Influence of ligand electronic effects on the structure of monovalent cobalt complexes

Article abstract of DOI:10.1021/ic800813u

The syntheses, spectroscopic properties, and structures of the monovalent cobalt complexes, [PhTt^tBu]Co(L), 1-L {PhTt^tBu = phenyltris[(tert-butylthio)methyl]borate; L = PPh₃, PMe₃, PEt₃, PMe₂Ph, PMePh₂, P(OPh)₃, CNBu^t}, are described. Complexes 1-L are prepared via the sodium amalgam reduction of [PhTt^tBu]CoCl in the presence of L. The complexes display magnetic moments and paramagnetically shifted ¹H NMR spectra consistent with triplet, S = 1, ground states. The molecular geometries, determined by X-ray diffraction methods, reveal that some of the complexes display structures in which the L donor is moved off of the inherent 3-fold axis. In the most extreme cases (e.g., 1-P(OPh)₃ or 1-CNBu^t), the geometries can be described as cis-divacant octahedra. The origin of the geometric distortions is a consequence of the electronic characteristics of L as first deduced by Detrich et al. for [Tp ^Np]Co(L) (J. Am. Chem. Soc. 1996, 118, 1703). The results establish a linear correlation between the magnitude of the structural distortion and the electronic parameter of the phosphine donor.

Full text of DOI:10.1021/ic800813u

Inorg. Chem. 2008, 47, 10700-10707
Influence of Ligand Electronic Effects on the Structure of Monovalent
Cobalt Complexes
Julie A. DuPont, Glenn P. A. Yap, and Charles G. Riordan*
Department of Chemistry and Biochemistry, UniVersity of Delaware, Newark, Delaware 19716
Received May 5, 2008
The syntheses, spectroscopic properties, and structures of the monovalent cobalt complexes, [PhTttBu]Co(L), 1-L
{PhTttBu ) phenyltris[(tert-butylthio)methyl]borate; L ) PPh3, PMe3, PEt3, PMe2Ph, PMePh2, P(OPh)3, CNBut}, are
described. Complexes 1-L are prepared via the sodium amalgam reduction of [PhTttBu]CoCl in the presence of L.
The complexes display magnetic moments and paramagnetically shifted 1H NMR spectra consistent with triplet, S
) 1, ground states. The molecular geometries, determined by X-ray diffraction methods, reveal that some of the
complexes display structures in which the L donor is moved off of the inherent 3-fold axis. In the most extreme
cases (e.g., 1-P(OPh)3 or 1-CNBut), the geometries can be described as cis-divacant octahedra. The origin of the
geometric distortions is a consequence of the electronic characteristics of L as first deduced by Detrich et al. for
[TpNp]Co(L) (J. Am. Chem. Soc. 1996, 118, 1703). The results establish a linear correlation between the magnitude
of the structural distortion and the electronic parameter of the phosphine donor.
Scheme 1. Depiction of the R Angle as Defined by Theopold5
Introduction
The development of monovalent cobalt complexes sup-
ported by the tris(thioether) ligand [PhTttBu] {PhTttBu
)
phenyltris[(tert-butylthio)methyl]borate} was motivated ini-
tially to provide benchmark complexes with which to
compare the monovalent nickel complexes, [PhTtR]Ni(L).1
In the latter studies, we demonstrated that [PhTtR]Ni(CO)
(R ) But, Ad) reacts with dioxygen yielding discrete,
thermally sensitive adducts. Specifically, oxygenation of
[PhTttBu]Ni(CO) resulted in the interception of the purple
bis-µ-oxo dinickel(III), [(PhTttBu)Ni]2(µ-O)2.2 Alternatively,
addition of dioxygen to [PhTtAd]Ni(CO) stopped at the 1:1
adduct formulated as Ni2+ superoxo, [PhTtAd]Ni(O2), based
on its electron paramagnetic resonance (EPR) and Ni X-ray
absorption spectroscopy (XAS) characteristics as supported
by density functional theory (DFT) results and reactivity
studies.3 More recent studies have extended chalcogen
activation to sulfur; the reaction of [PhTttBu]Ni(CO) and S8
produces the disulfido dinickel(II) complex, [(PhTttBu)Ni]2(µ-
Scheme 2. Synthesis of Monovalent [PhTttBu]Co(L) from
[PhTttBu]CoCl
S2).4 Herein, we report the preparation and structural analyses
of a series of monovalent cobalt complexes, [PhTttBu]Co(L)
[L ) PPh3, PMe3, PEt3, PMe2Ph, PMePh2, P(OPh)3, CNBut].
Some of these complexes display geometries wherein the
donor ligand, L, resides far off of the inherent 3-fold axis
(i.e., nonlinear B · · · Co-P angles). This structural phenom-
* To whom correspondence should be addressed. E-mail: riordan@
udel.edu. Fax: 302-831-6335.
(1) Schebler, P. J.; Mandimutsira, B. S.; Riordan, C. G.; Liable-Sands,
L. M.; Incarvito, C. D.; Rheingold, A. L. J. Am. Chem. Soc. 2001,
123, 331.
(2) Mandimutsira, B. S.; Yamarik, J. L.; Brunold, T. C.; Gu, W. W.;
Cramer, S. P.; Riordan, C. G. J. Am. Chem. Soc. 2001, 123, 9194.
(3) Fujita, K.; Schenker, R.; Gu, W. W.; Brunold, T. C.; Cramer, S. P.;
Riordan, C. G. Inorg. Chem. 2004, 43, 3324.
(4) Cho, J.; Van Heuvelen, K. M.; Yap, G. P. A.; Brunold, T. C.; Riordan,
C. G. Inorg. Chem. 2008, 47, 3931.
10700 Inorganic Chemistry, Vol. 47, No. 22, 2008
10.1021/ic800813u CCC: $40.75  2008 American Chemical Society
Published on Web 10/15/2008

Ligand Electronic Effects on MonoWalent Cobalt Complexes
Table 1. Electronic Absorption and Room Temperature Magnetic
Susceptibility Data for 1-PPh3, 1-PMe3, 1-PEt3, 1-PMePh2, 1-PMe2Ph,
1-P(OPh)3, and 1-CNBut
λ
max, nm (ꢀ, M-1 cm-1
)
µeff, µB
1-PPh3
1-PMe3
1-PEt3
1-PMe2Ph
1-PMePh2
1-P(OPh)3
1-CNBut
614 (30), 901 (596)
686 (74), 893 (314)
711 (135), 905 (671)
706 (70), 896 (508)
710 (49), 896 (618)
573 (120), 718 (257)
2.9(2)
3.1(1)
3.0(3)
3.1(2)
2.9(2)
2.9(2)
3.0(1)
276 (235), 335 (192), 780 (17)
Figure 1. Proton NMR spectrum of [PhTttBu]Co(PPh3) in C6D6 (1).
