 cm-1
,
ν˜(CtN) of 3c 2275 cm-1). The ∆ν˜(free RNtC/coordi-
nated RNtC) value of the Me3C-CH2CMe2-NtC/
B(C6F5)3 3b addition product is even larger at ca. +170
cm-1 (ν˜CtN of 3b 2301 cm-1). Compound 3b was also
characterized by an X-ray crystal structure analysis.
The essential structural parameters are very similar to
those of 3a (see Figure 4). The isonitrile-CtN bond
length also is slightly decreased upon coordinating
Me3C-CH2CMe2-NtC/B(C6F5)3 (d(C1-N2) ) 1.136(3)
Å found for 3b), which shows that the observed struc-
tural and spectroscopic trend in the series of compounds
investigated in this study appears experimentally con-
sistent.
tetrahedral coordination geometry around boron as
revealed by the X-ray crystal structure analysis of 3a .
The average C(aryl)-B-C(aryl) angle amounts to 113.5-
(2)°; the average C1-B-C(aryl) angle is 105.1(2)° (see
Figure 3). As expected, the isonitrile-boron moiety is
linear (angle N2-C1-B 174.7(3)°), with both the carbon
atom C1 and the nitrogen center N2 probably being sp-
hybridized (angle C1-N2-B 173.1(3)°). The newly
formed B-C1 linkage has a bond length of 1.624(4) Å,
which is slightly below the typical B-C(sp2) bond
lengths of the adjacent B-C(aryl) linkages (average
value 1.633(4) Å). The NtC bond length of the coordi-
nated isonitrile ligand in compound 3a is 1.133(3) Å (i.e.
C1-N2), which is ca. 1% shorter than the typical -Nt
C triple bond length of the uncoordinated aliphatic
isonitrile (1.145(5) Å).28 Although this reduction of the
NtC bond length in 3a may seem small, it is clearly
noticeable and it represents a larger effect than ob-
served upon tert-butylisonitrile coordination to the
Finally, the Ph3P-B(C6F5)3 addition product 6, which
had already been prepared by Stone et al. early on, was
also characterized by X-ray diffraction for comparison.
For details see Figure 5 and the Experimental Section.
(29) Brackemeyer, T.; Erker, G.; Fro¨hlich, R. Organometallics 1997,
16, 531.
(30) J acobsen, H.; Berke, H.; Brackemeyer, T.; Eisenbla¨tter, T.;
Erker, G.; Fro¨hlich, R.; Meyer, O.; Bergander, K. Helv. Chim. Acta
1998, 81, 1692.
(28) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen,
A. G.; Taylor, R. J . Chem. Soc., Perkin Trans. 1987, 2, S1.
(31) Casanova, J .; Schuster, R. E. Tetrahedron Lett. 1964, 405.

1728 Organometallics, Vol. 18, No. 9, 1999
J acobsen et al.
interaction, ∆Eelstat, the Pauli repulsion, ∆EPauli, and the
orbital interaction term, ∆Eint
:
BEsnap ) -[∆Eelstat + ∆EPauli + ∆Eint
]
(2)
Since electrostatic effects will play a major role in our
discussion, we will describe the first term of eq 2 in more
detail. ∆Eelstat describes the classical Coulomb interac-
tion between the unmodified and interpenetrating charge
distributions of fragments A and B:33
ZAZB
R
FA(1) FB(2)
|r1 - r2|
∆Eelstat
)
+
dr1 dr2 +
∫
F igu r e 5. Molecular structure of Ph3P-B(C6F5)3 6. Se-
lected bond lengths (Å) and angles (deg): P-B 2.180(6),
B-C80 1.633(7), B-C90 1.636(7), B-C100 1.639(7), P-C40
1.831(5), P-C50 1.818(5), P-C60 1.821(5); P-B-C80
105.5(3), P-B-C90 105.4(3), P-B-C100 105.0(3), C80-
B-C90 112.8(4), C80-B-C100 112.6(4), C90-B-C100
114.5(4), B-P-C40 113.5(2), B-P-C50 113.3(2), B-P-
C60 114.1(2), C40-P-C50 105.3(2), C40-P-C60 104.2-
(2), C50-P-C60 105.4(2).
F (1)VB dr + F (1)VA dr (3)
∫
∫
A
N
1
B
N
1
In eq 3, the first two terms are the repulsive nucleus-
nucleus and electron-electron interactions, while the
last two terms are attractive interactions between the
electron density on one nucleus and the positive point
charge of the other nucleus. ∆Eelstat is in general
attractive, since the second term decreases when A and
B start to overlap, leading to a dominance of the
electron-nucleus interaction terms.
∆EPauli in eq 2 is termed exchange repulsion or Pauli
repulsion and takes into account the destabilizing two-
orbital four-electron interactions between occupied or-
bitals on both fragments. ∆Eelstat and ∆EPauli are often
combined to yield the steric interaction term ∆E°, but
here we will deal with these contributions separately.
The last term in eq 2, ∆Eint, introduces the attractive
orbital interaction between occupied and virtual orbitals
on the two fragments and includes polarization and
charge-transfer contributions.
We conclude that the examples experimentally inves-
tigated in the course of this study show that coordina-
tion of the nitriles as well as the isonitriles to the
neutral Lewis acid tris(pentafluorophenyl)borane leads
to a substantial increase in the CtN bond strength. This
is evident from a marked shift of the ν˜CtN IR band to
higher wavenumbers, and this interpretation is sup-
ported by the observed small but experimentally sig-
nificant decrease of the CtN bond length observed by
X-ray diffraction. Both these effects are markedly
stronger for coordination of the nitriles and isonitriles
to the neutral B(C6F5)3 Lewis acid than to the positively
charged organometallic d0-Cp3M+ system (M ) Zr, Hf).
Electrostatic effects have quite convincingly been shown
to be dominant in the latter charged systems and must
probably be held responsible for the bond-strengthening
effect in the [Cp3Zr-CtNR]+ (4) and [Cp3Zr-NtCR]+
(2) systems. The question remains whether electrostatic
features are similarly effective in causing the analogous
but even more pronounced bond-strengthening effects
in the neutral adduct systems 1 and 3, respectively, or
if other reasons should be considered in these cases. A
detailed theoretical analysis has been carried out to
provide a reasonable answer to this problem.
To emphasize the importance of ∆Eelstat for chemical
bonding, one might combine ∆EPauli and ∆Eint into the
orbital term ∆Eorb
:
∆Eorb ) ∆EPauli + ∆Eint
(4)
This definition can be justified if we keep in mind that
both terms on the right side of eq 4 basically stem from
interactions between orbitals on fragments A and B; the
repulsive interaction between occupied orbitals on both
fragments gives rise to ∆EPauli, and the attractive
interaction between occupied orbitals on one center and
virtual ones on the other originates in ∆Eint. For the
bond snapping energy we now have
Th eor etica l Stu d ies. Bon d in g An a lysis. Help to
answer the question as posed above is provided by an
energy decomposition scheme, which we use in our
bonding analysis.32 We will thus begin the theoretical
discussion with a concise description of this particular
procedure. We will examine the B-L bond energy in
R3B-L complexes by examining the bond-forming reac-
tion between the Lewis acid and the Lewis base frag-
ments:
BEsnap ) -[∆Eelstat + ∆Eorb
]
(5)
Since the equilibrium geometry of the fragments
usually differs from their arrangement in the final
molecule, a geometric preparation energy ∆Eprep is
needed to get the fragments ready for bonding. For
R3B-L systems, this is mainly the energy required to
distort the BR3 units from the planar ground state to
the pyramidal fragment geometry. The bond energy BE
then writes as
R3B + L f R3B-L
(1)
The energy associated with reaction 1 is called the
bond snapping energy BEsnap. It can be broken down
into three main components, namely the electrostatic
BE ) BEsnap - ∆Eprep
(6)
(32) (a) Ziegler, T.; Rauk, A. Theor. Chim. Acta 1977, 46, 1. (b)
Ziegler, T. Rauk, A. Inorg. Chem. 1979, 18, 1558. (c) Ziegler, T. NATO
ASI 1992, C378, 367. (d) Bickelhaupt, F. M.; Nibbering, N. M. M.; van
Wezenbeek, E. M.; Baerends, E. J . J . Phys. Chem. 1992, 96, 4864.
