Synthesis and characterization of 17-valence-electron [CpCr(NO)X2]- anions: Oxidatively induced loss of the nitrosyl ligand

Article abstract of DOI:10.1021/ja963464z

Several anionic and paramagnetic [CpCr(NO)X₂]^- complexes (X = I [1], O₃SCF₃ (OTf) [2], Br [3], Cl [4]) have been prepared as potential precursors to neutral CpCr(NO)X₂ species. Reaction of [CpCr(NO)I]₂ with [NBu₄]I provides [NBu₄][1], and halide abstraction from [NBu₄][1] with 2 equiv of AgOTf affords [NBu₄][2]. The weakly-bound OTf^- ligands of [NBu₄][2] are readily displaced by Br^- to produce [NBu₄][3]. The dichloro complexes [NEt₄][4] and [PPN][4] are obtained by treating [CpCr(NO)C1]₂ with [NEt₄]Cl and [PPN]C1, respectively. Use of acetonitrile as reaction solvent allows generation of the requisite [CpCr(NO)Cl]₂ directly from CpCr(NO)(CO)₂ and PCl₅, a marked improvement over previous synthetic routes to this dimer. Similar halogenations of Cp*Cr(NO)(CO)₂ in NCMe provide access to the previously unknown [Cp*Cr(NO)1]₂ (5), and [Cp*Cr(NO)Cl]₂ (6), halobridged, and dimers. The solid-state molecular structure of [PPN] [4]^.CH₂Cl₂ has been established by single-crystal X-ray crystallography to be a normal three-legged piano stool. The one-electron oxidation of [4] has been investigated both chemically (by reaction with [Cp₂Fe]⁺) and electrochemically (by cyclic voltammetry). These studies suggest that upon oxidation a high-spin CpCr(NO)Cl₂ complex is initially formed which then rapidly releases NO. Extended Huckel molecular-orbital calculations have been performed on [4], [CpCr(CO)₃]^-, and [CpCrCl₃]^-, three CpCr-containing anions with ligands of varying π-bonding capabilities. Correlations between their orbital energies and electron occupancies and a rationale for the lability of the NO ligand in neutral CpCr(NO)Cl₂ are provided.

Full text of DOI:10.1021/ja963464z

J. Am. Chem. Soc. 1997, 119, 3513-3522
3513
Synthesis and Characterization of 17-Valence-Electron
[CpCr(NO)X2]- Anions: Oxidatively Induced Loss of the
Nitrosyl Ligand1
Peter Legzdins,* W. Stephen McNeil, Steven J. Rettig, and Kevin M. Smith
Contribution from the Department of Chemistry, The UniVersity of British Columbia,
VancouVer, British Columbia, Canada V6T 1Z1
ReceiVed October 3, 1996X
Abstract: Several anionic and paramagnetic [CpCr(NO)X2]- complexes (X ) I [1], O3SCF3 (OTf) [2], Br [3], Cl
[4]) have been prepared as potential precursors to neutral CpCr(NO)X2 species. Reaction of [CpCr(NO)I]2 with
[NBu4]I provides [NBu4][1], and halide abstraction from [NBu4][1] with 2 equiv of AgOTf affords [NBu4][2]. The
weakly-bound OTf- ligands of [NBu4][2] are readily displaced by Br- to produce [NBu4][3]. The dichloro complexes
[NEt4][4] and [PPN][4] are obtained by treating [CpCr(NO)Cl]2 with [NEt4]Cl and [PPN]Cl, respectively. Use of
acetonitrile as reaction solvent allows generation of the requisite [CpCr(NO)Cl]2 directly from CpCr(NO)(CO)2 and
PCl5, a marked improvement over previous synthetic routes to this dimer. Similar halogenations of Cp*Cr(NO)-
(CO)2 in NCMe provide access to the previously unknown [Cp*Cr(NO)I]2 (5), and [Cp*Cr(NO)Cl]2 (6), halo-
bridged, and dimers. The solid-state molecular structure of [PPN][4]‚CH2Cl2 has been established by single-crystal
X-ray crystallography to be a normal three-legged piano stool. The one-electron oxidation of [4] has been investigated
both chemically (by reaction with [Cp2Fe]+) and electrochemically (by cyclic voltammetry). These studies suggest
that upon oxidation a high-spin CpCr(NO)Cl2 complex is initially formed which then rapidly releases NO. Extended
Hu¨ckel molecular-orbital calculations have been performed on [4], [CpCr(CO)3]-, and [CpCrCl3]-, three CpCr-
containing anions with ligands of varying π-bonding capabilities. Correlations between their orbital energies and
electron occupancies and a rationale for the lability of the NO ligand in neutral CpCr(NO)Cl2 are provided.
Introduction
or three unpaired electrons in a variety of relatively high
oxidation states.7-12 Furthermore, attempts to impose an 18e
The electronic configurations exhibited by stable organ-
otransition-metal complexes are inexorably linked to the
π-bonding capabilities of their ligands. Examination of the
development of monocyclopentadienyl chemistry since the first
“half-sandwich” compounds were reported in 1954 provides
numerous examples of this general principle. The initial,
explosive growth of the field can be partly attributed to the
predictable adherence of CpM(CO)nXm (Cp ) η5-C5H5; X )
halide, hydride, or alkyl)2 compounds to the eighteen-valence-
electron (18e) rule.3 Indeed, the presence of several π-acidic
ligands like CO in the metals’ coordination spheres so stabilizes
low-valent, inert-gas configurations4 that one-electron oxidations
of these and related complexes generally produce very reactive
metalloradical species.5,6 This strong preference for attaining
a diamagnetic electron configuration is markedly absent for
Cp′MLnXm compounds (Cp′ ) Cp or Cp*(η5-C5Me5), L )
phosphine, amine) which contain no π-acidic ligands. Recent
studies have demonstrated that stable, paramagnetic (<18e)
complexes of this latter type can be synthesized with one, two,
(7) M ) Mo: (a) Poli, R.; Krueger, S. T.; Abugideiri, F.; Haggerty, B.
S.; Rheingold, A. L. Organometallics 1991, 10, 3041. (b) Poli, R.; Owens,
B. E.; Linck, R. G. J. Am. Chem. Soc. 1992, 114, 1302. (c) Abugideiri, F.;
Keogh, D. W.; Poli, R. J. Chem. Soc., Chem. Commun. 1994, 2317. (d)
Cole, A. A.; Fettinger, J. C.; Keogh, D. W.; Poli, R. Inorg. Chim. Acta
1995, 240, 355. (e) Fettinger, J. C.; Keogh, D. W.; Poli, R. J. Am. Chem.
Soc. 1996, 118, 3617. (f) Abugideiri, F.; Fettinger, J. C.; Keogh, D. W.;
Poli, R. Organometallics 1996, 15, 4407.
(8) M ) Cr: (a) Grohmann, A.; Ko¨hler, F. H.; Mu¨ller, G.; Zeh, H. Chem.
Ber. 1989, 122, 897. (b) Theopold, K. H. Acc. Chem. Res. 1990, 23, 263.
(c) Thomas, B. J.; Noh, S.-K.; Schulte, G. K.; Sendlinger, S. C.; Theopold,
K. H. J. Am. Chem. Soc. 1991, 113, 893. (d) Bhandari, G.; Kim, Y.;
McFarland, J. M.; Rheingold, A. L.; Theopold, K. H. Organometallics 1995,
14, 738. (e) Bhandari, G.; Rheingold, A. L.; Theopold, K. H. Chem. Eur.
J. 1995, 1, 199. (f) Fettinger, J. C.; Mattamana, S. P.; Poli, R.; Rogers, R.
D. Organometallics 1996, 15, 4211. (g) Jolly, P. W. Acc. Chem. Res. 1996,
29, 544. (h) Liang, Y.; Yap, G. P. A.; Rheingold, A.; Theopold, K. H.
Organometallics 1996, 15, 5284. (i) White, P. A.; Calabrese, J.; Theopold,
K. H. Organometallics 1996, 15, 5471.
(9) M ) V: (a) Nieman, J.; Teuben, J. H.; Huffman, J. C.; Caulton, K.
G. J. Organomet. Chem. 1983, 255, 193. (b) Nieman, J.; Pattiasina, J. W.;
Teuben, J. H. J. Organomet. Chem. 1984, 262, 157. (c) Nieman, J.; Teuben,
J. H. Organometallics 1986, 5, 1149. (d) Hessen, B.; Lemmen, T. H.;
Luttikhedde, H. J. G.; Teuben, J. H.; Petersen, J. L.; Huffman, J. C.; Jagner,
S.; Caulton, K. G. Organometallics 1987, 6, 2354. (e) Hessen, B.; Buijink,
J.-K. F.; Meetsma, A.; Teuben, J. H.; Helgesson, G.; Ha˚kansson, M.; Jagner,
S.; Spek, A. L. Organometallics 1993, 12, 2268. (f) Buijink, J.-K. F.;
Kloetstra, K. R.; Meetsma, A.; Teuben, J. H.; Smeets, W. J. J.; Spek, A. L.
Organometallics 1996, 15, 2523.
(10) M ) Nb: (a) Siemeling, U.; Gibson, V. C. J. Organomet. Chem.
1992, 424, 159. (b) de la Mata, J.; Galakhov, M. V.; Go´mez, M.; Royo, P.
Organometallics 1993, 12, 1189. (c) Fettinger, J. C.; Keogh, D. W.; Poli,
R. Inorg. Chem. 1995, 34, 2343. (d) Fettinger, J. C.; Keogh, D. W.; Kraatz,
H.-B.; Poli, R. Organometallics 1996, 15, 5489.