Residual pentane (×). Inset: δ 110-100 range.
the methylene proton chemical shifts range from δ 129.2
for 1-P(OPh)3 to δ 55.2 for 1-PMePh2. A broad upfield
resonance at δ 0.43 is assigned to the equivalent tert-butyl
protons. A similar upfield shift was observed for the other
1-L complexes with the tert-butyl proton resonances ranging
from δ 0.60 for 1-PEt3 to δ -7.51 for 1-PMePh2. The meta,
ortho, and para protons of the borato ligand in 1-PPh3
resonate at δ 16.2, 10.9, and 10.0, respectively. These
assignments were made on the basis of signal integration
and by assuming that the relative signal positions reflect a
spin polarization mechanism. The meta, ortho, and para
protons of the PPh3 ligand were located at δ 9.76, 9.10, and
6.88, respectively. In the other 1-L complexes, the phosphine
protons were shifted significantly from values for the free
phosphine. The proton NMR spectra of these cobalt(I)
complexes are similar to those of the corresponding monova-
lent nickel species, [PhTttBu]Ni(L) (L ) PMe3, CO), syn-
thesized previously in this laboratory.1 The downfield shift
of the methylene protons and the upfield shift of the tert-
butyl protons of the borato ligand are comparable. The 31P
NMR spectra of the cobalt(I) complexes are devoid of
signals, which is a consequence of efficient nuclear spin
relaxation induced by the paramagnetic center. Similarly,
Peters’ [PhBP3]Co(PR3) derivatives do not display 31P NMR
signals.6
enon was noted and described in detail by Theopold and
Doren5 for [TpNp]Co(L) complexes with π-acceptor ligands
(i.e., L ) N2 and CO). In [TpNp]Co(CO), the CO ligand is
26.6° off of the 3-fold axis, where the deviation is defined
as the R angle (Scheme 1), whereas in [(TpNp)Co]2(µ-N2), R
equals 34.7 and 37.8°. In this report, analysis of the structures
of [PhTttBu]Co(L) as a function of the electronic properties
of the phosphine ligands provides an understanding of the
cobalt structural deviations complementing and extending
significantly the examination of the two [TpNp]Co(L) com-
plexes by expanding the observations to complexes possess-
ing the [PhTttBu]Co fragment and to a systematic analysis
of six phosphine donors.
Results
Ia. Synthesis of [PhTttBu]Co(L) [L ) PPh3, PMe3,
PEt3, PMe2Ph, PMePh2, P(OPh)3]. Under a nitrogen
atmosphere, 1 equiv of the appropriate ligand L [L ) PPh3,
PMe3, PEt3, PMe2Ph, PMePh2, P(OPh)3] was added to a
diethyl ether solution of [PhTttBu]CoCl and stirred, resulting
in a color change from blue to green for L ) PMe3, PEt3,
PMe2Ph, and PMePh2; the addition of PPh3 or P(OPh)3 did
not induce a color change. Subsequently, 1 equiv of a Na/
Hg amalgam was added to the solution with vigorous stirring,
resulting in a rapid color change from blue to pale green for
L ) PMe3 and PEt3 and from blue to yellow-green for L )
PMe2Ph and PMePh2. For L ) PPh3, the solution color
changed to yellow, and for L ) P(OPh)3, the solution
changed to dark blue. Concomitant formation of a white
precipitate was noted in all cases. Removal of the volatiles
under reduced pressure, followed by extraction with pentane,
afforded the corresponding monovalent cobalt species, 1-L,
in 40-66% isolated yields (Scheme 2). These thermally
sensitive, high-spin, 16-electron complexes decompose rap-
idly upon exposure to air or moisture. The complexes are
soluble in tetrahydrofuran (THF), ether, and pentane but react
with chlorinated hydrocarbons (i.e., CHCl3 or CH2Cl2)
regenerating [PhTttBu]CoCl.
The 1-L complexes have triplet ground states, S ) 1, as
indicated by their bulk magnetic moments determined in
solution by the Evans method. The values span a narrow
range, µeff ) 2.9-3.2 µB, close to the spin-only value. [TpR]5
6
R
and [PhBP3 ] ligated cobalt(I) complexes have similar
magnetic moments.
The electronic spectra of the complexes 1-L recorded in
pentane (Table 1) display two optical bands consistent with
a d8 metal center in a pseudotetrahedral environment [i.e.,
analogous to the features found for tetrahedral nickel(II)
complexes]. These transitions are consistent with similar
Co(I) complexes, namely, [(TpNp)Co]2(µ-N2) (λmax ) 783,
992 nm),5 [PhBP3]Co(PMe3) (680 nm), and [(PhBPiPr3)Co]2(µ-
N2) (745, 948 nm).7 The two d-d optical transitions for the
series of phosphine complexes range from λmax (ε, M-1 cm-1)
) 711 (135) nm and 905 (671) nm for 1-PEt3, to 573 (120)
nm and 718 (257) nm for 1-P(OPh)3. A rough correlation
between the λmax values of the higher-energy optical band
and those of the electronic donor aptitude of the phosphine
as defined by the Tolman electronic parameter is noted in
Ib. Proton NMR Spectroscopic Characteristics. The
proton NMR spectrum of 1-PPh3 is representative of the
series of paramagnetic complexes (Figure 1). A broad singlet
located downfield at δ 104.1 is assigned to the six methylene
protons of the borato ligand. For this series of derivatives,
(6) Jenkins, D. M.; Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2002,
124, 11238.
(7) Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2003, 125, 10782.
(5) Detrich, J. L.; Konecny, R.; Vetter, W. M.; Doren, D.; Rheingold,
A. L.; Theopold, K. H. J. Am. Chem. Soc. 1996, 118, 1703.
Inorganic Chemistry, Vol. 47, No. 22, 2008 10701

DuPont et al.
