Phosphorus-31 Chemical Shift Anisotropies in Solid, Octahedral Chromium(0) Triphenylphosphine Derivatives

Article abstract of DOI:10.1021/ic960816u

The ³¹P chemical shift anisotropies have been measured at 293 K for the triphenylphosphine ligands in solid pentacarbonyl(triphenylphosphine)chromium(0), Cr(CO)₅(PPh₃) (1), and cis- and trans-tetracarbonyl(triphenylphosphine)(thiocarbonyl)chromium(0), Cr(CO)₄(PPh₃)(CS) (2 and 3). The major changes in the shift tensors occur for the δ₁₁ and δ₂₂ components perpendicular to the Cr-P bond direction. The individual tensor components of the ³¹P chemical shifts are clearly more important than are the isotropic values in providing information on the chromium-phosphorus bonding. The crystal structure of 3 has been determined by single-crystal X-ray diffraction. The complex crystallizes in the monoclinic P2₁/n (No. 14) space group with cell constants (at 293 K) a = 9.558(5) A?, b = 15.275(2) A?, c = 15.341(2) A?, β = 96.66(2)°, V = 2225(1) A?³ and Z = 4; R = 0.069 and R_w = 0.067. The crystal structure of 1 has been reported previously and most of the structural features are quite similar to those for 3. For instance, the Cr-P distances are 2.422(4) A? for 1 and 2.424(4) A? for 3. The P-Cr-C(S) and Cr-C-S linkages in 3 are almost linear with the angles being 178.4(4) and 174.1(9)°, respectively. The most significant difference between 1 and 3 is that Cr-C(X) (X = O, S) distance trans to PPh₃ is appreciably shorter for the thiocarbonyl, viz., 1.79(1) vs 1.845(4) A?. This shortening would be expected if CS is a much better π-acceptor ligand than is CO, as is thought to be the case.

Full text of DOI:10.1021/ic960816u

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Inorg. Chem. 1997, 36, 435-438
435
Phosphorus-31 Chemical Shift Anisotropies in Solid, Octahedral Chromium(0)
Triphenylphosphine Derivatives
Yining Huang, Haewon L. Uhm, Denis F. R. Gilson,* and Ian S. Butler*
Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal,
Quebec, Canada H3A 2K6
ReceiVed July 11, 1996X
The 31P chemical shift anisotropies have been measured at 293 K for the triphenylphosphine ligands in solid
pentacarbonyl(triphenylphosphine)chromium(0), Cr(CO)5(PPh3) (1), and cis- and trans-tetracarbonyl(triphenylphos-
phine)(thiocarbonyl)chromium(0), Cr(CO)4(PPh3)(CS) (2 and 3). The major changes in the shift tensors occur
for the δ11 and δ22 components perpendicular to the Cr-P bond direction. The individual tensor components of
the 31P chemical shifts are clearly more important than are the isotropic values in providing information on the
chromium-phosphorus bonding. The crystal structure of 3 has been determined by single-crystal X-ray diffraction.
The complex crystallizes in the monoclinic P21/n (No. 14) space group with cell constants (at 293 K) a ) 9.558(5)
Å, b ) 15.275(2) Å, c ) 15.341(2) Å, â ) 96.66(2)°, V ) 2225(1) Å3 and Z ) 4; R ) 0.069 and Rw ) 0.067.
The crystal structure of 1 has been reported previously and most of the structural features are quite similar to
those for 3. For instance, the Cr-P distances are 2.422(4) Å for 1 and 2.424(4) Å for 3. The P-Cr-C(S) and
Cr-C-S linkages in 3 are almost linear with the angles being 178.4(4) and 174.1(9)°, respectively. The most
significant difference between 1 and 3 is that Cr-C(X) (X ) O, S) distance trans to PPh3 is appreciably shorter
for the thiocarbonyl, Viz., 1.79(1) Vs 1.845(4) Å. This shortening would be expected if CS is a much better
π-acceptor ligand than is CO, as is thought to be the case.
Introduction
investigate the variation of the principal components of the
shielding tensor with changes in molecular structure.