X Abstract published in AdVance ACS Abstracts, April 1, 1997.
(1) π-Bonding and Reactivity in Transition Metal Nitrosyl Complexes,
Part 2. For Part 1, see ref 6. Taken in part from: McNeil, W. S. Ph.D.
Dissertation, The University of British Columbia, 1995.
(2) Wilkinson, G.; Cotton, F. A. Prog. Inorg. Chem. 1959, 1, 1.
(3) (a) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G.
Principles and Applications of Organotransition Metal Chemistry; Univer-
sity Science Books: Mill Valley, CA, 1987; p 169. (b) Tolman, C. A.
Chem. Soc. ReV. 1972, 1, 337.
(4) Templeton, J. L.; Winston, P. B.; Ward, B. C. J. Am. Chem. Soc.
1981, 103, 7713.
(5) (a) Baird, M. C. Chem. ReV. 1988, 88, 1217. (b) Trogler, W. C.,
Ed. Organometallic Radical Processes; Elsevier: New York, 1990. (c)
Reference 6 and refs 2-8 contained therein.
(6) Legzdins, P.; McNeil, W. S.; Batchelor, R. J.; Einstein, F. W. B. J.
Am. Chem. Soc. 1995, 117, 10521.
(11) M ) W: (a) Liu, A. H.; Murray, R. C.; Dewan, J. C.; Santarsiero,
B. D.; Schrock, R. R. J. Am. Chem. Soc. 1987, 109, 4282. (b) O’Regan,
M. B.; Liu, A. H.; Finch, W. C.; Schrock, R. R.; Davis, W. M. J. Am.
Chem. Soc. 1990, 112, 4331. (c) Schrock, R. R.; Glassman, T. E.; Vale,
M. G.; Kol, M. J. Am. Chem. Soc. 1993, 115, 1760.
S0002-7863(96)03464-6 CCC: $14.00 © 1997 American Chemical Society

3514 J. Am. Chem. Soc., Vol. 119, No. 15, 1997
Legzdins et al.
Scheme 1
tion of [CpMo(NO)I2]2 to the {Mo(NO)}5, 17e [CpMo(NO)I2]-
anion20 suggests that oxidation of [CpCr(NO)X2]- {Cr(NO)}5
complexes might provide a synthetic route to neutral CpCr-
(NO)X2 {Cr(NO)}4 products.
In this paper, we detail the synthesis and characterization of
[CpCr(NO)X2]- compounds (X ) I [1], O3SCF3(OTf) [2], Br
[3], Cl [4]). The results of chemical and electrochemical
oxidation of [NEt4][4] are consistent with the transient formation
of the desired {Cr(NO)}4 CpCr(NO)Cl2 followed by rapid loss
of NO. Extended-Hu¨ckel molecular-orbital calculations have
been utilized to rationalize (a) the relative stability of 17e {Cr-
(NO)}5 complexes, (b) the proclivity of [CpCr(NO)Cl2]- to lose
NO upon oxidation, and (c) the significance of π-donation in
stabilizing Cp′Cr(NO)X2 species.
configuration on these carbonyl-free complexes via reduction
result in highly reactive, electron-rich species.13
Compounds like [CpCr(NO)L2]+ that contain only one
π-acceptor ligand do not fit readily into either of the above
categories. For example, [CpCr(NO)(NH3)2]+ is similar to the
reactive, substitutionally-labile CpCr(CO)3 radical species5a in
that both complexes are paramagnetic, 17e, and contain Cr
formally in the +1 oxidation state. However, unlike CpCr-
(CO)3, [CpCr(NO)(NH3)2]+ is kinetically stable and decomposes
upon reduction in the absence of trapping ligands. The
coordination of additional π-acidic ligands favors the inert-gas
configuration, as in the 18e CpCr(NO)(CO)2, which loses
carbonyl ligands when oxidized. In other words, the cationc,
17e, {Cr(NO)}5 manifold14 in the [CpCr(NO)L2]+/0 redox couple
is stabilized when L is a good σ-donor ligand while the neutral,
18e, {Cr(NO)}6 configuration is stabilized when L is a π-ac-
ceptor ligand.6
Comparison of CpCr(NO)-containing complexes with their
Mo congeners presents a third possible electronic configuration,
namely the {Cr(NO)}4 manifold. [CpCr(NO)L2]+ species and
other 17e chromium nitrosyl complexes are accessible via
[CpCr(NO)I]2,15 the {Cr(NO)}5 monoiodo dimer obtained by
the iodine oxidation of CpCr(NO)(CO)2.16 As shown in Scheme
1, the reaction of the congeneric CpMo(NO)(CO)2 with I2
proceeds differently, affording the {Mo(NO)}4 diiodo dimer,
[CpMo(NO)I2]2.17 This and related [CpMo(NO)X2]2 dihalo
dimers have served as synthetic precursors to a broad spectrum
of {Mo(NO)}4 products,18,19 most of which have no chromium
counterparts. However, the electrochemically reversible reduc-
Results and Discussion
Preparation of [CpCr(NO)X2]- Anions. Treatment of
known [CpCr(NO)I]2 with 1 equiv of [NBu4]I in CH2Cl2 effects
cleavage of the dimer in a manner similar to that of other Lewis
bases,6,15,21 to generate [NBu4][1] as shown in eq 1.
This synthesis and isolation of anion [1] suggest that a range
of such [CpCr(NO)X2]- complexes (X ) halide or pseudo-
halide) should also be stable species. However, attempts to
metathesize the iodide ligands in [1] with either Br- or Cl- in
order to obtain directly the dibromo [3] and dichloro [4]
complexes do not succeed because of scrambling of the halide
ligands.22 Attempts to isolate a single organometallic product
from these mixtures have been unsuccessful. However, treat-
ment of [NBu4][1] with 2 equiv of AgOTf in CH2Cl2 results in
the clean metathesis of the iodide ligands to give [NBu4][CpCr-
(NO)(OTf)2] ([NBu4][2]) in 85% yield (eq 2).
(12) M ) Fe: (a) Roger, C.; Hamon, P.; Toupet, L.; Rabaaˆ, H.; Saillard,
J.-Y.; Hamon, J.-R.; Lapinte, C. Organometallics 1991, 10, 1045. (b)
Tenorio, J. M.; Puerta, M. C.; Valerga, P. Organometallics 1994, 13, 3330.
(c) Leal, A. J.; Tenorio, J. M.; Puerta, M. C.; Valerga, P. Organometallics
1995, 14, 3839. (d) Hamon, P.; Toupet, L.; Hamon, J.-R.; Lapinte, C.
Organometallics 1996, 15, 10.
(13) In the absence of π-acceptor ligands to relieve the increased electron
density at the metal center, these low-valent compounds readily activate
unsaturated organic substrates. See, for example: (a) Hessen, B.; Meetsma,
A.; van Bolhuis, F.; Teuben, J. H.; Helgesson, G.; Jagner, S. Organometallics
1990, 9, 1925. (b) Abugideiri, F.; Kelland, M. A.; Poli, R.; Rheingold, A.
L. Organometallics 1992, 11, 1303. (c) Abugideiri, F.; Kelland, M. A.;
Poli, R. Organometallics 1992, 11, 1311. (d) Do¨hring, A.; Emrich, R.;
Goddard, R.; Jolly, P. W.; Kru¨ger, C. Polyhedron 1993, 12, 2671. (e) Jonas,
K.; Klusmann, P.; Goddard, R. Z. Naturforsch. B 1995, 50, 394.
(14) The Enemark-Feltham notation is a more accurate representation
of electronic configuration in metal-nitrosyl complexes than is an assign-
ment of formal oxidation states due to the covalent nature of the M-NO
linkage. The {Cr(NO)}5 configuration can correspond approximately to a
d5, [CrI(NO+)] fragment if the nitrosyl ligand is linear, as it is in the
compounds described here. Enemark, J. H.; Feltham, R. D. Coord. Chem.
ReV. 1974, 13, 339.
Subsequent metatheses of the triflate ligands in [2] for Br-
or Cl- are more straightforward than the corresponding met-
atheses of iodide in [1], such that reaction of [NBu4][2] with
either KBr (eq 3) or KCl results in the clean formation of only
[3] or [4], as judged by both IR and ESR spectroscopies.
However, while the dibromo species, [NBu4][3], can be isolated
in pure form by recrystallization from CH2Cl2/hexanes, the
dichloro salt, [NBu4][4], cannot be completely separated from
the KOTf byproduct.
(20) Herring, F. G.; Legzdins, P.; Richter-Addo, G. B. Organometallics
1989, 8, 1485.
(21) Legzdins, P.; McNeil, W. S.; Shaw, M. J. Organometallics 1994,
13, 562.
(22) For instance, reaction of either [CpCr(NO)I]2 or [NBu4][1] with
[NEt4]Cl in CH2Cl2 results in a reaction mixture which exhibits an ESR
spectrum with three signals, namely one broad singlet attributable to diiodo
[1], one resolved triplet due to dichloro [4], and a third of intermediate
G-value with an unresolved coupling presumably due to [CpCr(NO)(I)(Cl)]-.
(15) Legzdins, P.; Nurse, C. R. Inorg. Chem. 1985, 24, 327.
(16) (a) Fischer, E. O.; Beckert, O.; Hafner, W.; Stahl, O. Z. Naturforsch.
B 1955, 10, 598. (b) Chin, T. T.; Hoyano, J. K.; Legzdins, P.; Malito, J.