(33) van den Hoeck, P. J .; Kleyn, A. W.; Baerends, E. J . Comments
At. Mol. Phys. 1989, 23, 93.

Structure and Bonding in L-B(C6F5)3 Complexes
Organometallics, Vol. 18, No. 9, 1999 1729
F igu r e 6. Optimized bond distances (in Å) and bond angles for the ground-state geometries of the fragment molecules.
F igu r e 7. Optimized DFT-BP86 geometries (distances in Å) for H3B‚CO, F3B‚CO, and F3B‚CO with fixed B-C bond
distances. Comparison with Bauschlicher and Ricca’s DFT-B3LYP calculation (italics, ref 6) and experimental values (in
parentheses, ref 4b (H3B‚CO) and ref 7 (F3B‚CO)).
If the bond energy is corrected for the difference in
zero-point energy of the final molecule and the consti-
tuting fragments, we obtain the bond dissociation en-
thalpy at 0 K, D°:
of the BR3 fragment is concerned. The calculations do
represent the fact that F3B‚CO possesses an unusually
long B-C bond distance, which is however underesti-
mated by roughly 0.1 Å. This discrepancy has been
6
reasoned before; since the potential well for the B-C
D° ) BE - ∆EZPE
(7)
bond stretch is extremely shallow, small errors in well
depth already result in a sizable error in the geometry.
Of further interest is the geometry of the CO ligand.
For the donor complex with BH3 we find that dCO
elongates by about 0.01 Å. The experimental bond
distance is essentially the same as in the free ligand34
(dCO ) 1.128 Å). For the complex with BF3, it can be
seen that both the CO and BF3 undergo only insignifi-
cant changes in geometry when forming the van der
Waals complex.
The geometric changes of the fragments under bond
formation will be of importance not only for bond
energetics but also for bond analysis. Therefore, we
present the optimized geometries of our fragment
molecules in Figure 6. For important structural param-
eters, references to experimental data can be found in
the following sections. For further information, we refer
the reader to the recent literature.6
Information about chemical bonding can be provided
by an analysis of the stretching frequencies. Of particu-
lar interest are the CO as well as the B-C stretching
modes, for which the wavenumbers are compiled in
Table 1.
Our calculations indicate that for H3B‚CO the wave-
number for the CO stretch is almost identical to that of
free CO, whereas in the experiment a shift to higher
wavenumbers by 25 cm-1 can be observed. Furthermore,
the B-C stretching mode is overestimated by about 87
Str u ctu r e a n d Bon d in g of H3B‚CO a n d F 3B‚CO.
Since H3B‚CO has received considerable attention in the
literature, we will refrain from a detailed discussion of
all the features of this molecule. We will present only a
concise comparison of the bonding characteristics of
H3B‚CO and F3B‚CO, to judge the performance of our
theoretical approach, and to set the stage for a discus-
sion of tris(pentafluorophenyl)borane donor complexes.
Optimized geometries for R3B‚CO complexes are
displayed in Figure 7.
Our values conform with the DFT-B3LYP results of
Bauschlicher and Ricca, and both calculations do agree
well with the experiment, at least as far as the geometry
(34) Huber, K.-P.; Herzberg, G. Molecular Spectra and Molecular
Structure. Vol IV. Constants of Diatomic Molecules; Van Nostrand
Reinhold: New York, 1979.

1730 Organometallics, Vol. 18, No. 9, 1999
J acobsen et al.
Ta ble 1. Selected Str etch in g Wa ven u m ber sa for
Although our calculated value for H3B‚CO lies within
the range of experimentally established values, it is
more likely that the true bond dissociation energy lies
around 90 kJ /mol. This is supported by the very ac-
curate coupled cluster calculations on the CCSD(T) level
by Bauschlicher and Rica, which yield for the bond
dissociation energy of H3B‚CO a value of 88 kJ /mol.6 It
seems that DFT calculations indeed overestimate the
H3B‚CO bond energy; this effect is somewhat stronger
at the BP86 than at the B3LYP level. For F3B‚CO, the
DFT calculations verify the experimental finding of a
very weak B-C linkage, on the order of magnitude of a
few kJ /mol.
H3B‚CO a n d F 3B‚CO
ν˜CtO
ν˜B-C
this work BRb
exp
this work BRb
exp
CO
2126
2211 2143c
2138.7d
H3B‚CO
F3B‚CO
2125
2147
2219 2164.7e
2236 2150.7d
778
73
753 691.4e
86
65(8)f
In cm-1
.
Ref 6. c Ref 34. Ref 35. Obtained in Ar-matrix at
a
b
d
T ) 15-30 K. Ref 4f. f Ref 7.
d
Ta ble 2. Bon d Dissocia tion En er giesa D° for
R3B‚CO Com p lexes, R ) H, F
BE
∆EZPE
D°
D° (BR)b
D°(expt)
We will now turn to the bond analysis, according to
eq 2. Besides the scheme presented here, there exist
several others in the literature, and H3B‚CO has been
subjected to the KM (Kitaura-Morokuma),5a,37 NEDA
(natural energy decomposition analysis),5e and CSOV
(constrained-space-orbital variation)6,38 approaches; the
latter was also applied to F3B‚CO. Since this molecule
is only weakly bound, as discussed before, we perform
the analysis with a fixed distance dC-B that corresponds
to the calculated values for the hypothetical complex
(C6F5)3B‚CO (vide supra). The results of this analysis
are summarized in Table 3, and the conclusions reached
are similar to those obtained from other bond analysis
schemes. Characteristic features for the BF3 complex
in comparison with its BH3 analogue are an increase
in electrostatic binding, a weaker orbital interaction,
and a stronger Pauli repulsion. The analysis for F3B‚
CO is somewhat arbitrary, since it is not performed for
the equilibrium geometry. The general trends, however,
become clear. It should be noted that in terms of BEsnap
both molecules H3B‚CO and F3B‚CO are energetically
stable. It is the contribution of the preparation energy
∆Eprep that renders a geometry with a short B-C
contact unstable in the case of F3B‚CO.