Phosphorus-31 NMR spectroscopy has been widely used in
organometallic chemistry since the chemical shift is extremely
sensitive to electronic, steric, and geometric environments of
the 31P nuclei.1 Upon coordination to a metal, the chemical
shifts are also influenced by such factors as the bonding
interaction between the phosphorus ligand and the metal, the
nature and the oxidation state of the metal, the coordination
number, the influence of the trans ligand, and the position of
the phosphorus ligand within the coordination sphere. Most of
the theoretical and experimental work on coordinated phos-
phorus compounds, however, has been limited to studies of
isotropic chemical shifts in solution. Only a few investigations
have dealt fully with the tensorial nature of the chemical shift
interactions. The use of isotropic chemical shifts results in the
loss of important information because the principal axial
components are inherently more informative than are the
isotropic values. For example, in some cases, a change in
molecular structure may lead to a small or negligible change in
isotropic shift, while the individual principal components may
undergo substantial variations in opposing directions. Therefore,
the individual tensor components, δii [or in some cases, the
anisotropy, ∆δ ) δ11 - (δ22 + δ33)/2], are more sensitive to
the structured parameters. The first step to improve the
understanding of 31P chemical shielding interactions is to
Many transition metals have quadrupole moments, and the
solid-state NMR spectra can be complicated by interactions
between the shift tensor, the quadrupole tensor, and the dipolar
and scalar couplings.2 Chromium, however, has only one
isotope with spin, 53Cr (I ) -3/2), which is only 10% abundant,
and so the spectra are much simpler. The results of measure-
ments of the 31P shielding tensors for three chromium(0)
complexes, Cr(CO)5(PPh3) (1) and cis- and trans-Cr(CO)4(CS)-
(PPh3) (2 and 3), are presented here, together with the X-ray
crystal structure of 3 for comparison with the known crystal
structure of 1.3
Experimental Section
Triphenylphosphine (Omega Chemical Co.) and trimethylamine
N-oxide hydrate (Aldrich Chemical Co.) were used without further
purification. All solvents used were of reagent-grade quality (Aldrich
Chemical Co.). The solvents were dried over sodium/benzophenone
and distilled under N2 immediately prior to use. All the synthetic
procedures were performed under N2 atmosphere in Schlenk-type
apparatus and the solvents and solutions were transferred by means of
syringes.
Complexes 1-3 were prepared as follows. A mixture of Cr(CO)5-
(CS) (0.192 g, 0.81 mmol),4 PPh3 (0.218 g, 0.83 mmol), and the ligand
substitution promoter5,6 trimethylamine N-oxide (Me3NO‚2H2O; 0.120
g, 1.08 mmol) in toluene (50 mL) was heated at 50 °C for 1 day. The
resulting solution was allowed to cool and filtered. The solvent was
removed from the filtrate under reduced pressure (Buchi rotavapor).
The oily residue remaining was adsorbed onto silica gel (Merck, grade
X Abstract published in AdVance ACS Abstracts, January 1, 1997.
(1) For example, see: (a) Lindner, E.; Fawzi, R.; Mayer, H. A.; Eichele,
K.; Pohmer, K. Inorg. Chem. 1991, 30, 1102. (b) Dixon, K. R. In
Multinuclear NMR; Mason, J., Ed.; Plenum Press: New York, 1987.
Phosphorus-31 NMR Spectroscopy in Stereochemical Analysis: Or-
ganic Compounds and Metal Complexes; Verkade, J. G., Quin, L. D.,
Eds.; VCH Publishers: Deerfield Beach, FL, 1987. (c) Gorenstein,
D. G. Prog. Nucl. Magn. Reson. 1983, 16, 1. (d) Meek, D. W.,
Mazanec, T. J. Acc. Chem. Res. 1981, 14, 266. (e) Pregosin, P. S.;
Kunz, R. W. 31P and 13C NMR of Transition Metal Phosphine
Complexes in NMR Basic Principles and Progress; Springer-Verlag:
Berlin, 1979; Vol. 16.
(2) Harris, R. K.; Olivieri, A. C. Prog. NMR Spectrosc. 1992, 24, 435.
(3) Plastas, H. J.; Stewart, J. M.; Grim, S. O. Inorg. Chem. 1973, 12,
265.