T. Inorg. Synth. 1990, 28, 196.
(17) (a) King, R. B. Inorg. Chem. 1967, 6, 30. (b) Seddon, D.; Kita,
W. G.; Bray, J.; McCleverty, J. A. Inorg. Synth. 1976, 16, 24.
(18) McCleverty, J. A. Chem. Soc. ReV. 1983, 12, 331.
(19) Legzdins, P.; Veltheer, J. E. Acc. Chem. Res. 1993, 26, 41.

Synthesis of 17-Valence-Electron [CpCr(NO)X2]- Anions
J. Am. Chem. Soc., Vol. 119, No. 15, 1997 3515
Table 1. Numbering Scheme, Color, Yield and Elemental Analysis Data
elemental analysis found (calcd)
complex
compd no.
color (yield, %)
C
H
N
[NBu4][CpCr(NO)I2]
[NBu4][CpCr(NO)(OTf)2]
[NBu4][CpCr(NO)Br2]
[NEt4][CpCr(NO)Cl2]
[PPN][CpCr(NO)Cl2]a
[Cp*Cr(NO)I]2
[Cp*Cr(NO)Cl]2
[NBu4][1]
[NBu4][2]
[NBu4][3]
[NEt4][4]
[PPN][4]
5
red-brown (90)
brown (85)
black (62)
black (53)
black (41)
green (62)
green (89)
39.35 (39.20)
40.30 (40.17)
45.91 (45.91)
44.86 (44.84)
65.57 (65.22)
35.29 (34.90)
47.86 (47.53)
6.47 (6.42)
6.05 (6.01)
7.67 (7.52)
7.33 (7.24)
5.22 (5.23)
4.38 (4.39)
6.14 (5.98)
4.23 (4.35)
4.03 (4.07)
5.00 (5.10)
8.04 (8.04)
3.19 (3.38)
4.36 (4.07)
5.49 (5.54)
b
6
a Sample for elemental analysis contains one molecule of THF of crystallization per ion pair. b Reported elemental analysis is the average of
three separate runs.
Table 2. Mass Spectral, Infrared, and ESR Data for 17e Species
FAB-MS (m/z)
IR (cm-1
)
ESR(CH2Cl2)
coupling (G)
complex
(+)
(-)
ν
NO(Nujol)
ν
NO(CH2Cl2)
g-value
[NBu4][1]
[NBu4][2]
[NBu4][3]
[NEt4][4]
[PPN][4]
5
242 [P+]
242 [P+]
242 [P+]
130 [P+]
538 [P+]
688 [P+]
474 [P+-NO]
401 [P-]
1637
1697
1641
1626
1622
1647
1648
1644
1690
2.051
1.978
2.011
1.986
1.986
445 [P-]
AN ) 5.1; ACr ) 21.3
AN ) 5.7; ACr ) 18.6
AN ) 5.4; ACr ) 20.3
AN ) 5.4; ACr ) 20.4
305 307 309 [P-]a
217 219 [P-]b
217 219 [P-]b
1641
1650
1645
6
a This envelope of peaks is a 1:2:1 isotopic cluster as expected for a mixture of 79Br and 81Br isotopomers, the 79Br/81Br form being most
abundant. b This envelope of peaks is a 3:2 isotopic cluster as expected for a mixture of 35Cl and 37Cl isotopomers, the 35Cl/35Cl form being most
abundant.
directly with chloride addition to prepare [4], since the successful
isolation of [4] is not hampered by any contaminating CpCr-
(NO)2Cl which might be present.
With this preparative route to [4] thwarted, a route analogous
to that shown in eq 1 and utilizing [CpCr(NO)Cl]2 as the initial
reactant appeared to offer an appealing alternative. The problem
thus became how best to prepare the requisite bridging-chloro
precursor.23
Physical and Spectroscopic Properties of CpCr(NO)X2
Treatment of CpCr(NO)(CO)2 with Cl2 in CH2Cl2, analogous
to the I2 reaction in Scheme 1, does not afford [CpCr(NO)-
Cl]2.15 We have now found that effecting the chlorination with
PCl5 as the chlorinating agent and MeCN as the solvent does
indeed produce the desired bridging-chloro dimer. Though
[CpCr(NO)Cl]2 isolated from the final reaction mixture as a
bright green powder exhibits the expected spectroscopic proper-
ties,24 the IR spectra of these samples also indicate contamina-
tion with ∼5% of CpCr(NO)2Cl.25 The amount of the dinitrosyl
contaminant present may be reduced by performing the reaction
at lower temperatures, and it can be effectively removed by
washing the crude product with Et2O and recrystallizing the
[CpCr(NO)Cl]2 from toluene at -30 °C. In this connection it
should be noted that [CpCr(NO)Cl]2 is not as thermally robust
as its iodo analogue, [CpCr(NO)I]2.26
Anions. Anions [1] to [4] are monomeric, 17e organometallic
species, which as their tetra(alkyl)ammonium salts are essentially
insoluble in Et2O or aliphatic solvents, but are soluble in CH2-
Cl2 or THF. The yields and analytical data for these complexes
are collected in Table 1, and the spectroscopic data are presented
in Table 2. The anions are generally more air-sensitive than
are their cationic [CpCr(NO)L2]+ counterparts,6 both as solids
and in solutions. The diiodo salt, [NBu4][1], may be handled
in air as a solid for short periods of time, but the bis(triflate),
[NBu4][2], and dibromo, [NBu4][3], salts both decompose in
the atmosphere within minutes to viscous oils. IR spectra of
air-exposed solutions show features indicative of the formation
of dinitrosyl complexes. As solids, the salts may be stored
indefinitely at ambient temperatures under an inert atmosphere
without the occurrence of noticeable decomposition.
Finally, reaction of [CpCr(NO)Cl]2 with [NEt4]Cl in toluene
affords [NEt4][CpCr(NO)Cl2] ([NEt4][4]) in 50-55% yields
from CpCr(NO)(CO)2 (eq 4). For preparative purposes it is
more efficient to use crude [CpCr(NO)Cl]2 and then carry on
Each anion exhibits a single strong nitrosyl-stretching absorp-
tion in the IR range expected for a linear nitrosyl group. In
Nujol-mull spectra, these bands are all between 1626 and 1641
cm-1 for the dihalide anions, while the triflate complex, [NBu4]-
[2], exhibits a νNO at 1697 cm-1. This higher-energy value is
a manifestation of the greater electron-withdrawing ability of
the triflate ligands. The FAB mass spectra of each of the salts
display discrete peaks due to the cation and anion parent ions
as well as diagnostic isotope patterns for the dibromo and
dichloro anions. As expected for monomeric 17e complexes,
each of the anions [1] to [4] exhibits a room-temperature ESR
spectrum. The spectrum of the dichloro salt, [NEt4][4], in CH2-
Cl2 is typical and is shown in Figure 1. Compounds [2] to [4]
(23) [CpCr(NO)Cl]2 has been obtained in an impure form as the lowest-
yield component of four chromium-containing products isolated from the
reaction of ClNO with [CpCr(CO)3]2 and as a pure solid in low overall
yield by treating [CpCr(NO)(OEt)]2 with HCl gas.24 However, neither of
these reactions represents a viable route for further preparative chemistry.
(24) Kolthammer, B. W. S.; Legzdins, P.; Malito, J. T. Inorg. Chem.
1977, 16, 3173.
(25) (a) Piper, T. S.; Wilkinson, G. J. Inorg. Nucl. Chem. 1956, 2, 38.
(b) Hoyano, J. K.; Legzdins, P.; Malito, J. T. Inorg. Synth. 1978, 18, 126.
(26) Greenhough, T. J.; Kolthammer, B. W. S.; Legzdins, P.; Trotter, J.
Inorg. Chem. 1979, 18, 3548.

3516 J. Am. Chem. Soc., Vol. 119, No. 15, 1997
Legzdins et al.
a,b
Table 3. Crystallographic Data for [PPN][4]‚CH2Cl2
Formula
fw
C42H37Cl4CrN2P2O
841.52
294
temp, K
color, habit
crystal size, mm
crystal system
space group
a, Å
green, prism
0.25 × 0.30 × 0.45
triclinic
P1h
12.729(2)
15.480(2)
11.778(2)
101.25(1)
101.40(1)
111.129(10)
2029.9(6)
2
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
F
calc, g/cm3
1.377
866.00
6.59
F(000)
µ(Mo KR), cm-1
trans. factors (relative)
scan type
Figure 1. ESR spectrum of [NBu4][4] in CH2Cl2.
0.939-1.000
ω-2θ
scan range, deg in ω
scan rate, deg/min
data collected
2θmax, deg
crystal decay, %
total no. of reflcns
no. of unique reflcns
Rmerge
1.05 + 0.35 tan θ
16.0
display spectra which are qualitatively similar and exhibit
coupling to both the nitrosyl nitrogen atom (I ) 1), which results
in a 1:1:1 triplet, and to the 9.55% abundant 53Cr nucleus (I )
3/2), which gives rise to satellite signals on either side of the
main feature. The coupling constants for these spectra are in
the ranges of 5-6 and 18-22 G for the nitrogen and chromium
couplings, respectively (Table 2). The ESR spectrum of the
diiodo anion is an exception in that it is a broad and featureless
singlet. Finally, there is a distinct increase in the G-values
exhibited by the dihalide anions as the halide ligand is changed
from Cl to Br to I, increasing from 1.986 to 2.011 to 2.051.