Figure 2. Thermal ellipsoid plots of 1-PMe3 and 1-PMe2Ph with atom labeling. Thermal ellipsoids are drawn to 30% probability. Hydrogen atoms have
been omitted for clarity.
Table 2. Crystallographic Data for Complexes 1-PPh3, 1-PMe3, 1-PEt3, 1-PMePh2, 1-PMe2Ph, 1-P(OPh)3, and 1-CNBut
1-PPh3
1-PMe3 ·2C6H6
1-PEt3
1-PMePh2 ·0.5C6H6
1-PMe2Ph
1-P(OPh)3
1-CNBut
formula
fw
cryst syst
space group
a, Å
b, Å
c, Å
R, deg
ꢀ, deg
γ, deg
V, Å3
C39H53BCoPS3 C36H59BCoPS3
C27H53BCoPS3 C34H51BCoPS3
C32H52BCoPS3 C39H53BCoO3PS3 C26H47BCoNS3
718.70
monoclinic
P2(1)/n
10.302(2)
23.003(4)
15.793(2)
90
688.72
monoclinic
P2(1)/n
9.676(2)
24.678(5)
16.249(3)
90
574.58
monoclinic
P2(1)/n
15.147(3)
11.447(2)
18.383(4)
90
656.64
633.63
monoclinic
P2(1)/n
15.713(4)
13.756(3)
16.610(4)
90
766.70
monoclinic
P2(1)/c
10.667(2)
17.983(3)
20.792(4)
90
539.57
triclinic
monoclinic
P2(1)/n
16.520(2)
9.277(1)
20.108(3)
90
j
P1
13.644(3)
15.072(3)
17.226(3)
88.543(3)
86.444(3)
84.079(3)
3516.2(11)
4
91.475(3)
90
94.471(3)
90
90.05(5)
90
103.606(3)
90
97.251(3)
90
105.256(3)
90
3741.3(10)
4
3868.8(12)
4
3187.4(11)
4
3489.5(14)
4
3956.5(12)
4
3972.9(7)
4
Z
T, K
120
1.276
1.77-28.29
120
1.182
1.65-28.24
0.669
120
1.197
1.11-21.99
0.799
0.0912, 0.2176 0.0710,0.1928
2
120
1.240
1.18-28.32
0.733
120
1.206
1.94-28.28
0.736
120
1.287
1.92-28.32
0.667
120
1.206
1.85-28.31
0.802
Dcalcd, g cm-3
2θ range, deg
µ(Mo KR), mm-1 0.695
R1, wR2a
0.0614, 0.1412 0.0419,0.1053
0.0461, 0.1131 0.0386, 0.0879
0.0730, 0.1453
1
a
2
2
Quantity minimized ) R(wF2) ) ∑[w(Fo - Fc )2]/∑[(wFo )2] /2; R ) ∑∆/∑(Fo), ∆ ) |(Fo - Fc)|.
Table 3. Selected Metric Parameters for 1-PMe3, 1-PEt3, and
Table 1. The complexes with the most electron-releasing
phosphines, 1-PMe3 and 1-PEt3, have among the lowest-
energy optical bands. Complex 1-P(OPh)3, with the electron-
withdrawing phosphite, displays the highest-energy electronic
transition. This trend is attributed, at least in part, to the
change in orbital overlap that accompanies the geometry
changes of the complexes. Specifically, moving the L donor
off of the inherent 3-fold axis (i.e., increasing R angle values)
increases Co-L π-orbital overlap, resulting in larger ligand
field splitting and higher-energy optical transitions.
1-PMe2Ph
1-PMe3
length, Å
angle, deg
Co-S(1)
Co-S(2)
Co-S(3)
Co-P
Co-S(1)
Co-S(2)
Co-S(3)
Co-P
Co-S(1)
Co-S(2)
Co-S(3)
Co-P
2.2872(6)
2.2845(6)
2.2841(6)
2.2561(6)
2.293(4)
2.318(4)
2.296(4)
2.272(4)
2.297(1)
2.305(1)
2.318(1)
2.2570(7)
B · · ·Co-P
S(1)-Co-P
S(2)-Co-P
S(3)-Co-P
B · · ·Co-P
S(1)-Co-P
S(2)-Co-P
S(3)-Co-P
B · · ·Co-P
S(1)-Co-P
S(2)-Co-P
S(3)-Co-P
176.6(1)
127.76(2)
121.32(2)
120.23(2)
178.3(1)
124.8(1)
125.3(2)
122.1(2)
179.0(1)
123.05(3)
125.20(3)
123.99(3)
1-PEt3
1-PMe2Ph
Ic. Molecular Structures of 1-PMe3, 1-PEt3, and
1-PMe2Ph. The molecular structures of these three com-
plexes, each containing electron-rich phosphine donors, are
largely similar (Figure 2). Selected bond lengths and angles
are contained in Tables 2 and 3. The common and most
conspicuous structural feature is the nearly linear B · · ·Co-P
angle. The R angle equals 3.4, 1.7, and 1.0° for 1-PMe3,
1-PEt3, and 1-PMe2Ph, respectively. As a consequence, the
P-Co-S angles are all quite similar averaging 123° for
1-PMe3 and 124° for 1-PEt3 and 1-PMe2Ph. The Co-S
bond distances average 2.285, 2.302, and 2.306 Å for
1-PMe3, 1-PEt3, and 1-PMe2Ph, respectively. These dis-
tances are slightly shorter than those found in [PhTttBu]-
Co(Me),8 with an average Co-S bond distance of 2.35 Å.
(8) DuPont, J. A.; Coxey, M. B.; Schebler, P. J.; Incarvito, C. D.;
Dougherty, W. G.; Yap, G. P. A.; Rheingold, A. L.; Riordan, C. G.
Organometallics 2007, 26, 971.