(4) (a) English, A. M.; Plowman, K. R.; Baibich, I. M.; Hickey, J. P.;
Butler, I. S. J. Organomet. Chem. 1981, 205, 177. (b) Uhm, H. L.
Ph.D. Thesis, McGill University, Montreal, Quebec, Canada, 1992.
(5) Albers, M. O.; Coville, N. J. Coord. Chem. ReV. 1984, 53, 227.
(6) Luh, T.-Y. Coord. Chem. ReV. 1984, 60, 255.
S0020-1669(96)00816-6 CCC: $14 00 © 1997 American Chemical Society

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436 Inorganic Chemistry, Vol. 36, No. 3, 1997
Huang et al.
60, 230-400 mesh; 1.3 g) and this mixture was placed on top of a 33
× 1.7 cm silica gel chromatography column. A 1:20 ether-hexanes
mixture was used to elute the reaction products from the chromatog-
raphy column. Subsequent solvent removal under reduced pressure
afforded a yellow solid (0.146 g). Slow recrystallization of this material
from a 1:1 THF-hexanes mixture gave crystallographic-grade yellow
crystals of 3 (0.64 g) and lesser amounts of compounds 1 and 2. The
identity of each complex was established by comparison of its IR
[ν(CO) region] and 31P-NMR spectra in solution with those already in
the literature.7 Data for 2 are as follows. IR (hexanes): ν(CO) 2045
Table 1. 31P Chemical Shift Anisotropies of Selected
Triphenylphosphine Derivativesa,b
compound
δ11 δ22
δ33
δiso ∆δh spani
c
PPh3
d
9
6
127
109
9
6
-42 -8 26 -33
-43 -10 25 -37
Cr(CO)5(PPh3) (1)
cis-Cr(CO)4(PPh3)(CS) (2)
trans-Cr(CO)4(PPh3)(CS) (3) 101
ClRh(PPh3)3
80 -42
74 -30
70 -37
38 -59
73 -100
37 -52
39 -43
38 -38
19 -52
27 -49
54 108
85
52 87 139
46 85 138
23 102 150
50 185 271
32 118 162
26 85 126
29 87 125
15 96 131
20 94 132
24 121 135
e
91
171
110
83
87
79
w, 1992 w, 1960 vs, 1945 w; ν(CS) 1244 m cm-1
.
31P-NMR
f
(CDCl3): 52.43 ppm (s). Data for 3 are as follows. IR (hexanes):
cis-PtMe2(PPh3)2
ν(CO) 2063 vvw, 1960 vs; ν(CS) 1244 m cm-1
.
13C-NMR (CDCl3):
330.94 (1C, s, CS), 212.88 ppm (4C, s, CO). 31P-NMR (CDCl3): 46.18
ppm (s).
cis-PtPh2(PPh3)2
f
83
g
trans-Ir(CO)Cl(PPh3)2
trans-Pt(PPh3)2Cl2
cis-Pt(PPh3)2Cl2
105 -2 -30
85 -7 -13
89 -10 -41
86 -18 -35
The IR spectra were recorded on a Bomem MB-100 FT spectrometer
equipped with a KBr beamsplitter and a DTGS detector. The 13C- and
31P-NMR spectra were measured for concentrated solutions on a Varian
XL-300 spectrometer. The solid-state 31P-NMR spectra were obtained
on a Chemagnetics CMX-300 spectrometer operating at 121.28 MHz,
with cross-polarization (CP) and high-power proton decoupling. The
90° pulse width was about 2.5 µs. The CP-MAS spectra were recorded
for samples packed into a bullet-type zirconia rotor (7.5 mm diameter).