This is exactly the same trend that has been observed for the
analogous molybdenum anions, for which the G-values increase
from 1.982 to 2.010 to 2.060.20 The trend is also mirrored in
complexes of the type CpMoX2(PMe3)227 and CpMoX2(dppe)28
(X ) Cl, Br, I), and in the latter cases the diiodo compounds
also exhibit much broader ESR signals than do the other species.
These effects are likely due to a combination of a varying degree
of ligand-atomic-orbital character in the SOMO and an increas-
ing halide-orbital contribution to the moment of the electron
spin in going from Cl to Br to I.27,28
+h,(k,(l
55
0
9770
9323
0.048
no. of reflcns with I g 3σ(I)
2843
0.052
0.044
1.92
R
Rw
gof
max ∆/σ (final cycle)
0.008
residual density, e/Å3
-0.44 to 0.35 (near Cr)
2
2
a Function minimized ∑w(|Fo| - |Fc|)2 where w ) 4Fo /σ2(Fo ), R
) ∑||Fo| - |Fc||/∑|Fo|, Rw ) (∑w(|Fo| - |Fc|)2/∑w|Fo| )1/2, and gof )
2
[∑w(|Fo| - |Fc|)2/(m - n)]1/2. Values given for R, Rw, and gof are
based on those reflections with I g 3σ(I). b Rigaku AFC6S diffracto-
meter, MO KR radiation (λ ) 0.71069 Å), graphite monochromator,
takeoff angle 6.0°, aperture 6.0 × 6.0 mm at a distance of 285 mm
from the crystal, stationary background counts at each end of the scan
(scan/background time ratio 2:1, up to 9 rescans), σ2(F2) ) [S2(C +
4B)]/Lp2 (S ) scan rate, C ) scan count, B ) normalized background
count).
Table 4. Selected Metrical Parametersa for [PPN][4]CH2Cl2
Solid-State Molecular Structure of [4]. Crystals of [PPN]-
[4]]‚CH2Cl2 suitable for X-ray crystallographic analysis were
obtained by reacting [CpCr(NO)Cl]2 with [PPN]Cl and subse-
quent recrystallization of the crude material from CH2Cl2/
hexanes. The crystallographic data for [PPN][4]‚CH2Cl2 are
listed in Table 3, and selected bond lengths and bond angles
are collected in Table 4. The solid-state molecular structure of
the anion in [PPN]CpCr(NO)Cl2]‚CH2Cl2 is depicted in Figure
2 with only one of the three possible forms found in the
disordered structure being shown. As a result of the disorder
and the restraints applied during the solution of the structure
(see Experimental Section), the geometry involving the chloride
and the nitrosyl ligands is probably less precise than indicated
by the standard deviations. Nevertheless, it is clear that [4]
has a typical three-legged piano-stool structure with intra-
molecular metrical parameters similar to those exhibited by
related CpCr(NO)-containing complexes.6,29
bond
length (Å)
bond
angles (deg)
atoms
atoms
Cr(1)-Cpb
Cr(1)-Cl(1)
Cr(1)-Cl(2)
Cr(1)-N(1c)
1.88
Cp-Cr(1)-Cl(1)
Cp-Cr(1)-Cl(2)
Cp-Cr(1)-N(1c)
Cr(1)-N(1c)-O(1c)c
120.4
117.7
119.8
172(4)
2.315(4)
2.314(4)
1.685(7)
a ESD’s in parentheses b Cp refers to the unweighted centroid of the
cyclopentadienyl ligand. c Restrained during refinement.
were not susceptible to electron loss, we now report the 1e
oxidation of CpCr(NO)X2 anions.
(a) Cyclic Voltammetry. The cyclic voltammogram of
[NBu4][1] scanned at 0.4 V/s exhibits an irreversible oxidation
feature in THF at Ep,a ) 0.53 V vs SCE. The dichloro salt,
[NEt4][4], also displays an irreversible oxidation feature under
the same conditions at a similar voltage, Ep,a ) 0.60 V. If the
solvent is changed to CH2Cl2, which allows a wider oxidation
window, more features become evident. An oxidative scan of
[NEt4][4] in CH2Cl2 at 0.1 V/s reveals three irreversible
oxidation waves at Ep,a ) 0.56, 1.15, and 1.74 V. As the scan
rate is increased, the first feature remains completely irreversible,
and the intensity of this current increases linearly with the square
root of the scan rate, as expected. However, at faster scan rates,
the current maximum of the second oxidation wave does not
increase to the same degree as that of the first feature. This
observation indicates that [NEt4][CpCr(NO)Cl2] decomposes
Oxidation Chemistry of the Dihalide Anions. We have
previously discussed the redox chemistry of {Cr(NO)}5 species
in our studies of the reduction of neutral CpCr(NO)(L)X21 and
6
cationic CpCr(NO)L2 compounds. While these complexes
(27) Krueger, S. T.; Poli, R.; Rheingold, A. L.; Staley, D. L. Inorg. Chem.
1989, 28, 4599.
(28) Krueger, S. T.; Owens, B. E.; Poli, R. Inorg. Chem. 1990, 29, 2001.
(29) (a) Chin, T. T.; Legzdins, P.; Trotter, J.; Yee, V. C. Organometallics
1992, 11, 913. (b) Greenhough, T. J.; Kolthammer, B. W. S.; Legzdins,
P.; Trotter, J. Acta Crystallogr. 1980, B36, 795.

Synthesis of 17-Valence-Electron [CpCr(NO)X2]- Anions
J. Am. Chem. Soc., Vol. 119, No. 15, 1997 3517
Scheme 2
Figure 2. Molecular structure of [4] as it occurs in [PPN][4]‚CH2Cl2;
33% probability thermal ellipsoids shown.
Scheme 2 comes from the fact that the last step can be effected
independently, treatment of [NEt4][4] in THF with gaseous NO
converting it to CpCr(NO)2Cl.32
rapidly upon oxidation and that the second oxidation feature is
due to a decomposition product of the initially formed CpCr-
(NO)Cl2 species, which has only a transient existence.
(b) Chemical Oxidation of [NEt4][CpCr(NO)Cl2]. The
chemical oxidation of [4] demonstrates that the initial oxidation
product does indeed decompose, and it results in the isolation
of two organochromium products. Thus, treatment of
[NEt4][CpCr(NO)Cl2] with [Cp2Fe][PF6] results in the formation
of a green mixture which contains CpCr(NO)2Cl25 as the only
nitrosyl-containing product (as judged by the solution IR
spectrum). Only one-half an equivalent of oxidant is required
to consume completely all the starting anion; any excess [Cp2-
Fe]+ remains unreacted. Chromatography of the final reaction
mixture effects the separation of two organochromium products
derived from the decomposition of the initially generated CpCr-
(NO)Cl2, namely the dinitrosyl complex CpCr(NO)2Cl and the
Cr(III) dimer [CpCrCl(µ-Cl)]2.30 A balanced equation for this
transformation is shown in eq 5.
The loss of nitric oxide from metallonitrosyl complexes is
currently a matter of some interest, due to the recently
discovered and far-ranging biological effects of NO,33 including
its role as the principal regulator of human blood pressure.34
Nitroprusside, Na2[Fe(CN)5(NO)], has been used for decades
to lower blood pressure during surgery, and is currently the only
clinically employed metal-nitrosyl complex.35 Loss of nitric
oxide from this species is triggered by a one-electron reduction
process, and is unfortunately accompanied by the release of 2
equiv of toxic cyanide for every molecule of NO, thus requiring
concomitant use of a cyanide antidote with the vasodilator.36
Consequently, design of new metallonitrosyl pharmaceuticals
is a subject of some interest, particularly with respect to
determination and control of conditions under which nitric oxide
is lost from the metal.37 The development of oxidatively-
triggered nitric-oxide-releasing metallonitrosyl compounds could
conceivably provide different and complementary pharmaceuti-
cal activity to the reductively-triggered nitroprusside ion.
Oxidation of Cp′M(NO)(CO)2 with Halogen Sources. The
evident instability of CpCr(NO)Cl2 also helps to account for
the metal-dependent difference in products observed when
Cp′M(NO)(CO)2 (M ) Cr, Mo) complexes possessing {M-
(NO)}6 configurations are reacted with halogenating agents such
as PCl5,38 Cl2, Br2, or I2 in noncoordinating sovlents. For
molybdenum, these reactions are remarkably general, and
{Mo(NO)}4 products of composition [Cp′Mo(NO)X2]2 are
isolable in good yields for both Cp17 and Cp* 39 derivatives. In
contrast, no analogous neutral dihalo {Cr(NO)}4 complexes are
obtained for chromium. Only in the single, specific case
A plausible mechanism that provides a rationale for the
formation of the isolated products, for the usual stoichiometry
between the reactants, and for the transfer of a nitrosyl ligand
is presented in Scheme 2. Removal of an electron from
[CpCr(NO)Cl2]- by [Cp2Fe]+ results in the formation of
ferrocene and CpCr(NO)Cl2, a 16e {Cr(NO)}4 complex. How-
ever, unlike the analogous {Mo(NO)}4 compound,17b CpCr-
(NO)Cl2 is not stable, as indicated by the irreversible electro-
chemical oxidation of its 17e anionic precursor. Loss of NO
from this complex would yield CpCrCl2, one-half of the
Cr(III) dimer isolated from the final reaction mixture. The
second product, CpCr(NO)2Cl, could then be formed if the
liberated nitric oxide displaces a chloride ligand from unoxidized
[NEt4][4].31 Support for the proposed mechanism outlined in
(32) It has been shown previously that [CpCr(NO)Cl]2 also forms CpCr-
(NO)2Cl upon reaction with nitric oxide.24
(33) (a) Culotta, E.; Koshland, D. E. Science 1992, 258, 1862. (b)
Feldman, P. L.; Griffith, O. W.; Stuer, D. J. Chem. Eng. News 1993, 71,
26. (c) Nelson, R. J.; Demas, G. E.; Huang, P. L.; Fishman, M. C.; Dawson,
V. L.; Dawson, T. M.; Snyder, S. H. Nature 1995, 378, 383.
(34) Snyder, S. H.; Bredt, D. S. Sci. Am. 1992, 266, 68.