10702 Inorganic Chemistry, Vol. 47, No. 22, 2008

Ligand Electronic Effects on MonoWalent Cobalt Complexes
Table 4. Selected Metric Parameters for 1-PMePh2, 1-PPh3, and
1-P(OPh)3
length, Å
angle, deg
1-PMePh2 (1) Co(1)-S(1) 2.322(1) B(1) · · ·Co(1)-P(1) 163.85(9)
Co(1)-S(2) 2.296(1) S(1)-Co(1)-P(1)
Co(1)-S(3) 2.313(1) S(2)-Co(1)-P(1)
Co(1)-P(1) 2.252(1) S(3)-Co(1)-P(1)
112.26(4)
139.37(5)
114.77(4)
1-PMePh2 (2) Co(2)-S(6) 2.279(1) B(2) · · ·Co(2)-P(2) 173.99(9)
Co(2)-S(5) 2.298(1) S(4)-Co(2)-P(2)
Co(2)-S(4) 2.304(1) S(5)-Co(2)-P(2)
Co(2)-P(2) 2.268(1) S(6)-Co(2)-P(2)
128.78(5)
120.83(5)
119.89(5)
166.6(1)
117.62(3)
113.45(3)
136.30(3)
156.44(4)
145.25(2)
110.00(2)
105.01(2)
1-PPh3
Co-S(1)
Co-S(2)
Co-S(3)
Co-P
2.2968(9) S(1)-Co-S(2)
2.3290(9) S(1)-Co-P
2.2962(9) S(2)-Co-P
2.2656(9) S(3)-Co-P
2.3079(6) B · · ·Co-P
2.3366(6) S(1)-Co-P
2.3197(6) S(2)-Co-P
2.1448(6) S(3)-Co-P
1-P(OPh)3
Co-S(1)
Co-S(2)
Co-S(3)
Co-P
105.01(2)°, while the third angle is much larger, 145.25(4)°.
The Co-S bond distances are unexceptional, averaging
2.302(2), 2.310(1), and 2.321(1) Å for 1-PMe2Ph, 1-PPh3,
and 1-P(OPh)3, respectively. The Co-P bond distances are
2.252(1) Å (1-PMePh2), 2.259(9) Å (1-PPh3), and 2.1448(6)
Å [1-P(OPh)3]. To the best of our knowledge, the Co-P
bond distance in 1-P(OPh)3 is much shorter than those found
for any of the other cobalt(I) derivatives. The difference
derives from the high π-acceptor aptitude of P(OPh)3.
IIa. Synthesis and Properties of 1-CNBut. The addition
of 1 equiv of tert-butyl isocyanide to a stirring diethyl ether
solution of [PhTttBu]CoCl resulted in a rapid color change
from blue to green. Subsequent addition of 1 equiv of a Na/
Hg amalgam produced a further color change to yellow with
concomitant precipitation of a white solid, NaCl. Removal
of the volatiles, followed by extraction with pentane, afforded
the corresponding monovalent cobalt species, 1-CNBut, in
60% yield (Scheme 2). This high-spin, S ) 1 complex is
thermally stable but decomposes rapidly upon exposure to
air or moisture. 1-CNBut is soluble in THF, ether, and
pentane. It reacts with chlorinated hydrocarbons generating
[PhTttBu]CoCl.
Figure 3. Thermal ellipsoid plots of 1-PMePh2, 1-PPh3, and 1-P(OPh)3.
Thermal ellipsoids are drawn to 30% probability. Hydrogen atoms have
been omitted for clarity.
IIb. Analysis of 1-CNBut: Proton NMR, Electronic
Absorption, and Infrared (IR) Spectra. The proton NMR
spectrum of paramagnetic 1-CNBut is similar to those of
the other four-coordinate 1-L complexes studied herein as
well as the [PhTttBu]Ni(CNBut) analogue. A broad singlet
located at δ 114.5 is assigned to the six methylene protons
of the borato ligand. A broad upfield resonance at δ 0.18 is
ascribed to the thee equivalent tert-butyl groups. The meta,
ortho, and para protons of the borato ligand are located at
δ 15.3, 10.3, and 9.83, respectively. The tert-butyl isocyanide
proton resonance, δ 12.0, is shifted downfield from free
isocyanide. The triplet ground state of 1-CNBut is confirmed
by its magnetic moment, µeff ) 3.1(1) µB.9
The longer Co-S distances in the latter cobalt(II) complex
are a consequence of the strong σ-donor ability of the methyl
ligand. The Co-P bond distances of 2.2561(6) Å (1-PMe3),
2.272(4) Å (1-PEt3), and 2.2570(7) Å (1-PMe2Ph) are
similar to those of the other 1-L analogues synthesized in
this series yet do not exhibit a discernible trend (Table 3).
Id. Molecular Structures of 1-PMePh2, 1-PPh3, and
1-P(OPh)3. Thermal ellipsoid plots of 1-PMePh2, 1-PPh3,
and 1-P(OPh)3 are given in Figure 3, and selected metric
parameters are listed in Table 4. 1-PMePh2 crystallized with
two independent molecules in the asymmetric unit, only one
of which is shown in Figure 3; metric parameters for each
independent molecule are provided in Table 4. These
complexes exhibit a clear distortion away from idealized C3
symmetry. Specifically, the R angles equal 11.2 (average of
the two independent molecules), 13.4, and 23.6° for 1-P-
MePh2, 1-PPh3, and 1-P(OPh)3, respectively. Necessarily,
the off-axis nature of the phosphine ligand results in very
different S-Co-P angles. For example, in 1-P(OPh)3, two
of the S-Co-P angles are similar, measuring 110.00(2) and
The terminal CNBut ligand is characterized by an intense
infrared band, νCN ) 2198 cm-1. Coordination results in a
shift to higher energy relative to free CNBut, νCN ) 2142
cm-1 (CHCl3). The νCN stretch for the nickel analogue
[PhTttBu]Ni(CNBut), νCN ) 2115 cm-1, in which the CNBut
(9) (a) Evans, D. F. J. Chem. Soc. 1959, 2003. (b) Live, D. H.; Chan,
S. I. Anal. Chem. 1970, 42, 791.
Inorganic Chemistry, Vol. 47, No. 22, 2008 10703

DuPont et al.