Typical conditions employed were as follows: recycle delay, 10 s;
contact time, 2 ms; number of data acquisitions, 3000-6000. The
principal values of the chemical shift tensor were determined from the
spinning sideband intensities in the MAS spectra by the graphical
method of Herzfeld and Berger8 from the CP-MAS spectra acquired at
two different spinning rates. The tensor components of the 31P nuclei
in PPh3 were obtained from the nonspinning spectra only, using a phase-
cycled Hahn echo pulse sequence (90°-t-180°).9 The undistorted echo
spectrum was obtained by taking a Fourier transform of the data starting
at the echo maximum. All the measurements were conducted at 293
K, except that spectra of PPh3 were also recorded at low temperature,
where the sample temperature was regulated to within (2 K by a RKC
REX-C1000 temperature controller.
h
22 95
98
h
13 115 130
11 113 121
8
58
3
-37
75
95
a Values of the principal elements and isotropic shifts are in ppm,
relative to external 85% H3PO4. b Uncertainties are (10 and (5 ppm
for the data obtained from the MAS and static spectra, respectively.
c Data measured at 298 K. d Data measured at 133 K. e From ref 24.
f From ref 23. g From ref 25. h ∆δ defined as δ11 - (δ22 + δ33)/2.
i Defined as δ11 - δ33 (see: Mason, J. Solid state NMR 1993, 2, 285).
phosphorus site does not exist in the crystal lattice,14 the question
arises whether averaging of the perpendicular components results
from large-amplitude motions of the phenyl rings.15 A low-
temperature measurement, at 133 K, was performed, which
showed that the shift anisotropy and span were essentially
unchanged. The measured values for the individual shift tensor
elements are given in Table 1 and are in good agreement with
those reported previously.13
The principal components of the shielding tensors and
anisotropies of the 31P nuclei in complexes 1-3 determined
experimentally are listed in Table 1. The isotropic values of
the 31P chemical shifts in the solid-state for these triphenylphos-
phine-chromium(0) complexes are very close to the values
reported previously for the complexes in solution.4,16 This
suggests that there are probably no significant changes in the
structures of these compounds on going from solution to the
solid state. It is apparent from the data in Table 1 that, upon
coordination to the metal, the 31P anisotropies in the PPh3 ligand
become quite large (often more than tripled) compared with
that of free PPh3. However, the values of δ33 change very little.
The degeneracy of δ11 and δ22 is lifted and these two
components are much more deshielded in the complexes than
is the perpendicular component in the free PPh3 molecule.
Details of the crystal structure determination of complex 3
are shown in Table 2. The final atomic positions and selected
bond lengths and bond angles are listed in Tables 3-5,
respectively. An ORTEP view of the molecule is shown in
Figure 1, while the crystal packing is illustrated in Figure 2.
There are remarkably few differences between the structures
of 13 and 3. The former crystallizes in a triclinic unit cell with
Z ) 2 compared to the monoclinic structure of 3 with Z ) 4.
The Cr-P bond distances are the same in both cases: 2.422(4)
and 2.424(4) Å for the pentacarbonyl and thiocarbonyl, respec-
tively. The average Cr-C(O) distance of the four cis-carbonyls,
however, is longer in the thiocarbonyl complex, 1.91(1) Vs
The single-crystal, X-ray measurements were performed on a Rigaku
AFC6S diffractometer, with graphite-monochromated Mo KR radiation.
A yellow parallelepiped crystal with approximate dimensions 0.20 ×
0.18 × 0.04 mm was mounted on a glass fiber. Cell constants and an
orientation matrix for data collection were obtained from a least-squares
refinement using the setting angles of 25 carefully centered reflections
in the range 15.02 < 2θ < 22.95°. The data collected using the ω-2θ
scan technique to a maximum 2θ value of 50° at a scan rate of 16°/
min. Of the 4353 reflections collected, 4904 were unique. After every
150 reflections, the intensities of three representative reflections were
measured and these remained constant throughout the data collection.
An empirical adsorption correction, using the program DIFABS,10 was
applied and resulted in transmission factors ranging from 0.77 to 1.11.
Corrections for Lorentz and polarization effects were also applied. The
structure was solved by direct methods11 with the non-hydrogen atoms
being refined anisotropically. The final cycle of the full-matrix least-
squares refinement, based on 1499 observed reflections [I > 3.00σ(I)]
and 271 variable parameters, converged with unweighted and weighted
agreement factors of R ) 0.070 and Rw ) 0.067. All calculations were
performed using the TEXSAN-TEXRAY crystallographic software
package.12
Results and Discussion
The shielding tensor of PPh3 has been investigated previously,
but only at room temperature, and an axially symmetric powder
pattern was observed.13 Since the precise C3 symmetry at the
(7) Dombek, B. D.; Angelici, R. J. Inorg. Chem. 1976, 15, 1089.