(35) For other potential medicinal uses for metallonitrosyl complexes
currently in development, see: (a) Fricker, S. P. Platinum Metals ReV. 1995,
39, 150. (b) Schoch, T. K.; Hubbard, J. L.; Zoch, C. R.; Yi, G.-B.; Sørlie,
M. Inorg. Chem. 1996, 35, 4383.
(30) (a) Ko¨hler, F. H.; de Cao, R.; Ackermann, K.; Sedlmair, J. Z.
Naturforsch. B 1983, 38, 1406. (b) Bra¨unlein, B.; Ko¨hler, F. H.; Strauss,
W.; Zeh, H. Z. Naturforsch. B 1995, 50, 1739.
(31) This mechanism assumes that the NO-transfer mechanism is fast
relative to the initial oxidation of [NEt4][CpCr(NO)Cl2]. A perceptive
reviewer noted that the oxidation potentials of [NEt4][4] and Cp2Fe indicate
that the ferrocenium oxidation is endergonic and thus is expected to be
slow.
(36) Butler, A. R.; Glidewell, C. Chem. Soc. ReV. 1987, 16, 361.
(37) Clarke, M. J.; Gaul, J. B. Struct. Bonding (Berlin) 1993, 81, 148.
(38) For the use of PCl5 as a chlorinating agent in organometallic
syntheses, see: (a) Reference 11a. (b) Dryden, N. H.; Legzdins, P.;
Batchelor, R. J; Einstein, F. W. B. Organometallics 1991, 10, 2077. (c)
Reference 28.
(39) Gomez-Sal, P.; de Je´sus, E.; Michiels, W.; Royo, P.; de Miguel, A.
V.; Martinez-Carrera, S. J. Chem. Soc., Dalton Trans. 1990, 2445.

3518 J. Am. Chem. Soc., Vol. 119, No. 15, 1997
Legzdins et al.
Scheme 3
illustrated in Scheme 1 (i.e. Cp′ ) Cp; X2 ) I2) are any
mononitrosyl chromium products isolable from these reactions.
In all other cases, Cp′Cr(NO)2X products are obtained in <50%
yields as the only NO-containing species.15,21 This lack of a
general synthetic route to [Cp′Cr(NO)X]2 compounds from the
treatment of Cp′Cr(NO)(CO)2 with X2 can now be attributed
to formation of Cp′Cr(NO)X2 species, as shown in Scheme 3.
Once generated, {Cr(NO)}4 intermediates like CpCr(NO)Cl2
transfer NO to 17e, {Cr(NO)}5 complexes to form the isolated
Cp′Cr(NO)2X products, a reaction sequence analogous to that
presented in Scheme 2.
Figure 3. EHMO energy-level diagram for [CpCrCl3]-, [CpCr(CO)3]-,
and [CpCr(NO)Cl2]-.
X ) I; 6, X ) Cl), as summarized in eq 6 below
Examination of the published IR monitoring of the reaction
of CpCr(NO)(CO)2 with I2 in CH2Cl2 reveals how the relative
rates of reaction of the various chromium species in solution
with iodine favor the clean formation of [CpCr(NO)I]2. The
key feature is that only after all the CpCr(NO)(CO)2 is consumed
does any excess I2 present react with the 17e species present to
generate absorptions due to CpCr(NO)2I.15 In other words, I2
reacts with CpCr(NO)(CO)2 faster than with CpCr(NO)(CO)I
or [CpCr(NO)I]2, and so the presence of the dicarbonyl {Cr-
(NO)}6 reactant prevents the formation of CpCr(NO)I2 and the
subsequent NO-transfer reactions. Clean generation of other
[Cp′Cr(NO)X]2 complexes by the same synthetic methodology
is not possible because the relative rates of oxidation encoun-
tered with I2 do not extend to related systems. Thus, when
CpCr(NO)(CO)2 is treated with PCl5, Cl2, or Br2 in CH2Cl2, or
when Cp*Cr(NO)(CO)2 is reacted with any halogen source, IR
bands due to Cp′Cr(NO)2X species are observed before all of
the {Cr(NO)}6 dicarbonyl reactant has been consumed.15,21 In
these cases, therefore, X2 reacts with the various 17e chromium-
containing intermediates at rates comparable to its reaction with
Cp′Cr(NO)(CO)2, thus hindering isolation of the {Cr(NO)}5
bridging-monohalo dimers.
π-Bonding in Cyclopentadienyl Chromium Complexes.
The preceding two sections of this report have addressed the
oxidation chemistry of {Cr(NO)}5 species by invoking that
transient CpCr(NO)X2 intermediates are unstable with respect
to nitric oxide loss. This instability may appear unusual when
contrasted with the substitutionally-inert nature of the {Cr(NO)}5
precursors6,21 and the well-established stability of the analogous
[Cp′Mo(NO)X2]n complexes.18,19 To account for this mode of
reactivity, we have examined the frontier molecular orbitals of
three related CpCrL3 anions, namely [CpCr(CO)3]-,40
[CpCrCl3]-,41 and [CpCr(NO)Cl2]-, using extended-Hu¨ckel MO
calculations.42,43 These calculations suggest that the [CpCrL3]-
complexes can be profitably viewed as being pseudo-tetrahedral
complexes,44 with all five of the Cr d-orbitals free to engage in
π-bonding (Figure 3).
For [CpCr(CO)3]-, the alignment of the z-axis along the
pseudo-3-fold rotation axis through the Cp centroid allows for
(40) (a) Pasynskii, A. A.; Eremenko, I. L.; Abdullaev, A. S.; Orazsa-
khatov, B.; Nefedov, S. E.; Stomakhina, E. E.; Ellert, O. G.; Katser, S. B.;
Yanovskii, A. I.; Struchkov, Y. T. Russ. J. Inorg. Chem. 1990, 35, 1285.
(b) Rohrmann, J.; Herrmann, W. A.; Herdtweck, E.; Riede, J.; Ziegler, M.;
Sergeson, G. Chem. Ber. 1986, 119, 3544.
(41) (a) Fischer, E. O.; Ulm, K.; Kuzel, P. Z. Anorg. Allg. Chem. 1963,
319, 253. (b) Reference 30a.
This overoxidation has been avoided by the use of σ-donor
ligands (L) to stabilize the 17e, {Cr(NO)}5 configuration6 in
order to synthesize complexes having the composition Cp′Cr-
(NO)(L)X (Scheme 3). Thus, both known 17e Cp*Cr(NO)-
(L)I complexes were prepared by adding a trapping ligand (L
) PPh3 or pyridine) immediately after I2 addition of Cp*Cr-
(NO)(CO)2 without attempting to isolate the bridging monoio-
dide dimer.21 The σ-donor ligand may also be bound to the
chromium center prior to oxidation, as illustrated by the clean
conversion of CpCr(NO)(PR3)(CO) by X2 to 17e CpCr(NO)-
(PR3)X.15 We have now employed a similar synthetic strategy
to obtain [Cp′Cr(NO)X]2 dimers by treating Cp′Cr(NO)(CO)2
with PCl5 or I2 in a coordinating solvent such as acetonitrile.
In MeCN these reactions proceed cleanly for both Cp and Cp*
derivatives, with only minimal amounts of Cp′Cr(NO)2X
byproducts being evident by IR spectroscopy. The coordinated
MeCN is readily removed in vacuo from the Cp′Cr(NO)-
(NCMe)X intermediates to obtain the appropriate dimer, includ-
ing the previously inaccessible [Cp*Cr(NO)X]2 compounds (5,
(42) The qualitative, readily-visualized bonding picture achieved through
extended-Hu¨ckel calculations provides the flexible conceptual framework
required for the comparisons we wish to draw, although this is accompanied
by an unavoidable loss of quantitative precision. The d-orbital labels are
assigned to imply the general shape and relative orientation of the metal-
based component of the bonding orbitals, not their exact, quantitative orbital
composition. For further discussion of the bonding in Cp′M(NO) complexes
(M ) Cr or Mo) at a variety of computational levels, see ref 43.
(43) (a) Hoffmann, R. Science 1981, 211, 995. (b) Schilling, B. E. R.;
Hoffmann, R.; Faller, J. W. J. Am. Chem. Soc. 1979, 101, 592. (c) Reference
14. (d) Herrmann, W. A.; Hubbard, J. L.; Bernal, I.; Korp, J. D.; Haymore,
B. L.; Hillhouse, G. L. Inorg. Chem. 1984, 23, 2978. (e) Legzdins, P.;
Rettig, S. J.; Sa´nchez, L.; Bursten, B. E.; Gatter, M. G. J. Am. Chem. Soc.
1985, 107, 1411. (f) Ashby, M. T.; Enemark, J. H. J. Am. Chem. Soc.