Table 5. Comparison of R Angles for 1-L Complexes and Related
[TpR]Co(L′) and [PhBP3]Ni(L) Complexes
R, deg
1-PPh3
1-PMe3
1-PEt3
1-PMe2Ph
13.4(1)
3.4(1)
1.7(1)
1.0(1)
1-PMePh2
6.0(1),16.1(1),av)11.1(3)
1-P(OPh)3
23.6(4)
26.5(1)
34.7,37.9
25.7
1-CNBut
[(TpNp)Co]2(µ-N2)5
[TpNp]Co(CO)5
[PhBP3]Ni(S-p-tBu-Ph)21
29.2
Table 6. Selected Bond Lengths (Å) and Bond Angles (deg) for
1-CNBut
length, Å
angle, deg
Co-S(1)
Co-S(2)
Co-S(3)
Co-C(22)
C-N
2.322(1)
2.307(1)
2.314(1)
1.857(4)
1.161(5)
B · · ·Co-P (av)
S(1)-Co-C(22)
S(2)-Co-C(22)
S(3)-Co-C(22)
S(1)-Co-S(2)
153.5(1)
148.5(1)
110.1(1)
103.5(1)
93.72(4)
Table 7. Steric and Electronic Parameters of Tertiary Phosphine
Figure 4. Thermal ellipsoid plot of 1-CNBut. Thermal ellipsoids are drawn
to 30% probability. Hydrogen atoms have been omitted for clarity.
Ligands Used in This Investigation
+
cone angle
(deg)
pKa of HPR3
(aqueous)
A1 νCO,
ligand
cm-1
ligand resides on the 3-fold axis,10 is at a much lower energy.
The d9 nickel(I) ion participates in greater π-backbonding,
when compared to the d8 cobalt(I) ion, in part because of its
higher electron count. The νCN value is higher for 1-CNBut
than for CNBut, suggesting that σ-donation predominates
over π-back-donation. A secondary effect that also may
increase the νCN energy is kinematic coupling of the C-N
and M-C oscillators,11 although this phenomenon cannot
account fully for the differences observed here.
PMe3
PEt3
PMe2Ph
PMePh2
PPh3
118
132
122
136
145
128
8.65
8.69
6.50
4.57
2.73
2.00
2064.1
2061.7
2065.3
2067.0
2068.9
2085.0
P(OPh)3
[PhTttBu]Co(CO), 1-CO. This species, which is in equilibrium
with the low-spin dicarbonyl [PhTttBu]Co(CO)2,8 has yet to be
structurally characterized.
The electronic absorption spectrum of 1-CNBut displays
a single, weak d-d transition at 780 (70) nm. The expected
lower-energy ligand field transitions were not observed for
1-CNBut and are presumably shifted to the near-IR region
beyond 1100 nm. The energy of the latter transition ranged
from 718 to 905 nm in the 1-L phosphine complexes. The
similar complexes, [TpNp]Co(CO) and [(TpNp)Co]2(µ-N2),
displayed transitions at λmax ) 1135 and 992 nm, respec-
tively.5
IIc. Molecular Structure of 1-CNBut. Examination of the
structure of 1-CNBut reveals a significant deviation from C3
symmetry with R equaling 26.5°, the largest angle among the
series of complexes reported (Figure 4 and Table 6). This
structural deviation results in two similar and more acute
S-Co-C bond angles of 103.5(1) and 110.1(1)° and one much
more open S-Co-C(22) angle of 148.5(1)°. The Co-C bond
distance of 1.857(1) Å is similar to those of other cobalt(I)
isocyanide complexes. For example, [Co(CNC6H5)5]+ has
Co-C bond lengths averaging 1.84(3) Å,12 and [Co(CNCH3)5]+
has Co-C bond lengths averaging 1.88(2) Å.13 The distortion
of the CNBut ligand off of the B · · ·Co-C vector is of equal
magnitude as that seen in Theopold’s [TpNp]Co(CO), where R
equals 26.6°. A similarly distorted structure is presumed for
Discussion
Cobalt Structural Distortions as a Function of
Phosphine Ligand. The structural characterization of the
series of [PhTttBu]Co(L) complexes permits for the examina-
tion of the correlation of the R angle with phosphine donor
characteristics. Such bent geometries have been observed in
other four-coordinate cobalt(I) complexes. Most notably,
Theopold and Doren5 described a similar deviation from ideal
C3 symmetry in [TpNp]Co(L) complexes (Table 5). They
employed the extended Hu¨ckel molecular orbital and density
functional theories to provide a detailed framework to
understand the structures of [TpNp]Co(L) complexes.5 The
latter approach accurately reproduced the molecular geom-
etries determined by X-ray diffraction analyses. By examina-
tion of complexes in the formal Co(I), Co(II), and Co(III)
oxidation states, it was determined that the d8, Co(I),
configuration was necessary but not sufficient to access the
bent geometry. Examination of the frontier molecular orbitals
in the context of a Walsh diagram that considers bending of
the L ) CO ligand off-axis (i.e., reduction of the molecular
symmetry from C3V to Cs) proved instructive.5 In summary,
the bent geometry is ascribed to a preferred electronic
structure anticipated for d8 complexes possessing ligands that
are both good σ-donors and π-acceptors. In the studies by
Theopold, both L ) CO and N2 satisfy this requirement.
(10) Mandimutsira, B. S.; Riordan, C. G. Unpublished results.
(11) Cotton, F. A.; Zingales, F. J. Am. Chem. Soc. 1961, 83, 351.
(12) Jurnak, F. A.; Greig, D. R.; Raymond, K. N. Inorg. Chem. 1975, 14,
2585.
(13) Cotton, F. A.; Wood, J. S.; Dunne, T. G. Inorg. Chem. 1964, 3, 1495.
10704 Inorganic Chemistry, Vol. 47, No. 22, 2008

Ligand Electronic Effects on MonoWalent Cobalt Complexes
Figure 5. Plot of the Tolman cone angle versus the R angle for 1-L
complexes. The R angle was determined by X-ray diffraction.
Figure 6. Plot of pKa (of the conjugate acid of PR3) versus R angle. The
line represents a best-fit linear regression, R2 ) 0.962.