(8) Herzfeld, J.; Berger, A. E. J. Chem. Phys. 1980, 73, 6021.
(9) Rance, M.; Byrd, R. A. J. Magn. Reson. 1983, 52, 221.
(10) Walker, N.; Stuart, D. Acta Crystallogr. 1983, 39A, 158.
(11) (a) Gilmore, C. J. J. Appl. Crystallogr. 1984, 17, 42. (b) Beirskens,
P. T. Direct Methods for Difference Structures. Technical Report 1;
Crystallography Laboratory: Toernooiveld, 6525 Ed Nijmegan, The
Netherlands, 1984.
(13) Penner, G. H.; Wasylishen, R. E. Can. J. Chem. 1989, 67, 1909.
(14) (a) Daly, J. J. J. Chem. Soc. 1964, 3799. (b) Dunn, B. J.; Orpen, A.
G. Acta Crystallogr. 1991, 47C, 345.
(15) Schaefer, T.; Sebastian, R.; Hruska, F. E. Can. J. Chem. 1993, 71,
639.
(12) TEXSAN-TEXRAY Structure Analysis Package; Molecular Structure
Corp.: The Woodlands, TX, 1985 and 1991.
(16) Grim, S. O.; Wheatland, D. A.; McFarlane, W. J. Am. Chem. Soc.
1987, 89, 5537.

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Chromium(0) Triphenylphosphine Derivatives
Table 2. Crystallographic Data for trans-Cr(CO)4(CS)(PPh)3
Inorganic Chemistry, Vol. 36, No. 3, 1997 437
Table 5. Intramolecular Bond angles (deg) Involving
Non-Hydrogen Atoms with Estimated Standard Deviations in the
Least Significant Figure Given in Parentheses
chem formula: C23H15CrO4PS
lattice params
fw ) 470.4
space group: P21/n (No. 14)
T ) 298 K
a ) 9.558 (5) Å
b ) 15.275 (2) Å
c ) 15.341 (2) Å
â ) 95.66 (2)°
P-Cr-C(19)
178.4(4)
92.5(4)
92.0(4)
90.2(4)
92.9(4)
88.1(6)
86.6(6)
89.2(6)
88.5(5)
86.4(6)
C(4)-C(5)-C(6)
P-C(6)-C(1)
120.4(13)
117.7(9)
λ ) 0.710 69 Å
ρcalcd ) 1.402 g cm-3
µ ) 4.84 cm-1
R ) 0.069a
P-Cr-C(20)
P-Cr-C(21)
P-C(6)-C(5)
124.2(10)
118.2(12)
120.9(12)
121.5(13)
117.6(12)
121.1(13)
P-Cr-C(22)
C(1)-C(6)-C(5)
C(8)-C(7)-C(12)
C(7)-C(8)-C(9)
C(8)-C(9)-C(10)
C(9)-C(10)-C(11)
V ) 2225 (1) Å3
Z ) 4
P-Cr-C(23)
Rw ) 0.067b
C(19)-Cr-C(20)
C(19)-Cr-C(21)
C(19)-Cr-C(22)
C(19)-Cr-C(23)
C(20)-Cr-C(21)
a R ) ∑||Fo| - |Fc||/∑|Fo|. b Rw ) [(∑w(|Fo| - |Fc|)2)/∑w|Fo| ]1/2
Table 3. Positional Parameters and B(eq) Values (Å2) for 3 with
Estimated Standard Deviations in the Least Significant Figure Given
in Parentheses
.