1986, 108, 730. (g) Hunter, A. D.; Legzdins, P.; Einstein, F. W. B.; Willis,
A. C.; Bursten, B. E.; Gatter, M. G. J. Am. Chem. Soc. 1986, 108, 3843.
(h) Bursten, B. E.; Cayton, R. H. Organometallics 1987, 6, 2004. (i)
Hubbard, J. L.; McVicar, W. K. Inorg. Chem. 1992, 31, 910. (j) Reference
6.
(44) Bonding in pseudo-tetrahedral complexes has been recently re-
viewed; see: Gibson, V. C. J. Chem. Soc., Dalton Trans. 1994, 1607.

Synthesis of 17-Valence-Electron [CpCr(NO)X2]- Anions
J. Am. Chem. Soc., Vol. 119, No. 15, 1997 3519
[CpCr(NO)X2]- complexes, which possess one nonbonding
orbital, relative to the 17e CpCr(CO)3 metalloradical, which has
no such orbital.
The remarkable mixing of Cp and NO π-bonding illustrated
in Figure 4 is due to the ability of the strong π-acceptor nitrosyl
to compete with the π-donor cyclopentadienyl for the available
metal π-symmetry orbitals. Similar extensive orbital reorga-
nization resulting from competition between two strong π-donor
ligands in four-coordinate compounds has been reported by
Schrock and co-workers,48 who find that “in complexes that
contain two different types of 2π, 1σ ligands, the 2π, 1σ ligands
are best treated as a unit rather than individually”.49 Research
on such compounds has since been largely restricted to high-
valent d0 and d2 complexes of the early transition metals
containing strong π-donors such as [η5-C5R5]-, [NR]2-, and
[CR]3- ligands.44 The EHMO calculations described here
suggest that a sufficiently strong π-acceptor ligand may compete
significantly with a strong π-donor ligand, leading to M(1σ,2π)2
electronic cores analogous to those found in pseudo-tetrahedral
compounds with two π-donor ligands. In other words, our
calculations indicate that CpCr(NO) may be considered to be a
M(1σ,2π)2 fragment, comparable to Cp′2Ti,50 CpV(NR),51
Cr(NR)2,52 and Cp′Cr(CR).53
Figure 4. Pictorial representation of the Cp and NO π-bonding
interactions with the five Cr 3d orbitals in [CpCr(NO)Cl2]-.
Armed with these perspectives on the bonding in CpCr
complexes, we can now more fully appreciate the consequences
of performing a one-electron oxidation of [CpCr(NO)Cl2]-. The
oxidation presumably effects the removal of the unpaired
electron from the high-energy, singly occupied orbital, thereby
generating a 16e compound. For the chromium species, this is
an irreversible process with removal of the unpaired electron
triggering nitrosyl-ligand dissociation. In the case of the
congeneric molybdenum compounds, however, this is a revers-
ible process both chemically and electrochemically. To explain
this dramatic difference, there must be some electronic property
that distinguishes the chromium complex from its molybdenum
congener. A strong possibility is that the transient CpCr(NO)-
Cl2 is an open-shell complex, as illustrated in Figure 5.
A recent review of “open-shell organometallics” has high-
lighted the importance of release of spin-pairing energy in
stabilizing paramagnetic first-row transition-metal complexes
as compared to their typically diamagnetic second-row conge-
maximum overlap of two orbitals (dxz and dyz) with the two Cp
π-symmetry orbitals, while the remaining three orbitals (dxy,
2
2
2
dx -y , and dz ) each form π-bonds with all three carbonyl
ligands. All five orbitals are doubly occupied, with the CpCr
π-orbitals being slightly lower in energy than the CrCO orbitals.
These results agree well with previous molecular-orbital calcula-
tions of CpM(CO)3 compounds.45 The same orbital orientation
holds for the [CpCrCl3]- complex. The dxz and dyz orbitals are
still filled and strongly π-bonding with the Cp ligand, but the
2
2
2
dxy, dx -y , and dz are now high in energy, singly occupied, and
weakly π-antibonding to the filled p-orbitals of the three Cl
ligands.
Finally, in the case of [CpCr(NO)Cl2]-, the orbitals mix and
reorient in order to maximize Cp and NO π-bonding at the
expense of any interaction with the Cl ligands.46 Four of the
five Cr d-orbitals combine with the two Cp and the two NO
π-symmetry orbitals to result in four low-energy, filled orbitals,
each of which are π-bonding to Cp and NO (Figure 4). The
remaining high-energy, singly-occupied d-orbital is nonbonding
to the strong π-ligands, although there is some π-antibonding
interaction with the weak π-donor Cl ligands.47
(48) Williams, D. S.; Schofield, M. H.; Anhaus, J. T.; Schrock, R. R. J.
Am. Chem. Soc. 1990, 112, 6728.
(49) Williams, D. S.; Schofield, M. H.; Schrock, R. R. Organometallics
1993, 12, 4560.
Comparison of the relative energies, occupancies, and bonding
character of the orbitals in Figure 3 suggests an appealing
explanation for the ligand control of electronic stability in these
cyclopentadienyl chromium complexes. It appears that non-
bonding d-orbitals in CpCrL3 complexes can readily accom-
modate single, unpaired electrons, thereby resulting in stable
paramagnetic species having fewer than 18e. This feature
accounts for the stability of 17e [CpCr(NO)L2]+ and
(50) (a) Pattiasina, J. W.; Heeres, H. J.; van Bolhuis, F.; Meetsma, A.;
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Filippou, A. C.; Wo¨ssner, D.; Lungwitz, B.; Kociok-Ko¨hn, G. Angew.
Chem., Int. Ed. Engl. 1996, 35, 876.
(45) (a) Lichtenberger, D. L.; Fenske, R. F. J. Am. Chem. Soc. 1976,
98, 50. (b) Albright, T. A.; Hofmann, P.; Hoffmann, R. J. Am. Chem. Soc.
1977, 99, 7546. (c) Hofmann, P.; Schmidt, H. R. Angew. Chem., Int. Ed.
Engl. 1986, 25, 837. (d) Reference 5a.
(46) A similar orbital shift in response to ligand asymmetry in CpMLL′X
complexes is lucidly described in ref 43b.
(47) Reexamination of the EHMO calculations of [CpCr(NO)(NH3)2]+
reveals that it possesses an identical “four-below-one” splitting pattern,
including two additional Cp and NO π-bonding orbitals lower in energy
than the two previously reported.6 The SOMO is around 0.8 eV lower in
energy than the corresponding orbital in [CpCr(NO)Cl2]-. While the
difference in charge undoubtedly plays an important role in this variation
between the two {Cr(NO)}5 complexes, the nature of the ligands (π-donating
chloride compared to the purely σ-bonding ammonia) is likely also a
contributing factor to this energy difference and the resulting ease of
oxidation of the dichloro anion over the bis-ammonia cation.

3520 J. Am. Chem. Soc., Vol. 119, No. 15, 1997
Legzdins et al.
complex, the π-overlap will be maximized if the Ph-N-Ph
plane is parallel to the M-NO vector, and the structure of this
compound shows the N-M-N-C(ipso) dihedral angles to be
less than 4.9° and 10.1°, actually putting one of the phenyl
groups in a sterically unfavorable position with respect to the
Cp ring. Consequently, this orientation must be ascribed to an
electronic interaction, namely that of the amide group function-
ing as a π-donor ligand to the metal center.
The existence of these two CpCr(NO)X2 complexes suggests
a continuum of three electronic configurations for CpCr(NO)
compounds, each selectively stabilized by a particular ligand
type: {Cr(NO)}6 compounds are stabilized by π-acidic ligands,
{Cr(NO)}5 configurations are favored for complexes with
π-neutral ligands, and {Cr(NO)}4 species are stabilized by
π-basic ligands. Studies are currently in progress to develop
rational synthetic routes to such π-donor stabilized {Cr(NO)}4
complexes.
Figure 5. Qualitative energy-level diagram for closed- and open-shell
CpM(NO)X2 complexes.
ners.54 The reduced orbital size, decreased orbital splitting, and
increased pairing energy of the lighter metals all contribute to
the formation of high-spin first-row species.55 Thus, it is
probable that whereas CpMo(NO)Cl2 is a low-spin, diamagnetic
complex, oxidation of [CpCr(NO)Cl2]- vacates an orbital of
sufficiently low energy that the high-spin configuration becomes
accessible to CpCr(NO)Cl2. In a transient open-shell CpCr-
(NO)Cl2 complex, the π-bonding orbitals to the nitrosyl are no
longer completely filled since one of the electrons in the M-NO
π-orbital has been promoted to the orbital above. This change
in electron configuration has the effect of weakening the metal-
nitrogen bond, thus promoting loss of the nitrosyl ligand.
Consequently, invoking the open-shell configuration for the
intermediate CpCr(NO)Cl2 complex explains both its instability
as compared to its molybdenum congener and its rapid release
of nitric oxide as a decomposition path.