To further validate and extend the analysis of the structural
distortions of the [TpNp]Co(L) (L ) N2, CO) complexes, we
consider the structural distortions exhibited by the series of
complexes, [PhTttBu]Co(L). The characteristics of the six
phosphine donors employed provide a means for deducing
the roles of phosphine steric and electronic effects on
molecular geometry. In this context, we are unaware of
phosphine derivatives of the type [TpR]Co(PR′3). Extensive
efforts by Tolman14 and others15 have quantified systemati-
cally the steric and electronic properties of phosphines. First,
we consider phosphine sterics as measured by the Tolman
cone angles (Table 7).16 A plot of the Tolman cone angle
versus the R angle (Figure 5) shows clearly that there is no
correlation. For example, PEt3 and P(OPh)3 have similar cone
angles, yet the R angles in 1-PEt3 and 1-P(OPh)3 differ by
about 22°. Furthermore, PPh3 has the largest cone angle in
the series at 145°, yet 1-PPh3 exhibits an R angle of 13.4°.
To explore a correlation between the electronic properties
of the phosphine ligands and the R angle, we consider two
different measures of the former. Henderson and Streuli17
have measured the relative basicity of substituted phosphines
by determining the pKa values of their conjugate acids (Table
7). A plot of pKa versus the R angle is provided in Figure 6.
This plot shows that a qualitative correlation exists with a
correlation coefficient of 0.962 (p e 0.0022). Specifically,
the more basic phosphines lead to [PhTttBu]Co(L) complexes
with small R angles, while those that are less basic lead to
bent [PhTttBu]Co(L) complexes. For example, PEt3 is the
most basic phosphine examined in this study with a pKa of
8.69, and 1-PEt3 displays a small R angle of 2.0°. Con-
versely, the least basic donor is P(OPh)3 with a pKa of -2.00.
Complex 1-P(OPh)3 shows a significantly bent structure with
a large R angle of 23.6°.
Figure 7. Plot of Tolman electronic parameter (νCO) versus R angle. The
line represents a best-fit linear regression, R2 ) 0.917.
aqueous pKa measurements are dependent upon the interac-
tions between the phosphine and a proton, rather than the
interaction between the phosphine and a polarizable metal.
Furthermore, pKa values are dependent on the solvation
energies of the phosphine itself.18 Thus, a more rigorous
picture of the electronic effects of the phosphines in
coordination chemistry is available by comparing the Tolman
electronic parameter (νCO) to the R angles. The former values
were derived via analysis of the series of complexes
Ni(CO)3(PR3) and use CO ligands to report on electron
density at the metal available for back-bonding. Examination
of this plot (Figure 7) establishes the correlation between
νCO and the R angle with a correlation coefficient of 0.917
(p e 0.011). Specifically, phosphines with electron-donat-
ing substituents with low CO stretching frequencies yield
[PhTttBu]Co(L) complexes with small R angles. PEt3, PMe3,
and PMe2Ph can be classified as pure σ-donating ligands with
negligible π-accepting ability. Accordingly, the correspond-
ing 1-L complexes display small R angles of 2, 4, and 1.5°,
respectively. The addition of electron-withdrawing groups
increases the π-accepting aptitude of the phosphine. Thus,
consistent with the arguments of Theopold and Doren,5
PMePh2 and PPh3 lead to monovalent cobalt complexes with
increasingly larger R angles of 11.1 and 13.4°. This is further
pKa may not be the most appropriate measure of the
electronic contribution of the phosphine ligand. These
(14) Tolman, C. A. J. Am. Chem. Soc. 1970, 92, 2956.
(15) (a) Rahman, M. M.; Liu, H. Y.; Prock, A.; Giering, W. P. Organo-
metallics 1987, 6, 650. (b) Rahman, M. M.; Liu, H. Y.; Eriks, K.;
Prock, A.; Giering, W. P. Organometallics 1989, 8, 1. (c) Suresh,
C. H.; Koga, N. Inorg. Chem. 2002, 41, 1573.
(16) Tolman, C. A. J. Am. Chem. Soc. 1970, 92, 2953.
(17) (a) Henderson, W. A., Jr.; Streuli, C. A. J. Am. Chem. Soc. 1960, 82,
5791. (b) Streuli, C. A. Anal. Chem. 1960, 32, 985.
(18) Meriwether, L. S.; Fiene, M. L. J. Am. Chem. Soc. 1959, 81, 4200.
Inorganic Chemistry, Vol. 47, No. 22, 2008 10705

DuPont et al.
Phosphorus NMR spectra were recorded on a Bruker 360 MHz
NMR spectrometer, using an aqueous solution of 85% phosphoric
acid as the external standard (0 ppm). Magnetic moments were
determined in solution via the method of Evans.9 Electronic spectra
were recorded on a Hewlett-Packard 8453 diode array spectrometer.
Infrared spectra were recorded on a Mattson Genesis Series Fourier
transform infrared (FT-IR) spectrometer under a N2 purge at ambient
temperature. Solid-state FT-IR samples were prepared as KBr
pellets, and solution-state FT-IR samples were prepared in pentane
between KBr discs. Melting points were determined with a Melt-
Temp melting point determination apparatus and are reported
uncorrected. Elemental analyses were performed by Desert Ana-
lytics, Inc., Tucson, AZ; Galbraith Laboratory, Inc., Knoxville, TN;
or Oneida Research Services, Inc., Whitesboro, NY. Crystal-
lographic data were collected on a Bruker-AXS APEX diffracto-
meter. All software and sources of the scattering factors are
contained in various versions of the SHELXTL program library
(G. Sheldrick, Siemens XRD, Madison, WI).
highlighted by the structure of 1-P(OPh)3, containing the
electron-withdrawing phosphite, which has the largest R
angle of 23.6°.
Lastly, we comment on the structure of 1-CNBut with an
R angle of 26.5°. Isocyanides are poorer π-acceptors than
CO but better π-acceptors than alkyl or aryl phosphines,
approaching the π-accepting ability of PF3. Thus, the large
R angle in 1-CNBut is qualitatively in accord with the
conclusions regarding the cobalt(I) phosphine complexes,
namely, that ligands with π-accepting ability lead to bent
[PhTttBu]Co(L) complexes with correspondingly large R
angles. Indeed, the structure of 1-CNBut likely serves as a
good model for the structure of 1-CO, a species detected in
solution, in equilibrium with 1-(CO)2, but not yet structurally
authenticated.
Summary
Synthesis of [PhTttBu]Co(L) Complexes. The preparation of
1-PPh3 is described in detail below and is exemplary of the
procedure employed in the synthesis of all complexes. Each reaction
commenced with 100 mg of [PhTttBu]CoCl.