2
C(10)-C(11)-C(12) 122.2(12)
P-C(12)-C(7)
124.1(9)
119.3(9)
116.6(11)
C(20)-Cr-C(22) 176.4(6)
P-C(12)-C(11)
C(7)-C(12)-C(11)
C(20)-Cr-C(23)
C(21)-Cr-C(22)
91.0(5)
95.9(5)
C(14)-C(13)-C(18) 120.5(13)
C(13)-C(14)-C(15) 120.2(14)
C(14)-C(15)-C(16) 120.2(13)
C(15)-C(16)-C(17) 120.0(13)
C(16)-C(17)-C(18) 121.6(13)
atom
x
y
z
B(eq)
0.11067(13) 0.57558(12) 3.63(9)
-0.0954(3) 0.6407(3) 9.8(3)
0.26379(21) 0.53847(20) 3.32(15)
C(21)-Cr-C(23) 174.5(5)
Cr
0.27320(21)
0.3210(6)
0.2377(3)
C(22)-Cr-C(23)
Cr-P-C(6)
86.4(5)
113.6(4)
115.2(4)
118.6(5)
104.4(6)
102.6(5)
100.4(5)
S
P
Cr-P-C(12)
Cr-P-C(18)
O1
O2
O3
O4
C1
C2
C3
C4
C5
C6
C7
C8
-0.0367(10)
0.2730(11)
0.5886(10)
0.2675(10)
-0.0504(14)
-0.1869(15)
-0.2145(16)
-0.1079(21)
0.0317(15)
0.0597(13)
0.1706(14)
0.1936(16)
0.3079(18)
0.4003(15)
0.3776(13)
0.2622(12)
0.3571(16)
0.4390(19)
0.5139(16)
0.5123(16)
0.4311(15)
0.3513(12)
0.3016(14)
0.0792(16)
0.2717(14)
0.4706(14)
0.2687(12)
0.0709(6)
0.1366(7)
0.1317(7)
0.0529(6)
0.2710(9)
0.2958(10)
0.3552(11)
0.3885(11)
0.3641(9)
0.3040(8)
0.3443(8)
0.3612(9)
0.3275(10)
0.2771(8)
0.2597(8)
0.2928(7)
0.4310(9)
0.4911(9)
0.4660(11)
0.3815(11)
0.3219(8)
0.3452(9)
-0.0014(8)
0.0902(8)
0.1341(8)
0.1245(9)
0.0763(8)
0.5745(7)
0.7786(5)
0.5748(6)
0.3852(5)
0.4977(9)
0.5063(10)
0.5660(12)
0.6211(11)
0.6130(9)
0.5522(8)
0.3718(8)
0.2859(8)
0.2510(9)
0.3047(9)
0.3888(8)
0.4253(7)
0.5725(9)
0.6230(11)
0.6975(10)
0.7234(8)
0.6743(9)
0.5988(8)
0.6058(8)
0.5723(9)
0.6991(9)
0.5751(8)
0.4550(8)
7.2(6)
7.5(6)
6.7(6)
5.8(5)
4.9(8)
6.1(9)
6.6(9)
7.8(10)
5.5(8)
3.9(6)
4.9(7)
5.2(7)
5.7(8)
5.3(8)
4.4(6)
3.5(6)
6.0(8)
7.3(10)
6.6(9)
6.8(9)
5.4(7)
3.8(6)
5.4(7)
5.0(7)
5.5(7)
4.3(7)
3.9(6)
P-C(18)-C(13)
P-C(18)-C(17)
122.8(10)
119.7(10)
C(6)-P-C(12)
C(6)-P-C(18)
C(12)-P-C(18)
C(13)-C(18)-C(17) 117.5(11)
Cr-C(19)-S
174.3(9)
C(2)-C(1)-C(6) 120.8(12) Cr-C(20)-O(1)
C(1)-C(2)-C(3) 121.0(14) Cr-C(21)-O(2)
C(2)-C(3)-C(4) 119.5(14) Cr-C(22)-O(3)
C(3)-C(4)-C(5) 120.1(14) Cr-C(23)-O(4)
173.8(12)
171.1(11)
179.2(12)
177.3(11)
C9
C10
C11
C12
C13
C14
C15
C16
C17
C18
C19
C20
C21
C22
C23
Table 4. Intramolecular Distances (Å) Involving Non-Hydrogen
Atoms with Estimated Standard Deviations in the Least Significant
Figure Given in Parentheses
Figure 1. Molecular structure of 3. ORTEP representations are at
30% probability.