The nature of the electronic interaction energies
might be analyzed in terms of a Mulliken fragment
population analysis. The total population of the formerly
empty virtual orbitals of the BR3 fragment in the final
molecule serves as a measure for σ donation, whereas
the electron donation to the virtual orbitals of the CO
molecule indicates the amount of π back-donation. BH3
and BF3 are comparably good electron acceptors; the
somewhat higher value for σ donation in the case of BH3
is explained by the shorter B-C bond distance. Our
analysis reveals that in the case of BH3 there is also π
back-bonding to the CO moiety, which stems from a
combination of ligand-based orbitals having the ap-
propriate symmetry properties. This effect induces an
elongation of the CO bond length, which counteracts the
electrostatic CO bond contraction. Overall, the calcu-
lated CO bond length in H3B‚CO appears to be slightly
elongated. In comparison with the experimental results,
H3B‚CO
155
13
142
114
141c
92d
97 ( 8e
e99f
79g
F3B‚CO
5
2
3
11
7.6 ( 0.3h
a
In kJ /mol. If necessary, literature values have been trans-
formed and rounded. Ref 6. DFT-B3LYP. c Ref 4k. Ref 4i. e Ref
b
d
4h. f Ref 4 g. Ref 4e. Ref 9.
g
h
cm-1. The results for F3B‚CO show better agreement
with the experiment. The observed shift to higher
wavenumbers for ν˜CtO is well reproduced, and the B-C
stretching mode at very low energy also closely matches
the result from a microwave study.35 Bauschlicher and
Ricca’s CO stretches show better qualitative agreement
with the experimental trends, although in absolute
terms the BP86 calculation is closer to the measured
values.
Goldmann and Krogh-J espersen18,36 have thoroughly
analyzed the origin of the increase in the CtO stretch-
ing force constant FCO, which corresponds to an increase
in ν˜CtO and a decrease in the CtO bond length upon
coordination of carbon monoxide to cationic species.
They conclude that electrostatic effects rather than
donation from the 5σ orbital of CO are responsible for
these effects. Although σ donation is an important factor
in the bonding of CO to transition metal centers,
Goldmann and Krogh-J espersen found that FCO values
can be quantitatively interpreted by using a model only
invoking M-CO π back-bonding and an electrostatic
parameter. In connection with this problem, they also
discuss the H3B‚CO molecule, and we refer the reader
back to their work18 for further details.
In what follows, we try to adapt some of the qualita-
tive concepts connected with the examination carried
out by Goldmann and Krogh-J espersen. At the onset,
we note that from our frequency analysis we might
conclude that the calculated B-C bond strength is
probably too high (compare ν˜B-C, experimental and
theoretical values) and that the reason for this is an
overestimation of π back-bonding, since an increase in
π bonding will strengthen the B-C bond, but will also
cancel the effect of increasing ν˜CtO due to electrostatic
effects.
it seems that it is this particular contribution to ∆Eint
,
namely the π back-donation, that is overestimated in
the DFT calculations, as previously concluded. The
back-bonding in F3B‚CO is present, but less pronounced
than that for H3B‚CO. The σ/π ratio for the first
amounts to 3.6, compared to 1.3 for the second. Conse-
In Table 2, computed dissociation energies and ex-
perimental data are presented.
(35) Gebicki, J .; Liang, J . J . Mol. Struct. 1984, 117, 283.
(36) A study similar to that of ref 18 was presented by Frenking
and co-workers: Lupinetti, A. J .; Fau, S.; Frenking, G.; Strauss, S. H.
J . Chem. Phys. 1997, 101, 9551.
(37) Kitaura, K.; Morokuma, K. Int. J . Quantum Chem. 1976, 10,
325.
(38) Bagus, P. W.; Hermann, K.; Bauschlicher, C. W., J r. J . Chem.
Phys. 1984, 80, 4378.

Structure and Bonding in L-B(C6F5)3 Complexes
Organometallics, Vol. 18, No. 9, 1999 1731
lengths for the isonitrile, nitrile, and carbonyl com-
plexes. We will limit our discussion to these two
parameters, which offer some insight into the chemical
bonding in these complexes. It should further be men-
tioned that in all cases the agreement between the
calculated and measured structures of the B(C6F5)3
units is high, with deviations of about 0.01 Å and 1° for
bond lengths and bond angles, respectively.
For the isonitrile complex, the calculation slightly
underestimates the B-L bond distance by about 0.01
Å, whereas for the boron nitrile adduct the computed
distance dBN falls short by 0.04 Å compared to the
experiment. The opposite effect is found for the phos-
phine complex, for which the calculation overestimates
the B-P separation by roughly the same amount
instead. One of the donor-acceptor bond characteristics
of BF3 is that the interatomic distances in the gas phase
tend to be larger than those for the solid state. An
intriguing example of this is the complex F3B‚NCCH3,
which has a B-N bond length of 2.011 Å in the gas
phase39 and a 1.630 Å bond length in the solid state.40
We might expect a similar but less pronounced effect
in tris(pentafluorophenyl)borane complexes, and on the
basis of these observations we could explain the longer
B-P bond of the gas-phase calculation as compared to
the solid-state structure. The fact that the gas-phase
calculations for the isonitrile and nitrile complexes yield
short B-L bonds might again indicate that our calcula-
tions overestimate the B-L bond strength in systems
containing good π acceptor ligands. On coordination to
the boron center, the angle (HPH) opens up by roughly
10°; the same is found in the experiment, where the
angle (HPH) changes from 93.345° in the free ligand41
to about 104° in the (C6F5)3B adduct. This indicates a
rehybridization of the P-center, which adopts a quasi
tetrahedral coordination. The same effect, although less
pronounced, is observed in (C6F5)3B‚PPh3, where the
angle changes from 102° for the free ligand42 to 105° in
the complex. Not only does the increased steric bulk of
the phosphine ligand influence the cone angle but it also
leads to a lengthening of the B-P bond, which in the
case of (C6F5)3B‚PPh3 is about 0.1 Å longer than in
phosphine. This highlights the fact that the potential
energy surface around the B-L equilibrium distance is
shallow, allowing for a large variation in the bond
length, a characteristic feature of dative bonds when
contrasted with covalent bonds.43
F igu r e 8. Selected geometric parameters (averged values,
distances in Å) for various (C6F5)3B‚L complexes from DFT
calculations and X-ray structure analysis (in parentheses).
For the isonitrile complex, the experimental values refer
to (C6F5)3B‚NC-C(CH3)3.
quently, the electrostatic contraction dominates, and we
find a shortened CO bond for the hypothetical F3B‚CO
geometry. We wish to reiterate that dominance of the
electrostatic terms in the bond energy over contributions
due to π back-bonding causes contraction of the CO bond
length in accordance with a shift of the ν˜CO stretch to
higher wavenumbers.
This in turn leads us to question how tris(pentafluo-
rophenyl)borane, B(C6F5)3, might interact with CO.
Intuitively, we would expect that B(C6F5)3 would un-
dergo strong electrostatic interactions comparable to or
even higher than those of BF3. On the other hand, we
would also anticipate a stronger orbital interaction due
to an enhanced π component, since the C-based orbitals
responsible for donation in B(C6F5)3 are higher in energy
than their F-based counterparts in BF3 and do provide
a better energetic match with the accepting π orbitals
of the CO group. Before we analyze the B(C6F5)3 bonding
with prototypical donor-acceptor ligands in detail, we
will begin the next section with a discussion of the
optimized molecular geometries when compared with
experimental results obtained from X-ray structure
analysis.
Both the experimental and the theoretical studies
result in a shortening of the CtX triple bond when
CNCR3, NCCH3, or CO coordinates to (C6F5)3B. These
changes are compiled in Table 4.