Summary
Despite the chemical and electrochemical reversibility of the
[CpMo(NO)X2]0/- redox couple, CpCr(NO)Cl2 is not isolable
from the 1e oxidation of [CpCr(NO)Cl2]-. Recovery of
CpCr(NO)2Cl and [CpCrCl2]2 from this reaction suggests that
the initial oxidation product, CpCr(NO)Cl2, is unstable with
respect to NO loss. The difference between stable, diamagnetic
{Mo(NO)}4 and NO-labile {Cr(NO)}4 species may be ascribed
to the greater spin-pairing energy of Cr over Mo. If the spin-
pairing energy is comparable to the HOMO-LUMO energy gap
for CrCr(NO)Cl2, there may be a high-spin configuration
available to Cr where it is inaccessible for Mo.54 The formation
of an open-shell, S ) 1 species relieves spin pairing and regains
the single occupancy of the nonbonding orbital by promoting a
metal-nitrosyl π-bonding electron, which weakens the Cr-
NO bond and enhances NO lability. Furthermore, the stability
of both known Cp′Cr(NO)X2 {Cr(NO)}4 compounds may now
be viewed as depending on π-donation from alkoxide or amide
ligands to reinforce the low-spin state.56
Recognition of the propensity of high-spin Cp′Cr(NO)X2
species to lose NO has permitted us to rationalize previously
inexplicable reaction products and further develop the synthetic
chemistry of Cp′Cr(NO)-containing complexes. Performing the
halogenation of Cp′Cr(NO)(CO)2 in the coordinating solvent
NCMe stabilizes the initial, 17e products, effectively preventing
overoxidation and providing a direct route to new [Cp′Cr-
(NO)X]2 dimers.15,21 The solvent dependence of this reaction
is of general significance since controlled oxidations of transi-
tion-metal carbonyls with halogens constitute an important route
to metal-halide complexes, synthetic precursors to a vast
number of organotransition-metal compounds.58
The apparently anomalous stability of paramagnetic, 17e
CpCr(NO)-containing compounds (despite the strong π-acceptor
nitrosyl ligand and a lack of an open-shell54 configuration) can
be rationalized through π-bonding considerations. When these
compounds are coordinated with π-neutral ancillary ligands, four
of the Cr d-orbitals are strongly π-bound to both Cp and NO
ligands, consistent with viewing the CpCr(NO) fragment as a
M(1σ,2π)2 unit.49 The fifth Cr d-orbital remains higher in
energy and essentially nonbonding. EHMO calculations on
CpCrL3 anions suggest that such orbitals readily accommodate
unpaired electrons, resulting in stable paramagnetic complexes
with <18e.
If the key difference between stable CpMo(NO)X2 species
and unstable CpCr(NO)X2 intermediates is their spin state,
perhaps {Cr(NO)}4 complexes could be stabilized if the open-
shell configuration was rendered inaccessible. One way of
doing this is by raising the energy of the empty orbital which
is nonbonding to Cp and NO and thereby increasing the energy
gap between the LUMO and the four filled π-bonding orbitals
of the CpCr(NO) unit. Strong interaction between π-donor X
ligands and the π-symmetry LUMO would accomplish just such
a distortion of the orbital energies. The importance of π-dona-
tion in stabilizing unsaturated complexes has been recently
reviewed.56 Although there is evidence for weak π-donation
from the Cl ligands in [CpCr(NO)Cl2]-,47 this is apparently
insufficient to enforce a closed-shell configuration on the
transient CpCr(NO)Cl2 species. On the other hand, the stronger
π-donation from alkoxide and arylamide ligands appears to be
43i
sufficient since Cp*Cr(NO)(O-i-Pr)2 and CpCr(NO)(NPh2)-
I57 have been isolated from halide metathesis reactions of
Cp′Cr(NO)2Cl. Both of these {Cr(NO)}4 compounds have been
characterized by X-ray crystallography,43i,57 and in both cases
alignment of the heteroatom-carbon bond with the Cr-nitrosyl
bond in their solid-state molecular structures indicates that the
filled heteroatom p-orbital is involved in a π-bonding interaction
with the empty metal orbital lying in the plane perpendicular
to the Cr-NO axis.43b,d,i,f This interaction results in a closed-
shell complex, as illustrated pictorially in Figure 5. The
importance of this π-donation is especially apparent in the CpCr-
(NO)(NPh2)I compound.57 In the case of the diphenylamide
(54) Poli, R. Chem. ReV. 1996, 96, 2135.
Experimental Section
(55) Shriver, D. F.; Atkins, P.; Langford, C. H. Inorganic Chemistry,
2nd ed.; W. H. Freeman: New York, 1994; p 250.
(56) Caulton, K. G. New J. Chem. 1994, 18, 25.
(57) Sim, G. A.; Woodhouse, D. I.; Knox, G. R. J. Chem. Soc., Dalton
Trans. 1979, 83.
All reactions and subsequent manipulations were performed under
anaerobic and anhydrous conditions using an atmosphere of dinitrogen.
(58) Poli, R. Chem. ReV. 1991, 91, 509.

Synthesis of 17-Valence-Electron [CpCr(NO)X2]- Anions
J. Am. Chem. Soc., Vol. 119, No. 15, 1997 3521
General procedures routinely employed in these laboratories have been
Improved Preparation of [CpCr(NO)Cl]2. Orange CpCr(NO)-
(CO)2 (812 mg, 4.00 mmol) was dissolved in MeCN (∼15 mL), and
the solution was cooled to -30 °C. PCl5 (407 mg, 1.95 mmol) was
added, and the stirred mixture was allowed to warm slowly to room
temperature. The warming was accompanied by a color change to deep
green and the evolution of gas. The solvent was removed from the
final mixture in vacuo, and the green residue was washed with pentane
(2 × 15 mL). The crude green powder thus isolated (674 mg) exhibited
spectroscopic properties characteristic of [CpCr(NO)Cl]2, but its IR
spectrum indicated the presence of a small amount of CpCr(NO)2Cl25
in the sample. The green powder was washed with Et2O (3 × 10 mL)
and then recrystallized from toluene to obtain green microcrystals of
analytically pure [CpCr(NO)Cl]2.
Preparation of [NEt4][CpCr(NO)Cl2] ([NEt4][4]). [CpCr(NO)-
Cl]2 was prepared as described above from 4.00 mmol of CpCr(NO)-
(CO)2. The resulting green powder and [NEt4]Cl (650 mg, 3.92 mmol)
were suspended in toluene and stirred for 3 h, whereupon a color change
from green to brown occurred. The solvent was removed from the
final mixture in vacuo, the remaining solid was extracted into CH2-
Cl2/hexanes (20 mL, 4:1 ratio), and the mixture was filtered. Cooling
of the filtrate to -30 °C resulted in the deposition of [NEt4][4] as narrow
black blocks.
15
described in detail previously.59 The complexes [CpCr(NO)I]2 and
16
CpCr(NO)(CO)2 were prepared by published procedures. [Cp2Fe]-
[PF6] was prepared by addition of Na[PF6] to an aqueous solution of
[Cp2Fe]2[SO4].60 Cp*Cr(NO)(CO)261 was synthesized by treatment of
Cp*Cr(CO)3H62 with Diazald (Aldrich) in THF. All other reagents
were used as received from commercial suppliers. Filtrations were
performed through Celite (1 × 2 cm) supported on a medium porosity
glass frit unless otherwise specified. Ambient-temperature X-band ESR
spectra were recorded on an ECS 106 spectrometer with the assistance
of Dr. F. G. Herring of this Department and with the microwave
frequency being determined with an EIP 625A CW microwave counter.
Color, isolated yields, and analytical data for all product complexes
are listed in Table 1. Mass, IR, and ESR spectral data are collected in
Table 2.
Electrochemical Measurements. General methodology employed
during cyclic voltammetry studies in our laboratories has been described
previously.20 The three-electrode cell consisted of either (a) a Pt-bead
working electrode (∼1 mm diameter), a coiled Pt-wire auxiliary
electrode, and a Ag/AgCl pseudoreference electrode or (b) a Pt-disk
working electrode, a Pt-wire auxiliary electrode, and a Ag-wire
pseudoreference electrode. Cyclic voltammetry studies employed THF
solutions of 1 mM analyte and 0.1 M [n-Bu4N][PF6] support electrolyte.
The electrochemical cell was assembled and used in a Vacuum
Atmospheres model HE-43-2 glovebox. Voltages are reported versus
the saturated calomel electrode (SCE) and were measured using an
internal ferrocene standard, the highly reversible Cp2Fe/[Cp2Fe]+ couple
occurring at E1/2 ) +0.55 V vs SCE in THF under these conditions.
Molecular Orbital Calculations. Extended-Hu¨ckel theory calcula-
tions were carried out using the commercially available HyperChem
for Windows Release 3 and ChemPlus extensions for HyperChem. In
all cases, single-point calculations were performed on the structures
obtained from single-crystal X-ray crystallographic analyses40a,41b using
an unweighted Hu¨ckel constant of 1.75.
Preparation of [NBu4][CpCr(NO)I2] ([NBu4][1]). [CpCr(NO)I]2
(548 mg, 2.00 mmol) and [NBu4]I (739 mg, 2.00 mmol) were dissolved
in CH2Cl2 (30 mL) and stirred for 1 h, and the solvent was then removed
in vacuo. The red-brown residue was triturated with Et2O (25 mL),
collected on a frit, and further washed with Et2O (2 × 25 mL). The
resulting red-brown powder was analytically pure [NBu4][1].
Preparation of [NBu4][CpCr(NO)(OTf)2] ([NBu4][2]). [CpCr-
(NO)I]2 (548 mg, 2.00 mmol) and [NBu4]I (739 mg, 2.00 mmol) were
dissolved in CH2Cl2 (20 mL) and stirred for 1 h, after which time the
solution was cannulated onto AgOTf (1.03 g, 3.99 mmol). A flocculent
precipitate formed immediately. The mixture was stirred for 5 min
and was then filtered. The brown filtrate was taken to dryness in vacuo,
and the residue was triturated with hexanes (2 × 15 mL) to obtain
[NBu4][2] as a brown powder.