The synthesis of a series of phosphine-ligated [PhTttBu]-
Co(L) complexes revealed systematic variations in geometric
structure. Detailed examination of the results confirms a
correlation between the crystallographically determined R
angle and the electronic properties of L as first proposed for
[TpNp]Co(L).5 Phosphines that are purely σ-donors lead to
monovalent cobalt complexes with correspondingly small R
angles. Phosphines with increasingly greater π-accepting
capacity yield complexes with progressively larger R angles.
Thus, the observed R angle can be tuned by changing the
σ-donating and π-accepting properties of the ligand. Ad-
ditionally, this can be further expanded to include non-
phosphine ligands (e.g., CNBut), which are strong σ-donating
and π-accepting ligands. Lastly, we note that such bent
structures should be available to other metal complexes with
d8 configurations such as Ni(II) and Fe(0), which is a topic
of some interest to us.
(i) 1-PPh3. [PhTttBu]CoCl (100 mg, 0.20 mmol) was dissolved
in 40 mL of diethyl ether. PPh3 (64 mg, 0.24 mmol) was added to
the mixture with vigorous stirring until the solid dissolved. One
equivalent of a 0.3% Na/Hg (5 mg of Na, 1.56 g of Hg) amalgam
was added to the reaction mixture and stirred, resulting in a color
change from blue-green to yellow and the formation of a white
precipitate, NaCl. The solution was stirred for 8-12 h. The reaction
mixture was filtered through a Celite pad on a medium porosity
filter frit, and the filtrate was collected. The solvent was removed
under reduced pressure. The yellow product was extracted with
pentane and eluted through a silica gel plug. The solvent was
removed in vacuo yielding a yellow solid. Yield: 95 mg (65%).
1
Mp: 150 °C dec. H NMR (C6D6): δ 102.8 [br, 6 H, (CH2)], 16.2
[s, 2 H, m-(C6H5)B], 10.9 [s, 2 H, o-(C6H5)B], 10.0 [s, 1 H,
p-(C6H5)B], 9.76 [s, 6 H, m-(C6H5)P], 9.10 [br, 6 H, o-(C6H5)P],
6.88 [s, 3 H, p-(C6H5)P], 0.43 [br, 27 H, (CH3)3S]. µeff (C6D6) )
2.9 µB. UV-vis (pentane), λmax (ε, M-1 cm-1): 614 (30), 901 (596).
Anal. Calcd for C39H53S3BCoP: C, 65.2; H, 7.43. Found: C, 65.4;
H, 7.16.
Experimental Section
General Considerations. Unless otherwise noted, all reactions
were carried out under an inert atmosphere of N2 using standard
Schlenk techniques or in an Ar-filled Vacuum Atmospheres
glovebox equipped with a gas purification system.19 All glassware
was dried at 140 °C for at least 4 h. Solvents were dried by passage
through a column of activated alumina, followed by thorough
degassing with N2. Triphenylphosphine, triethylphosphine, and tert-
butyl isocyanide were purchased from Acros Organics and used as
received. Dimethylphenylphosphine and methyldiphenylphosphine
were purchased from Strem Chemical and used as received.
Trimethylphosphine was purchased from Sigma Aldrich Chemical
and used as received. [PhTttBu]CoCl was prepared as described
previously.1 Deuterated solvents were purchased from Cambridge
Isotope Laboratories and dried over 4 Å molecular sieves.
Proton and 13C NMR spectra were recorded on either a Bruker
360 MHz spectrometer equipped with a Sun Workstation, a Bruker
AM 250 MHz spectometer, or a Bruker AC 250 MHz spectrometer.
All NMR signals were referenced to residual protio solvent signals.
Unless otherwise noted, all data were collected at ambient tem-
perature. Chemical shifts are quoted in δ (ppm) and coupling
constants in hertz. Abbreviations are as follows: s, singlet; br, broad.
1
(ii) 1-PMe3. Yield: 43 mg (40%). Mp: 120 °C dec. H NMR
(C6D6): δ 97.3 [br, 5 H, (CH2)], 44.1 [br, 9 H, (PCH3)], 15.1 [s, 2
H, m-(C6H5)B], 10.6 [s, 2 H, o-(C6H5)B], 9.57 [s, 1 H, p-(C6H5)B],
0.22 [br, 27 H, (CH3)3S]. µeff (C6D6) ) 2.9 µB. UV-vis (pentane),
λmax (ε, M-1 cm-1): 686 (74), 893 (314). Anal. Calcd for
C24H47S3BCoP: C, 54.1; H, 8.89. Found: C, 53.6; H, 8.54.
1
(iii) 1-PEt3. Yield: 50 mg (43%). Mp: 140 °C dec. H NMR
(C6D6): δ 95.4 [br, 6 H, (CH2)], 18.9 [br, 5 H, (PCH2CH3)], 16.2
[s, 2 H, m-(C6H5)B], 11.0 [s, 2 H, o-(C6H5)B], 9.91 [s, 1 H,
p-(C6H5)B], 4.02 [br, 9 H, (PCH2CH3)], 0.60 [br, 27 H, (CH3)3S].
µeff (C6D6) ) 3.0 µB. UV-vis (pentane), λmax (ε, M-1 cm-1): 711
(135), 905 (671). Anal. Calcd for C27H53S3BCoP: C, 60.8; H, 8.90.
Found: C, 60.2; H, 9.01.
(iv) 1-PMe2Ph. Yield: 68 mg (57%). Mp: 143 °C dec. 1H NMR
(C6D6): δ 100.8 (br, 5 H, BCH2), 48.6 (br, 5 H, PCH3), 16.6 [s, 2
H, m-(C6H5)B], 11.3 [s, 2 H, o-(C6H5)P], 10.8 [s, 2 H, o-(C6H5)B],
9.74 [s, 1 H, p-(C6H5)B], 9.00 [s, 1 H, p-(C6H5)P], 7.26 [s, 2 H,
p-(C6H5)P], 0.36 [br, 27 H, (CH3)3S]. µeff (C6D6) ) 3.1 µB. UV-vis
(pentane), λmax (ε, M-1 cm-1): 706 (70), 896 (508). Anal. Calcd
for C28H49S3BCoP: C, 57.4; H, 8.48. Found: C, 57.7; H, 8.04.