The bond contraction ∆dCtX is compatible for experi-
ment and theory and lies within 0.01-0.02 Å. The
calculation suggests that the contraction should be
larger for the isonitrile than for the nitrile, whereas the
(39) Dvorak, M. A.; Ford, R. S.; Suenram, R. D.; Lovas, F. J .;
Leopold, K. R. J . Am. Chem. Soc. 1992, 114, 108.
(40) (a) Hoard, J . L.; Owen, T. B., Buzzell, A, Salmon, O. N. Acta
Crystallogr. 1950, 3, 130. (b) Swanson, B.; Shriver, D. F.; Ivers, J . A.
Inorg. Chem. 1969, 8, 2182.
(41) Helms, D. A.; Gordy, W. J . Mol. Spectrosc. 1977, 66, 206.
(42) (a) Daly, J . J . J . Chem. Soc. 1964, 3799. (b) Dunne, B. J .; Orpen,
A. G. Acta Crystallogr. 1991, 47C, 345. (c) Cheklov, A. N. Kristal-
lografiya 1993, 38, 79. (d) Bruckmann, J .; Kru¨ger, C.; Lutz, F. Z.
Naturforsch. 1995, 50B, 351.
(43) Haaland, A. Angew. Chem. 1989, 101, 1017; Angew. Chem., Int.
Ed. Engl. 1989, 30, 1160.
Str u ctu r es of Don or Com p lexes of Tr is(p en -
ta flu or op h en yl)bor a n e. The calculated structures of
(C6F5)3B‚L complexes, L ) CNCR3, NCCH3, PH3, CO,
together with data from crystal diffraction are displayed
in Figure 8.
The most interesting structural parameters are the
B-L bond distances, as well as the CtX triple bond

1732 Organometallics, Vol. 18, No. 9, 1999
J acobsen et al.
Ta ble 3. Bon d An a lysisa a n d Mu llik en P op u la tion sb by Vir tu a l F r a gm en t Or bita ls for R3B‚CO Com p lexes,
R ) H, F
∆Eelstat
∆EPauli
∆Eint
BEsnap
∆Eprep
BE
σ(COfB)d
π(BfCO)d
H3B‚CO
F3B‚COc
-310
-327
635
670
-536
-386
211
43
56
86
155
-43
0.58
0.50
0.44
0.14
a
b
d
Energies in kJ /mol. In au. c Geometry optimized with fixed B-C bond distance (see text). B ) H3B or F3B, respectively.
Ta ble 4. Ch a n ges in th e CtX Bon d Len gth of
CNCR3, NCCH3, a n d CO u n d er Coor d in a tion to
(C6F 5)3B
∆G ≈ 0 kJ /mol at 200 K in the gas phase. This suggests
that the complex (C6F5)3B‚CO does not possess much
thermodynamic stability. The crystal environment,
however, might substantially stabilize the (C6F5)3B‚CO
molecule (vide infra) and might allow for its isolation
in the solid state.
dCtX experimenta
dCtX theorya
free coordinated
∆
free coordinated
∆
R3B-CNCMe3 1.145b
R3B-CNMe
R3B-NCMe
R3B-CO
1.133
1.124
0.012
1.176
1.156
1.151
1.131
0.020
0.010
0.005
The interaction between (C6F5)3B and CO also allows
us to assess the bonding properties of this Lewis acid
in comparison to the prototypes for the respectively soft
and hard acids BH3 and BF3. As we anticipated earlier,
the electrostatic component of the B-L bond even
exceeds the value for BF3 and BH3 by roughly 70 kJ /
mol. We also find that the term ∆Eint is comparable to
that for BH3 and significantly larger than that of BF3.
However, due to the extended size, and therefore to the
extended electronic core, we also find an increase in
Pauli repulsion so that (C6F5)3B ranks between BH3 and
BF3 in terms of overall bonding energy.
1.141c
0.017 1.161
1.136
a
b
In Å. Ref 28. c Ref 26.
Ta ble 5. Bon d An a lysisa for Va r iou s (C6F 5)3B‚L
Com p lexes
∆Eelstat ∆EPauli ∆Eint BEsnap ∆Eprep BE
R3B-CNMe
R3B-NCMe
R3B-PH3
R3B-CO
-483
-413
-330
-385
839
742
603
791
-562
-483
-419
-519
206
154
146
113
94
90
94
75
112
64
52
38
a
In kJ /mol.
A somewhat different situation is found for the
phosphine ligand. We now have significant smaller
bonding terms ∆Eelstat and ∆Eint, but also a smaller
repulsion term ∆EPauli. This is a direct consequence of
the much longer B-L bond distance. As a final result,
the subtle balance between attractive and repulsive
bond terms places the PH3 ligand between NCCH3 and
CO in terms of the B-L bond strength (see Table 5).
The strength of the B-P bond in (C6F5)3B‚PH3 has
been estimated in experimental studies. Crystals of this
compound lose phosphine when heated in vacuo, and
static vapor pressure measurements suggest an en-
thalpy of dissociation of about 80 kJ /mol.14 This value
is about one and a half times higher than the (uncor-
rected) computed value, which should decrease further
when the zero-point energy correction is taken into
account. A possible explanation for this may be that the
crystal environment prevents a spontaneous relaxation
of the (C6F5)3B into its equilibrium geometry. This would
imply that for an estimate of BE(B-P) we only have to
consider the preparation energy of PH3. When doing so,
we get for BE(B-P) a value of 129 kJ /mol. Applying
zero-point energy and temperature correction, as esti-
mated from a calculation on H3B‚PH3, we obtain for the
enthalpy of bond dissociation at 298 K ∆H ≈ 100 kJ /
mol, in fair agreement with the experimental value.
Weak B-L bonds might thus be substantially stabilized
in a crystal environment.
experiment shows a somewhat larger contraction for
acetonitrile compared to tert-butylisonitrile. The steric
bulk of the tert-butyl group offers a possible explanation
for this observation, as well as for the unusual long C-N
bond when compared with the computed values (com-
pare Figures 6 and 8). All this indicates that again
electrostatic effects are of importance in the bonding of
(C6F5)3B‚L complexes, which we will analyze in the next
section.