Cr-P
2.422(4)
C(2)-C(3)
1.337(25)
1.35(3)
Cr-C(19)
Cr-C(20)
Cr-C(21)
Cr-C(22)
Cr-C(23)
S-C(19)
1.786(13)
1.875(15)
1.931(14)
1.900(14)
1.918(14)
1.537(13)
1.843(13)
1.833(11)
1.830(12)
1.150(17)
1.218(16)
1.134(17)
1.128(16)
1.379(20)
1.362(19)
C(3)-C(4)
C-O distances are 1.147 Å in 1 and 1.158 ( 0.016 Å in 3.
C(4)-C(5)
1.406(24)
1.357(18)
1.385(18)
1.376(18)
1.371(23)
1.372(22)
1.359(18)
1.389(17)
1.384(21)
1.374(20)
1.33(3)
The geometry of the tertiary phosphine ligand is the same, with
C(5)-C(6)
14
average C-P-C angles of 102.7° (103.0° in the free PPh3
)
C(7)-C(8)
and Cr-P-C angles of 115.7° in both cases. The average P-C
distance in 3 is 1.834(1) Å, i.e., close to that normally found
for PPh3 as a ligand to transition metals (1.828 Å).18 Finally,
as for free PPh3,14 one of the C-P-C angles in the coordinated
PPh3 ligand in both 1 and 3 is slightly smaller (∼2°) than the
other two.
C(7)-C(12)
C(8)-C(9)
P-C(6)
C(9)-C(10)
C(10)-C(11)
C(11)-C(12)
C(13)-C(14)
C(13)-C(18)
C(14)-C(15)
C(15)-C(16)
C(16)-C(17)
C(17)-C(18)
P-C(12)
P-C(18)
O(1)-C(20)
O(2)-C(21)
O(3)-C(22)
O(4)-C(23)
C(1)-C(2)
C(1)-C(6)
A basic assumption, which is often used to discuss the
tensorial orientation, is that changes in the environment of the
phosphorus nucleus usually lead to changes in the values of
the principal components and leave the orientation of the
principal axes relatively unaffected, provided that the local
symmetry remains unchanged. For example, the direction of
the most shielded component, δ33, lies along the Rh-P bond
direction for all three 31P nuclei in ClRh(PPh3)3.19 Although
1.351(25)
1.365(19)
1.358(19)
1.88(1) Å, and the former is equal to the average Cr-C(O)
distance in Cr(CO)6 [1.909(3) Å].17 The trans-carbonyl Cr-
C(O) bond distance in 1 is 1.88(2) Å, while the corresponding
Cr-C(S) distance in 3 is 1.78(1) Å. This difference implies
that the π-back-bonding to the thiocarbonyl group is stronger
than to the trans-carbonyl ligand. The average cis-carbonyl
(18) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson, D.
G.; Taylor, R. J. Chem. Soc., Dalton Trans. 1989, 81.
(19) Naito, A.; Sastry, D. L.; McDowell, C. A. Chem. Phys. Lett. 1985,
115, 19.
(17) Whitaker, A.; Jeffrey, J. W. Acta Crystallogr. 1967, 23, 977.

+
+
438 Inorganic Chemistry, Vol. 36, No. 3, 1997
Huang et al.
observations are exactly the opposite. In transition metal-
phosphorus chemistry, PPh3 is generally considered to be a poor
π-acceptor ligand. However, Wovkulich and co-workers22 have
demonstrated recently that the π-bonding between Cr and PPh3
is quite significant for Cr(CO)4(PPh3)L complexes. Since the
PPh3 ligand competes for electron density with the ligand trans
to it, the importance of the π-bonding will depend on the nature
of the ligand L. The π-bonding between PPh3 and Cr is more
important when L is a strong σ-donor, and less so if L is a
good π-acceptor. Therefore, since CS is a better π-acceptor
ligand than is CO, the π-back-bonding interaction between PPh3
and Cr should be relatively stronger in 1 compared to that in 3.
Consequently, the larger 31P shielding frequency found for 1
compared to that for 3 suggests a larger anisotropy of electronic
density distribution around the phosphorus, in particular, in the
Cr-P direction, due to the slightly stronger π-bonding in 1.