(v) 1-PMePh2. Yield: 80 mg (60%). Mp: 147 °C dec. 1H NMR
(C6D6): δ 55.2 [br, 6 H, (CH2)], 24.3 [s, 3 H, (PCH3)], 15.6 [s, 4
(19) Shriver, D. F.; Drezdon, M. A. The Manipulation of Air SensitiVe
Compounds, 2nd ed.; Wiley: New York, 1986.
10706 Inorganic Chemistry, Vol. 47, No. 22, 2008

Ligand Electronic Effects on MonoWalent Cobalt Complexes
H, o-(C6H5)P], 14.1 [s, 2 H, m-(C6H5)B], 12.1 [s, 1 H, p-(C6H5)B],
8.21 [b, 2 H, p-(C6H5)P], 7.87 [br, 2 H, o-(C6H5)B], 6.73 [s, 4 H,
m-(C6H5)P], -7.51 [br, 27 H, (CH3)3S]. µeff (C6D6) ) 2.9 µB.
UV-vis (pentane), λmax (ε, M-1 cm-1): 710 (49), 896 (618). Anal.
Calcd for C34H51S3BCoP: C, 62.2; H, 7.83. Found: C, 62.0; H, 7.58.
(vi) 1-P(OPh)3. Yield: 102 mg (66%). Mp: 135 °C dec. 1H NMR
(C6D6): δ 129.2 [br, 6 H, (CH2)], 18.2 [s, 2 H, m-(C6H5)B], 17.9
[br, 2 H, p-(C6H5)P], 11.2 [s, 2 H, o-(C6H5)B], 10.7 [s, 1 H,
p-(C6H5)B], 9.43 [br, 4 H, m-(C6H5)P], 6.60 [s, 4 H, o-(C6H5)P],
-3.13 [br, 27 H, (CH3)3S]. µeff (C6D6) ) 2.9 µB. UV-vis (pentane),
λmax (ε, M-1 cm-1): 573 (120), 718 (257). Anal. Calcd for
C39H53S3O3BCoP: C, 61.1; H, 6.97. Found: C, 61.2; H, 7.06.
(i) 1-PPh3. Yellow, plate-like crystals were grown by slow
evaporation of a concentrated pentane solution.
(ii) 1-PMe3. Green plates were grown by slow evaporation of a
concentrated benzene solution at ambient temperature. Three
benzene solvent molecules were located co-crystallized in the
asymmetric unit with two centered at inversion with a net occupancy
of two benzene molecules per compound molecule.
(iii) 1-PEt3. Pale-green blocks were grown by slow evaporation
of a concentrated pentane solution at ambient temperature. The
compound consistently deposited as poorly diffracting, laminar
twins. The data representative of the best effort were obtained from
a pseudomerohedral crystal twinned along the unique monoclinic
axis diffracting only to 22° θ. The phenyl group was treated as a
rigid hexagon.
(iv) 1-PMe2Ph. Lime-green blocks were grown by slow evapora-
tion of a concentrated benzene solution at ambient temperature.
The thioether arms are disordered with 60/40 refined site occupan-
cies. A benzene solvent molecule is located at the inversion center
and was treated as a rigid hexagon at half-occupancy.
1
(vii) 1-CNBut. Yield: 66 mg (60%). Mp: 76 °C dec. H NMR
(C6D6): δ 114.5 [br, 6 H, (CH2)], 15.3 [br, 2 H, m-(C6H5)B], 12.0
[br, 9 H, (CN(CH3)3], 10.3 [br, 2 H, m-(C6H5)B], 9.84 [br, 1 H,
o-(C6H5)B], 0.18 [br, 27 H, (CH3)3S]. µeff (C6D6) ) 3.0 µB. UV-vis
(pentane), λmax (ε, M-1 cm-1): 276 (235) 335 (192), 780 (17). IR
(KBr): νCN ) 2198 cm-1. Anal. Calcd for C26H47S3BCoN: C, 57.9;
H, 8.78. Found: C, 57.8; H, 8.77.
X-ray Structural Solution and Refinement. Data were collected
on a Bruker-AXS APEX diffractometer equipped with graphite
monochromated Mo KR radiation (λ ) 0.7107 Å). The unit cell
parameters and orientation matrices were determined by gathering
reflections from three sets of 15-20 frames using 0.3° ω scans.
No symmetry higher than triclinic was observed for 1-PMePh2,
and solution in the centrosymmetric space group option yielded
chemically reasonable and computationally stable results of refine-
ment. The systematic absences in the diffraction data of the other
compounds are uniquely consistent with the reported space groups.
All structure factors, anomalous dispersion coefficients, and the
applied SADABS absorption correction program are contained in
various versions (5.1-6.12) of the SHELXTL program library.20
All non-hydrogen atoms were anisotropically refined using the
conventional full-matrix least-squares method. All hydrogen atoms
were treated as idealized contributions. The CIFs are available from
the Cambridge Crystallographic Data Center under the depository
numbers CCDC 671800-671806.
(v) 1-PMePh2. Green blocks were grown by slow evaporation
of a concentrated pentane solution at ambient temperature. Two
symmetry unique but chemically equivalent molecules are located
in the asymmetric unit.
(vi) 1-P(OPh)3. Dark blue blocks were grown by slow evapora-
tion of a concentrated pentane solution at ambient temperature.
(vii) 1-CNBut. Yellow plates were grown by slow evaporation
of a concentrated pentane solution at ambient temperature. The
methyl carbon atoms of the CNBut ligand display unresolvable
rotational disorder, and rigid bond restraints were applied.
Acknowledgment. William G. Dougherty, Jr. is thanked
for solving the structures of 1-PMePh2 and 1-P(OPh)3. These
studies were supported through funding provided by the
National Science Foundation (CHE-0213260 and CHE-
0518508).
Supporting Information Available: CIF files for all X-ray
structures. This material is available free of charge via the Internet
at http://pubs.acs.org.
(20) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112.
(21) MacBeth, C. E.; Thomas, J. C.; Betley, T. A.; Peters, J. C. Inorg.
Chem. 2004, 43, 4645.
IC800813U
Inorganic Chemistry, Vol. 47, No. 22, 2008 10707