Bon d in g of Don or Com p lexes of Tr is(p en ta flu o-
r op h en yl)bor a n e. The results of our bonding analysis
according to eq 2 are collected in Table 5. We will first
analyze the bonding for the three CtX donors. The
terms ∆Eint, ∆EPauli and ∆Eelstat are on the same order
of magnitude partly with different signs and show
similar differences, but only the last term parallels the
observed order of coordination effects. It is thus the
relative size of the electrostatic energy (CNMe > NCMe
> CO) and not the orbital interaction energy (CNMe >
CO > NCMe) that determines the ranking in the bond
snapping energy (CNMe > NCMe > CO). Furthermore,
the relative differences in ∆Eelstat for the different donors
L are similar to those found for BEsnap. From this we
might conclude that the bonding in (C6F5)3B‚L com-
plexes indeed is dominated by electrostatic effects. The
formation of all complexes requires the distortion of the
planar (C6F5)3B molecule into its pyramidal fragment
geometry, which leads to sizable contribution of the
preparation energy, and leads to a weakening of the
B-L by about 75-100 kJ /mol. Thus, the complex
(C6F5)3B‚CO has a bond energy of only 38 kJ /mol and
is rather weakly bound. If we apply the zero point
energy correction, taking the H3B‚CO value as a first
estimate, we arrive at a value of 25 kJ /mol for the bond
dissociation enthalpy at 0 K. If we further take entropy
terms into accountsagain obtained as estimates from
H3B‚COswe get for the reaction (C6F5)3B + CO f
(C6F5)3B‚CO a value of ∆G ≈ 40 kJ /mol at 298 K and
We conclude this section with a closer look at the
orbital interaction term ∆Eint, and we will once more
use a Mulliken fragment population analysis to assess
the relative importance of σ donation vs π back-
donation. Our results are compiled in Table 6. For
CtX ligands, the σ donation is about 5 times greater
than the π back-donation and represents the dominant
orbital interaction. This is in sharp contrast with the
case of classical transition metal complexes, in which
the π component is equal to, or more important than,

Structure and Bonding in L-B(C6F5)3 Complexes
Organometallics, Vol. 18, No. 9, 1999 1733
Ta ble 6. Mu llik en F r a gm en t P op u la tion An a lysisa
for Va r iou s (C6F 5)3B‚L Com p lexes
σ(COfB)b
π(BfCO)b
σ/π
R3B-CNMe
R3B-NCMe
R3B-PH3
R3B-CO
0.66
0.53
0.60
0.58
0.14
0.10
0.16
0.22
4.71
5.30
3.75
2.64
a
b
In au. B ) (C6F5)3B.
the σ contribution.19,44,45 Nevertheless, a π component
is present, and its nature is similar to that of the one
discussed for BH3. In relation to the σ/π ratio, we would
like to present another argument which corroborates our
findings for electrostatic effects being responsible for Ct
X bond shortening. One common argument used to
explain a C-X distance decrease under complex forma-
tion is that of increased σ donation. The 5σ orbital in
CO is assumed to possess some C-O antibonding
character.46 Hence, OCfB donation should remove
electron density from this orbital and consequently
strengthen and shorten the bond. On the other hand, π
back-donation counteracts this effect. This would imply
that the ligand showing the highest σ/π ratio should
show the largest bond contraction ∆dCtX. In contrast,
our calculation show that ∆dCtX does not correlate with
σ/π ratio, but rather with the ∆Eelstat size, in agreement
with Goldmann and Krogh-J espersen.18 The experimen-
tal data obtained from crystal structures do not quite
confirm this trend; but the change in the ν˜CtX might
clarify the picture. For the nitrile adducts, we find for
∆ν˜CtN values of about 100 cm-1, whereas for the
isonitriles, with the exception of 3c, ∆ν˜CtN increases to
about 170 cm-1. This observation reflects the theoretical
F igu r e 9. Energy decomposition (in kJ /mol) of the M-L
bond for (C6F5)3B‚L and [Cp3Zr‚L]+ complexes according
to orbital terms and electrostatic terms. The negative of
∆Eorb and ∆Eelstat is shown, so that in any case positive
energy values indicate stabilization, and vice versa. O
stands for ∆Eorb, E for ∆Eelstat, and B for the bond snapping
energy BEsnap
.
trend found for ∆Eelstat
.
Com p a r ison w it h Ca t ion ic Tr a n sit ion Met a l
System s. As we have noticed earlier, the effects of a
shift of ν˜CtN to higher wavenumbers and a shortening
of the dCtN are markedly stronger for coordination of
the nitriles and isonitriles to the neutral B(C6F5)3 Lewis
acid than to the positively charged organometallic d0-
Cp3Zr+ system. The bond analysis for the latter systems
also brought to light the importance of electrostatic
effects.30 This points to a necessary comparison between
the main group complexes R3B‚L and the transition
metal complexes [R3Zr‚L]+. The results of the bonding
analysis according to orbital terms and electrostatic
terms (eq 5) are presented in Figure 9.
It might be surprising that the ∆Eelstat component is
higher for the boron than for the zirconium complexes,
even though the latter ones carry a positive charge. This
is caused by the longer Zr-L bond of about 2.30 Å,
which enters the electrostatic term as 1/r. As anticipated
in the experiment, an enhanced electrostatic component
seems to be responsible for more pronounced ∆ν˜CtN and
∆dCtN. Furthermore, we have not only an increase in
∆Eelstat but an enhanced Pauli repulsion as well, which
is the dominant contribution to ∆Eorb. It is remarkable
that the trend and the magnitude of BEsnap are es-
sentially the same for the [R3Zr‚L]+ and for the R3B‚L
complexes. The bonding characteristics of the main
group and the transition metal compounds appear to
be strikingly similar, but the variation in ∆Eelstat
manifests itself in the trends as mentioned above.
The difference in Zr-L and B-L bonding is contrib-
uted by the preparation energy. The [Cp3Zr]+ fragment
does not significantly change its geometry under coor-
dination of the Lewis base. The acceptor orbital is an
2
empty, metal-based dz orbital, and rehybridization is
not required as it is in the R3B case. Thus, BEsnap is a
good approximation of the bond energy BE. The Zr-L
bonds are about 75-100 kJ /mol more stable than B-L
linkages, and the transition metal complex is more
likely to be formed.
Con clu sion
We have prepared and characterized a series of donor
adducts of tris(pentafluorophenyl)borane. For the nitrile
and isonitrile complexes, it was found that dCtN short-
ens and that ν˜CtN is shifted to higher wavenumbers
under adduct formation. A theoretical analysis revealed
that the bonding in (C6F5)3B‚L complexes is mainly
dominated by electrostatic interactions, which in turn
are responsible for the observed structural and spec-
troscopic changes.19 In this respect, B(C6F5)3 is similar
to the hard Lewis acid BF3, but an enhanced orbital
interaction provides additional bond stabilization. In
contrast to BF3, the complex with carbon monoxide
possesses some intrinsic thermodynamic stability and
might be further stabilized by a crystal environment.
(44) Kraatz, H.-B.; J acobsen, H.; Ziegler, T.; Boorman, P. M.
Organometallics 1993, 12, 76.
(45) (a) Li, J .; Schreckenbach, G.; Ziegler, T. J . Am. Chem. Soc. 1995,
117, 486. (b) Ziegler, T.; Tschinke, V.; Ursenbach, C. J . Am. Chem.
Soc. 1987, 109, 4825.
(46) Albright, T. A.; Burdett, J . K.; Whangboo, M.-H. Orbital
Interactions in Chemistry; J ohn Wiley: New York, 1985; p 78.

1734 Organometallics, Vol. 18, No. 9, 1999
J acobsen et al.
Gen er a l P r oced u r e for th e P r ep a r a tion of th e Com -
p lexes 1a , 1b, 3a , 3b, a n d 3c. To a solution of tris(pentafluo-
rophenyl)borane in 100 mL of pentane was added dropwise a
solution of the Lewis base (ca. 10% excess) in the same volume
of pentane. The product began to precipitate immediately.
After further stirring for 2 h at room temperature, the solid
product was isolated by filtration, washed twice with 20 mL
of pentane, and dried in vacuo. The products were obtained
as colorless powders.
The bonding characteristics of (C6F5)3B‚L complexes are
very similar to that of [R3Zr‚L]+, and the hypothetical
(C6F5)3B‚CO can indeed be looked at as the main group
analogue of a (nonclassical) transition metal carbonyl
complex.