Preparation of [NBu4][CpCr(NO)Br2] ([NBu4][3]). A mixture of
[NBu4][CpCr(NO)(OTf)2] (346 mg, 0.503 mmol) and KBr (120 mg,
1.01 mmol) in CH2Cl2 (15 mL) was stirred for 3 d. The final brown
solution was filtered, and the filtrate was taken to dryness in vacuo.
The brown residue was recrystallized from CH2Cl2/hexanes to obtain
black prisms of [NBu4][3].
Reaction of [NBu4][CpCr(NO)(OTf)2] with KCl. A mixture of
[NBu4][CpCr(NO)(OTf)2] (320 mg, 0.465 mmol) and KCl (69 mg, 0.93
mmol) in CH2Cl2 (20 mL) was stirred at ambient temperatures for 1 d,
the final green solution was filtered, and the filtrate was taken to dryness
in vacuo. The remaining green powder was triturated with hexanes (2
× 15 mL) and recrystallized from CH2Cl2/hexanes to obtain a green
crystalline material. This material exhibited spectroscopic properties
consistent with its formulation as a salt of [CpCr(NO)Cl2]-, but it could
not be obtained analytically pure.
Preparation of [PPN][CpCr(NO)Cl2] ([PPN][4]). [PPN]Cl (205
mg, 0.357 mmol) was added to recrystallized [CpCr(NO)Cl]2 in toluene
(∼5 mL), and the mixture was stirred at ambient temperature. After 1
h, the solvent was removed in vacuo, the residue was extracted with
CH2Cl2, the extracts were filtered through Celite, and a small amount
of hexanes was added. Cooling the solution to -30 °C resulted in the
crystallization of [PPN][4]‚CH2Cl2 as black blocks (112 mg, 41% yield)
suitable for X-ray crystallographic analysis.
Reaction of the Dichloro Salt, [Et4N][4], with [Cp2Fe][PF6]. THF
(∼10 mL) was vacuum transferred onto [Et4N][4] (347 mg, 1.03 mmol)
and [Cp2Fe][PF6] (170 mg, 0.514 mmol), and the resulting solution
was stirred at room temperature for 1 h. The solvent was removed in
vacuo, the residue was extracted into CH2Cl2 (2 × 15 mL), the
combined extracts were filtered through Florisil (1 × 2 cm), and the
Florisil plug was washed with an additional amount (45 mL) of CH2-
Cl2. Solvent was removed from the combined filtrates in vacuo, and
the remaining residue was extracted with Et2O (2 × 15 mL). The
insoluble green powder left behind was recrystallized from CH2Cl2/
hexanes to obtain [CpCrCl2]2 (25 mg, 13% yield based on [4]).30 The
Et2O extracts were combined and taken to dryness, and the resulting
yellow powder was dissolved in Et2O (15 mL). The Et2O solution
was transferred to the top of a column of Florisil (2 × 8 cm) made up
in hexanes. Elution of the column with Et2O led to the development
and collection of two bands, the first orange-yellow in color and the
second yellow-gold. The first eluate was stripped of solvent to obtain
bright orange Cp2Fe (35 mg, 37% yield). The second was taken to
dryness in vacuo to obtain CpCr(NO)2Cl (52 mg, 50% yield based on
NO).25
Reaction of the Dichloro Salt, [NEt4][4], with NO. [NEt4][4] (174
mg, 0.500 mmol) was suspended in THF (15 mL), and nitric oxide
was bubbled slowly through the mixture for a total of 30 min. The
dark brown-gold solution was taken to dryness under reduced pressure,
the residue was extracted with CH2Cl2 (2 × 20 mL), and the extracts
were filtered through neutral alumina I (2 × 3 cm). Solvent was
removed from the filtrate in vacuo to obtain (43 mg, 40% yield) CpCr-
(NO)2Cl.25
Preparation of [Cp*Cr(NO)I]2 (5). MeCN (∼30 mL) was vacuum
transferred onto Cp*Cr(NO)(CO)2 (123 mg, 0.450 mmol), and the
orange chromium complex dissolved as the solution was warmed in
an ice bath. Iodine (0.056g, 0.441 mmol) was added, and the solution
was stirred for 6 h while being slowly warmed to ambient temperature,
after which time only peaks attributable to Cp*Cr(NO)(CO)2 remained
in the solution’s IR spectrum. The solution was heated with a 55 °C
water bath for ∼35 min, and the solution was then taken to dryness in
vacuo. The remaining green residue was dissolved in THF (15 mL)
and taken to dryness four times, and then triturated with Et2O (30 mL,
also removed in vacuo) four times. The residue was washed extensively
with hexanes and then extracted into toluene, filtered through Celite,
and recrystallized at -30 °C from toluene:hexanes.
(59) Legzdins, P.; Rettig, S. R.; Ross, K. J.; Batchelor, R. J.; Einstein,
F. W. B. Organometallics 1995, 14, 5579.
(60) Jolly, W. L. The Synthesis and Characterization of Inorganic
Compounds; Prentice-Hall: Englewood Cliffs, NJ, 1970; p 487.
(61) (a) King, R. B.; Efraty, A.; Douglas, W. M. J. Organomet. Chem.
1973, 60, 125. (b) Malito, J. T.; Shakir, R.; Atwood, J. L. J. Chem. Soc.,
Dalton Trans. 1980, 1253.
(62) Leoni, P.; Landi, A.; Pasquali, M. J. Organomet. Chem. 1987, 321,
365.
Preparation of [Cp*Cr(NO)Cl]2 (6). MeCN (∼20 mL) was

3522 J. Am. Chem. Soc., Vol. 119, No. 15, 1997
Legzdins et al.
vacuum transferred onto Cp*Cr(NO)(CO)2 (1.48 g, 5.41 mmol) and
the orange solution was warmed to -30 °C. PCl5 (0.550 g, 2.64 mmol)
was added, and the reaction mixture was stirred at -30 °C for ∼20
min and then allowed to warm slowly to room temperature. After 90
min, Et2O (10 mL) was added to the green solution, and the solution
was filtered. The plug of Celite was washed with Et2O (3 × 5 mL),
and the combined filtrates were taken to dryness in vacuo. The
remaining residue was triturated with Et2O (10 mL) and washed
extensively with pentane (7 × 5 mL). Remaining solvent was removed
under static vacuum to obtain crude [Cp*Cr(NO)Cl]2 as a green powder
(1.22 g). Analytically pure samples of the complex were obtained as
dark green microcrystals by recrystallizing the powder overnight at -30
°C from a ∼2:1 toluene:hexanes solvent mixture.
Selected bond lengths and bond angles appear in Table 4. Tables
of final atomic coordinates and equivalent isotropic thermal parameters,
anisotropic thermal parameters, complete tables of bond lengths and
angles, torsion angles, intermolecular contacts, and least-squares planes
are included as Supporting Information.
Acknowledgment. This paper is dedicated to the memory
of Sir Geoffrey Wilkinson whose brilliant explorations at the
frontiers of inorganic chemistry will continue to illuminate
research in this field for many years to come. We are grateful
to the Natural Sciences and Engineering Research Council of
Canada for support of this work in the form of grants to P.L.
and a postgraduate scholarship to W.S.M. We also thank
Professor Rinaldo Poli for helpful discussions and for providing
preprints of refs 7e and 54 prior to publication, and we
acknowledge Dr. David J. Burkey for his constructive comments
during the preparation of this manuscript.
X-ray Structure Determination of [PPN][CpCr(NO)(Cl)2]‚CH2-
Cl2 ([PPN][4]‚CH2Cl2). Crystallographic data appear in Table 3. The
final unit-cell parameters were obtained by least-squares analysis on
the setting angles for 25 reflections with 2θ ) 18.0-25.9°. The
intensities of three standard reflections, measured every 200 reflections
throughout the data collections, showed only minor random fluctuations.
The data were processed63 and corrected for Lorentz and polarization
effects and absorption (empirical, based on azimuthal scans).
The structure was solved by the Patterson method. The structure
analysis was initiated in the centrosymmetric space group P1h, this choice
being confirmed by subsequent calculations. The NO and Cl ligands
are disordered, all three sites being occupied by both types of ligand.
Geometric restraints were employed for the three partial nitrosyl ligands.
As a result of the disorder and the restraints, the geometry involving
the chloride and nitrosyl ligands is probably less precise than indicated
by the standard deviations. All non-hydrogen atoms except the low-
occupancy nitrosyl N and O atoms were refined anisotropically. The
hydrogen atoms were fixed in calculated positions with C-H ) 0.98
Å and BH ) 1.2 Bbonded atom. No secondary extinction correction was
necessary. Neutral atom scattering factors and anomalous dispersion
corrections were taken from the International Tables for X-Ray
Crystallography.64
Supporting Information Available: Tables of fractional
coordinates and displacement parameters, intramolecular dis-
tances and angles, supplementary crystallographic data for the
structure determination, hydrogen atom coordinates, anisotropic
thermal parameters, torsion angles, nonbonded contacts out to
3.80 Å, and least-squares planes for [PPN][4]‚CH2Cl2 (27
pages). See any current masthead page for ordering and Internet
access instructions.
JA963464Z
(64) (a) International Tables for X-Ray Crystallography; Kynoch Press:
Birmingham, UK (present distributor Kluwer Academic Publishers: Boston,
MA), 1974; Vol. IV, pp 99-102. (b) International Tables for X-Ray
Crystallography; Kluwer Academic Publishers: Boston, MA, 1992; Vol.
C, pp 200-206.
(63) teXsan: Structure Analysis Package; Molecular Structure Corp.: The
Woodlands, TX, 1995.