The more deshielded δ11 and δ22 components may also arise
from slight differences in the paramagnetic contributions from
low-lying Cr-P σfπ*-transition energies.
The shielding tensor values for several transition metal-
triphenylphosphine complexes are also listed in Table 1. While
the anisotropy (∆δ) for complex 3 (85 ppm) is slightly lower,
that for complex 1 (108 ppm) is comparable to the 121 ppm
23
value for trans-Ir(CO)Cl(PPh3)2 and those for the two PPh3
ligands trans to each other in ClRh(PPh3)2 (102 and 118 ppm),
but they are substantially smaller than the 185 ppm anisotropy
for the PPh3 ligand trans to Cl in ClRh(PPh3)3.18 Upon
complexation to a metal, the δ33 values for all the complexes
listed, except for the PPh3 ligand trans to Cl in ClRh(PPh3)3,
do not vary markedly from the δ| value in free PPh3. The δ11
and δ22 components are most affected by metal-ligand bond
formation, and the degeneracy of these components is always
lifted upon complexation. The values of these two components
also depend on the nature of the metal. In conclusion, δ11 and
Figure 2. Crystal packing of 3.
the X-ray studies show that the 31P atoms in 1 and 3 do not lie
on a true crystallographic symmetry element, the Cr-PPh3
fragments in these complexes do have approximate C3V local
symmetry, and it is reasonable to assume that the δ33 direction
is along the Cr-P bond axis. This assumption has been
validated even for a number of less symmetric metal cyclic bis-
(phosphine) complexes.1a,20 The remaining two tensorial com-
ponents must be in a plane perpendicular to the δ33 direction.
Since an approximate local mirror plane at phosphorus requires
one of these components to be perpendicular to the plane, the
other one must lie in it. However, it is difficult to assign either
δ11 or δ22 specifically to one of these two directions.
δ
22 are particularly sensitive to the bonding between the metal
and the PPh3 ligand. The individual tensor components of the
chemical shift are more informative than are the isotropic values
in attempting to understand the nature of the metal-ligand
bonding.
The paramagnetic components of the tensor are sensitive to
changes in the current densities induced in the plane perpen-
dicular to the tensorial direction. Since δ33 is directed along
the Cr-P bond, the differences in the bonding between the
chromium and phosphorus in complexes 1 and 3 should be
reflected in the values of δ11 and δ22. Indeed, δ11 and δ22 are
more deshielded for 1 than for 3. As mentioned earlier, the
X-ray diffraction studies have shown that the structures of 1
and 3 are closely similar and do not provide any information
that might explain the differences in the chemical shift tensor
components.
The CS ligand is considered to be a much better π-acceptor
than is CO,21 and removes electron density from both the Cr
and P atoms, especially when the latter are trans to CS. On
this basis, more deshielded values of δ11 and δ22 would be
expected for complex 3 than for 1, but the experimental
Acknowledgment. This research was supported by grants
from the NSERC (Canada) and the FCAR (Que´bec). Y.H.
thanks McGill University for the award of a Faculty of Graduate
Studies and Research Fellowship. The assistance of Drs. F.
Morin and J. Britten in the X-ray diffraction work and solid-
state NMR measurements, respectively, is gratefully acknowl-
edged.
Supporting Information Available: Tables for the X-ray structural
determination of 3, including details of the data collection, anisotropic
thermal parameters, hydrogen atom parameters, and torsion angles (5
pages). Ordering information is given on any current masthead page.
IC960816U
(22) Wovkulich, M. J.; Atwood, J. L.; Canada, L.; Atwood, J. D.
Organometallics 1985, 4, 867.
(23) Randall, L. H.; Carty, A. J. Inorg. Chem. 1989, 28, 1194.
(24) Harris, R. K.; McNaught, I. J.; Reams, P. Magn. Reson. Chem. 1991,
S60, 29.
(20) Lindner, E.; Fawzi, R.; Mayer, H. A.; Eichele, K.; Hiller, W.
Organometallics 1992, 11, 1033 and the references therein.
(21) Butler, I. S. Acc. Chem. Res. 1977, 77, 313.
(25) Power, W. P.; Wasylishen, R. E. Inorg. Chem. 1992, 31, 2176.