Com p u ta tion a l P r oced u r e
All calculations are based on the local density approximation
(LDA) in the parametrization of Vosko, Wilk, and Nussair,47
with the addition of gradient corrections due to Becke48 and
Perdew49 (BP86), which were included self-consistently (NL-
SCF). The calculations utilized the Amsterdam Density Func-
tional package ADF,50 release 2.3. Use was made of the frozen
core approximation. For all elements, except for H and for the
atoms of the pentafluorophenyl group, the valence shells were
described using a triple ú-STO basis, augmented by one d and
one f STO polarization function (ADF database V). H was
treated with a triple-ú STO basis and one additional p and d
STO polarization function (ADF database V), whereas for the
members of the C6F5 moiety a double-ú STO basis with one d
polarization function was employed. For systems containing
the BH3 or BF3 unit, calculations were performed in C3v
symmetry, the accuracy parameter for the numerical inte-
gration50b was chosen as 6.0, and final gradients in the
geometry optimization were less than 2.5 × 10-4 au/Å. For
B(C6F5)3 systems, no symmetry constraints were employed, the
numerical accuracy was set to 4.5, and final gradients were
2.5 × 10-3 au/Å and better. Default settings51 were used for
frequency calculations. The computations were carried out on
DEC AlphaStations 500.
Tr is(p en t a flu or op h en yl)b or a n e-Acet on it r ile, Com -
p lex 1a . Tris(pentafluorophenyl)borane (1.00 g, 1.95 mmol)
and 90 mg (2.19 mmol) of acetonitrile give the product 1a .
Yield of 1a : 1.02 g (94%), mp 219 °C (DSC). C20H3BF15
N
(553.0): calcd C 43.43, H 0.55, N 2.53; found C 42.67, H 0.83,
N 2.00. 1H NMR (200.1 MHz, benzene-d6): δ ) 0.32 (s, 3H,
1
CH3). 13C NMR (50.3 MHz, benzene-d6): δ ) 148.5 (dm, J CF
1
) 246 Hz, B(C6F5)3 (o-C)), 141.0 (dm, J CF ) 252 Hz, B(C6F5)3
(p-C)), 137.8 (dm, 1J CF ) 254 Hz, B(C6F5)3 (m-C)), -0.89 (CH3),
not observed CtN and B(C6F5)3 (ipso-C). 11B NMR (64.2 MHz,
benzene-d6): δ ) -10.3 (ν1/2 ) 330 ( 10 Hz). 19F NMR (282.4
MHz, benzene-d6): δ ) -134.7 (m, 2F, (m-F)), -154.8 (broad,
1F, (p-F)), -163.2 (m, 2F, (o-F)). IR (KBr): ν˜ ) 2367 (CtN)
cm-1. X-ray crystal structure analysis of 1a : formula C20H3-
NBF15, M ) 553.04, colorless, 0.40 × 0.20 × 0.10 mm, a )
10.944(1) Å, b ) 9.288(1) Å, c ) 19.384(1) Å, â ) 90.39(1)°, V
) 1970.3(4) Å3, Fcalc ) 1.864 g cm-3, µ ) 2.09 cm-1, empirical
absorption correction via ψ scan data (0.983 e C e 0.998), Z
) 4, monoclinic, space group P21/n (No. 14), λ ) 0.710 73 Å, T
) 223 K, ω/2θ scans, 7993 reflections collected ((h, +k, (l),
[(sin θ)/λ] ) 0.62 Å-1, 4001 independent and 2017 observed
reflections [I g 2 σ(I)], 335 refined parameters, R ) 0.037, wR2
) 0.078, max. residual electron density 0.22 (-0.27) e‚Å-3
hydrogens calculated and riding.
,
Tr is(p e n t a flu or op h e n yl)b or a n e-p -Me t h ylb e n zon i-
tr ile, Com p lex 1b. Tris(pentafluorophenyl)borane (1.00 g,
1.95 mmol) and 250 mg (2.13 mmol) of p-methylbenzonitrile
give the product 1b. Yield of 1b: 1.13 g (92%), mp 180 °C
(DSC). C26H7BF15N (629.1): calcd C 49.64, H 1.12, N 2.23;
found C 49.22, H 1.47, N 2.28. 1H NMR (200.1 MHz, benzene-
Exp er im en ta l Section
Gen er a l. All reactions were carried out under an argon
atmosphere using Schlenk-type glassware or in a glovebox.
Solvents (including deuterated solvents used for NMR spec-
troscopy) were dried and distilled under argon prior to use.
The materials either were commercial products [acetonitrile,
p-methylbenzonitrile, p-nitrobenzonitrile, tert-butylisocyanide,
1,1,3,3-tetramethylbutylisocyanide, 2,6-dimethylphenylisocya-
nide, triphenylphoshine] or were prepared following literature
procedures [tris(pentafluorophenyl)borane].20
3
d6): δ ) 6.90 (d, 2H, J HH ) 8.11 Hz, Ph (m-H)), 6.38 (d, 2H,
3J HH ) 8.11 Hz, Ph (o-H)), 1.68 (s, 3H, CH3). 13C NMR (50.3
1
MHz, benzene-d6): δ ) 148.7 (dm, J CF ) 245 Hz, B(C6F5)3
1
(o-C)), 143.4 (Ph (p-C)), 141.0 (dm, J CF ) 252 Hz, B(C6F5)3
1
(p-C)), 137.9 (dm, J CF ) 254 Hz, B(C6F5)3 (m-C)), 133.2 (Ph
The following instruments were used for spectroscopic and
physical characterization of the compounds: Bruker AC 200P
and Bruker ARX 300 NMR spectrometers. Chemical shifts are
referred to Me4Si [δ1H (C6D5H) ) 7.15, δ13C (C6D6) ) 128.0],
neat BF3*OEt2 [δ11B ) 0 for ¥(11B) ) 32.084 MHz], neat
MeNO2 [δ14N ) 0 for ¥(14N) ) 7.224 MHz, δ15N ) 0 for ¥(15N)
) 10.133 MHz], CFCl3 [δ19F ) 0 for ¥(19F) ) 94.077 MHz],
80% H3PO4 in D2O [δ31P ) 0 for ¥(31P) ) 40.4807 MHz]; Nicolet
5 DXC FT-IR spectrometer; melting points, DSC 2910 (Thermo
Analysis/Du Pont); elemental analyses, Foss-Heraeus CHN-
Rapid; X-ray crystal structure analyses, Enraf-Nonius CAD4
and MACH3 diffractometers (programs used: data reduction
MolEN, structure solution SHELXS-86, structure refinement
SHELXL-93, graphics SCHAKAL-92). Crystals of 1a , 3a , 3b,
and 6 were obtained from benzene solution by the diffusion
method (pentane diffusion from the gas phase). Single crystals
of 1c were obtained from benzene-d6.
(o-C)), 130.6 (Ph (m-C)), 21.7 (CH3); not observed CtN, C-Me,
Ph (ipso-C), and B(C6F5)3 (ipso-C). 11B NMR (64.2 MHz,
benzene-d6): δ ) -9.6 (ν1/2 ) 470 ( 10 Hz). 19F NMR (282.4
MHz, benzene-d6): δ ) -134.7 (m, 2F, (m-F)), -155.6 (m, 1F,
(p-F)), -163.2 (m, 2F, (o-F)). IR (KBr): ν˜ ) 2322 (CtN) cm-1
.
T r is (p e n t a flu o r o p h e n y l)b o r a n e -p -N it r o b e n zo n i-
tr ile, Com p lex 1c. Tris(pentafluorophenyl)borane (249 mg,
0.49 mmol) and 72 mg (0.49 mmol) of p-nitrobenzonitrile were
mixed as solids, dissolved in 15 mL toluene, and stirred for
30 min at room temperature. After removing all volatile
substances in vacuo 308 mg (0.47 mmol, 96%) of the pure
product was obtained as a yellowish powder. Yield of 1c: 0.308
1
g (96%), mp 183 °C (DSC). H NMR (200.1 MHz, benzene-d6):
δ ) 7.23 (m, 2H, AA′), 6.54 (m, 2H, BB′). 13C NMR (50.3 MHz,
benzene-d6): δ ) 151.3 (broad, C-NO2), 148.6 (dm, 1J CF ) 239
1
Hz, B(C6F5)3 (o-C)), 141.2 (dm, J CF ) 252 Hz, B(C6F5)3 (p-C)),
1
137.8 (dm, J CF ) 249 Hz, B(C6F5)3 (m-C)), 133.8, 124.0 (Ph),
114.9, 113.2 (broad, dCCN/CN (without assignment)), 114.7
(broad, Ph (ipso-C)). 11B NMR (64.2 MHz, benzene-d6): δ )
-10.2 (ν1/2 ) 570 ( 10 Hz). 14N NMR (14.5 MHz, benzene-d6):
δ ) -11 (ν1/2 ) 1550 ( 50 Hz). 19F NMR (282.4 MHz, benzene-
d6): δ ) -134.7 (broad, 2F, (m-F)), -154.8 (broad, 1F, (p-F)),
(47) Vosko, S. J .; Wilk, M.; Nussair, M. Can. J . Phys. 1980, 58, 1200.
(48) Becke, A. D. Phys. Rev. 1988, A38, 3098.
(49) Perdew, J . P. Phys. Rev. 1986, B33, 8822.
(50) (a) Baerends, E. J .; Ellis, D. E.; Ros, P. E. Chem. Phys. 1973,
2, 41. (b) teVelde, G.; Baerends, E. J . J . Comput. Phys. 1992, 99, 84.
(c) Fonseca Guerra, C.; Visser, O.; Snijders, J . G.; te Velde, G.; Baerends
E. J . In Methods and Techniques in Computational Chemistry: ME-
TECC-95; Clementi, E., Corongiu, G., Eds.; STEF: Cagliari, 1995; p
305.
-162.8 (broad, 2F, (o-F)). IR (KBr): ν˜ ) 2334 (CtN) cm-1
X-ray crystal structure analysis of 1c: formula C25H4N2O2BF15
.
,
M ) 660.11, light yellow, 0.20 × 0.20 × 0.20 mm, a ) 13.175-
(1) Å, b ) 12.073(1) Å, c ) 15.083(1) Å, â ) 92.46(1)°, V )
2396.9(3) Å3, Fcalc ) 1.829 g cm-3, µ ) 17.88 cm-1, empirical
(51) te Velde, G. ADF 2.1 User’s Guide; Vrije Universiteit: Amster-
dam, 1996.

Structure and Bonding in L-B(C6F5)3 Complexes
Organometallics, Vol. 18, No. 9, 1999 1735
absorption correction via ψ scan data (0.949 e C e 0.998), Z
) 4, monoclinic, space group P21/c (No. 14), λ ) 1.541 78 Å, T
) 223 K, ω/2θ scans, 9913 reflections collected ((h, (k, -l),
[(sin θ)/λ] ) 0.62 Å-1, 4893 independent and 3725 observed
reflections [I g 2σ(I)], 407 refined parameters, R ) 0.044, wR2
ylphenylisocyanide give the product 3c. Yield of 3c: 1.19 g
(95%), mp 130 °C (DSC). C27H9BF15N (643.2): calcd C 50.43,
H 1.41, N 2.18; found C 50.59, H 1.70, N 2.31. 1H NMR (200.1
3
MHz, benzene-d6): δ ) 6.63 (d, 1H, J HH ) 7.55 Hz, (p-H)),
3
6.36 (d, 2H, J HH ) 7.55 Hz, Ph (m-H)), 1.80 (s, 6H, CH3). 13C
1
) 0.124, max. residual electron density 0.25 (-0.19) e‚Å-3
,
NMR (50.3 MHz, benzene-d6): δ ) 148.5 (dm, J CF ) 247 Hz,
hydrogens calculated and riding.
B(C6F5)3 (o-C)), 141 (dm, 1J CF ) 252 Hz, B(C6F5)3 (p-C)), 137.8
1
Tr is(p e n t a flu or op h e n yl)b or a n e -t e r t -Bu t ylisocya -
n id e, Com p lex (3a ). Tris(pentafluorophenyl)borane (1.00 g,
1.95 mmol) and 180 mg (2.17 mmol) of tert-butylisocyanide give
the product 3a . Yield of 3a : 1.03 g (89%), mp 175 °C (DSC).
(dm, J CF ) 253 Hz, B(C6F5)3 (m-C)), 132.3 (Ph (p-C)), 137.4
(Ph (o-C)), 128.7 (Ph (m-C)), 17.2 (CH3); not observed NtC,
Ph (ispo-C) and B(C6F5)3 (ipso-C). 11B NMR (64.2 MHz,
benzene-d6): δ ) -21.0 (ν1/2 ) 14 ( 1 Hz). 19F NMR (282.4
3
C
23H9BF15N (595.1): calcd C 46.42, H 1.52, N 2.35; found C
MHz, benzene-d6): δ ) -132.3 (m, 2F, (m-F)), -155.2 (t, J FF
1
46.02, H 1.92, N 2.60. H NMR (200.1 MHz, benzene-d6): δ )
) 21 Hz, 1F, (p-F)), -162.7 (m, 2F, (o-F)). IR (KBr): ν˜ ) 2275
0.70 (s, 9H, (CH3)3C). 13C NMR (50.3 MHz, benzene-d6): δ )
(NtC) cm-1
.
1
1
148.6 (dm, J CF ) 240 Hz, B(C6F5)3 (o-C)), 140.8 (dm, J CF
)
T r is (p e n t a flu o r o p h e n y l)b o r a n e -T r ip h e n y lp h o s -
p h in e, Com p lex 6. A solution of 560 mg (2.14 mmol) of
triphenylphoshine in 50 mL of pentane was added dropwise
to a solution of 1.00 g (1.95 mmol) of tris(pentafluorophenyl)-
borane in the same volume of pentane. The reaction mixture
was refluxed until the product precipitated. After further
stirring for 1 h at room temperature, the solid product was
isolated by filtration, washed twice with 20 mL of pentane,
and dried in vacuo. The product was obtained as a colorless
powder. Yield of 6: 1.24 g (82%), mp 203 °C (DSC, decomp.).
1
253 Hz, B(C6F5)3 (p-C)), 137.7 (dm, J CF ) 251 Hz, B(C6F5)3
(m-C)), 80.7 (CH3)3C-), 27.7 ((CH3)3C-); not observed NtC
and B(C6F5)3 (ipso-C). 11B NMR (64.2 MHz, benzene-d6): δ )
-21.8 (ν1/2 ) 10 ( 1 Hz). 19F NMR (282.4 MHz, benzene-d6):
δ ) -134.8 (m, 2F, (m-F)), -155.3 (t, 3J FF ) 21 Hz, 1F, (p-F)),
-162.9 (m, 2F, (o-F)). IR (KBr): ν˜ ) 2310 (NtC) cm-1. X-ray
crystal structure analysis of 3a : formula C23H9NBF15, M )
595.12, colorless, 0.40 × 0.30 × 0.10 mm, a ) 9.345(2) Å, b )
11.545(2) Å, c ) 12.053(2) Å, R ) 61.45(1)°, â ) 85.75(1)°, γ )
88.23(1)°, V ) 1139.1(4) Å3, Fcalc ) 1.735 g cm-3, µ ) 1.88 cm-1
,
C36H15BF15P (774.3): calcd C 55.84, H 1.95; found C 55.19, H
empirical absorption correction via ψ scan data (0.945 e C e
0.999), Z ) 2, triclinic, space group P1h (No. 2), λ ) 0.710 73 Å,
T ) 223 K, ω/2θ scans, 4849 reflections collected ((h, +k, (l),
[(sin θ)/λ] ) 0.62 Å-1, 4609 independent and 2493 observed
reflections [I g 2σ(I)], 364 refined parameters, R ) 0.041, wR2
1.97. 1H NMR (200.1 MHz, benzene-d6): δ ) 7.33-7.24 and
7.10-7.05 (2m, 15H, PPh3). 13C NMR (50.3 MHz, benzene-
1
d6): δ ) 148.8 (dm, J CF ) 253 Hz, B(C6F5)3 (o-C)), 140.8 (dm,
1
1J CF ) 262 Hz, B(C6F5)3 (p-C)), 137.2 (dm, J CF ) 247 Hz,
3
B(C6F5)3 (m-C)), 133.3 (d, J PC ) 10 Hz, Ph (m-C)), 130.2 (d,
) 0.095, max. residual electron density 0.20 (-0.28) e‚Å-3
hydrogens calculated and riding.
,
2J PC ) 13 Hz, Ph (o-C)), 128.8 (d, J PC ) 7 Hz, Ph (p-C)); not
4
observed Ph3P (ipso-C) and B(C6F5)3 (ipso-C). 11B NMR (64.2
MHz, benzene-d6): δ ) -2.5 (ν1/2 ) 110 ( 10 Hz). 31P NMR
(81.0 MHz, benzene-d6): δ ) -5.2. 19F NMR (282.4 MHz,
Tr is(p en ta flu or op h en yl)bor a n e-1,1,3,3-Tetr a m eth yl-
bu tylisocya n id e, Com p lex (3b). Tris(pentafluorophenyl)-
borane (1.00 g, 1.95 mmol) and 300 mg (2.15 mmol) of 1,1,3,3-
tetramethylbutylisocyanide give the product 3b. Yield of 3b:
1.25 g (98%), mp 129 °C (DSC). C27H17BF15N (651.2): calcd C
49.80, H 2.63, N 2.15; found C 49.64, H 2.48, N 2.10. 1H NMR
(200.1 MHz, benzene-d6): δ ) 1.04 (s, 2H, -CH2-), 0.89 (s,
6H, (CH3)2C-), 0.57 (s, 9H, (CH3)3C-). 13C NMR (50.3 MHz,
3
benzene-d6): δ ) -134.8 (m, 2F, (m-F)), -157.3 (t, J FF ) 21
Hz, 1F, (p-F)), -164.4 (m, 2F, (o-F)). X-ray crystal structure
analysis of 6: formula C36H15BF15P, M ) 774.26, colorless, 0.30
× 0.25 × 0.10 mm, a ) 32.194(3) Å, b ) 8.655(1) Å, c ) 22.673-
(2) Å, â ) 105.52(1)°, V ) 6087.2(11) Å3, Fcalc ) 1.690 g cm-3
,
µ ) 2.12 cm-1, empirical absorption correction via ψ scan data
(0.983 e C e 0.992), Z ) 8, monoclinic, space group C2/c (No.
15), λ ) 0.710 73 Å, T ) 223 K, ω/2θ scans, 3682 reflections
collected (+h, +k, (l), [(sin θ)/λ] ) 0.54 Å-1, 3611 independent
and 1979 observed reflections [I g 2 σ(I)], 478 refined
parameters, R ) 0.040, wR2 ) 0.077, max. residual electron
density 0.26 (-0.30) e‚Å-3, hydrogens calculated and riding.
1
benzene-d6): δ ) 148.5 (dm, J CF ) 241 Hz, B(C6F5)3 (o-C)),
1
1
140.9 (dm, J CF ) 253 Hz, B(C6F5)3 (p-C)), 137.8 (dm, J CF
)
255 Hz, B(C6F5)3 (m-C)), 63.8 ((CH3)2C-), 52.0 (-CH2-), 31.0
((CH3)3C-), 29.9 ((CH3)3C-), 28.8 ((CH3)2C-); not observed Nt
C and B(C6F5)3 (ipso-C). 11B NMR (64.2 MHz, benzene-d6): δ
) -21.7 (ν_ ) 12 ( 1 Hz). 19F NMR (282.4 MHz, benzene-d6):
δ ) -133.2 (m, 2F, (m-F)), -155.3 (t, 3J FF ) 21 Hz, 1F, (p-F)),
-162.9 (m, 2F, (o-F)). IR (KBr): ν˜ ) 2301 (NtC) cm-1. X-ray
crystal structure analysis of 3b: formula C27H17NBF15, M )
651.23, colorless, 0.30 × 0.20 × 0.10 mm, a ) 9.018(1) Å, b )
10.800(1) Å, c ) 14.871(2) Å, R ) 77.38(1)°, â ) 76.53(1)°, γ )
Ackn owledgm en t. Financial support from the Swiss
National Science Foundation SNSF (H.B.) and from the
Fonds der Chemischen Industrie and the Deutsche
Forschungsgemeinschaft (G.E.) is gratefully acknowl-
edged.
73.16(1)°, V ) 1330.3(3) Å3, Fcalc ) 1.626 g cm-3, µ ) 1.68 cm-1
,
empirical absorption correction via ψ scan data (0.943 e C e
0.999), Z ) 2, triclinic, space group P1h (No. 2), λ ) 0.710 73 Å,
T ) 223 K, ω/2θ scans, 5704 reflections collected ((h, -k, (l),
[(sin θ)/λ] ) 0.62 Å-1, 5403 independent and 2655 observed
reflections [I g 2 σ(I)], 402 refined parameters, R ) 0.046, wR2
Su p p or tin g In for m a tion Ava ila ble: Additional material
concerning the X-ray structure analyses of the complexes 1a ,
1c, 3a , 3b, and 6 including complete listings of X-ray param-
eters, bond lengths and angles, and positional and thermal
parameters, and a listing of calculated total bonding energies.
This material is available free of charge via the Internet at
http://pubs.acs.org.
) 0.098, max. residual electron density 0.20 (-0.24) e‚Å-3
hydrogens calculated and riding.
,
Tr is(p en ta flu or op h en yl)bor a n e-2,6-Dim eth ylp h en yl-
isocya n id e, Com p lex (3c). Tris(pentafluorophenyl)borane
(1.00 g, 1.95 mmol) and 280 mg (2.13 mmol) of 2,6-dimeth-
OM981033E