Structural, Spectroscopic, and Electrochemical Studies of the Complexes [Ni2(μ-CNR)(CNR)2(μ-dppm)2]n+ (n = 0, 1, 2): Unusual Examples of Nickel(0)-Nickel(I) and Nickel(0)-Nickel(II) Mixed Valency

Article abstract of DOI:10.1021/ic035021j

Reaction of Ni(COD)₂ (COD = cyclooctadiene) with dppm (dppm = bis(diphenylphosphino) methane) followed by addition of alkyl or aryl isocyanides yields the class of nickel(0) dimers Ni₂(μ -CNR)(CNR)₂(μ-dppm)₂ (R = CH₃ (1), n-C ₄H₉ (2), CH₂C₆H₅ (3), i-C₃H₇ (4), C₆H₁₁ (5), f-C ₄H₉ (6), p-IC₆H₄ (7), 2,6-(CH ₃)₂C₆H₃ (8)). The cyclic voltammograms of the dimers exhibit two sequential single electron oxidations to the +1 and +2 forms. Specular reflectance infrared spectroelectrochemical (IRSEC) measurements demonstrate reversible interconversions between the neutral Ni(0) dimers and their +1 and +2 forms. Bulk samples of the +2 forms are prepared by chemical oxidation using [FeCP₂][PF₆], while the +1 forms are prepared by the comproportionation of neutral and +2 forms. The neutral complexes 6 and 8 were characterized by X-ray diffraction as symmetric, locally tetrahedral binuclear Ni(0) complexes. The +2 forms of these complexes, 6²⁺ and 8²⁺, have asymmetric structures with one locally square planar and one locally tetrahedral metal center, evidence for a Ni(II)-Ni(0) mixed valence state. The X-ray structural characterization of 6⁺ is symmetrical and qualitatively similar to that of the neutral complex 6. The +1 forms all exhibit intense near IR electronic absorptions that are assigned as intervalence charge transfer (IVCT) bands. On the basis of structural, spectroscopic, and electrochemical data, the +1 forms of the complexes, 1⁺-8⁺, are assigned as Robin-Day class III, fully delocalized Ni(+0.5)-Ni(+0.5) mixed valence complexes.

Full text of DOI:10.1021/ic035021j

Inorg. Chem. 2004, 43, 1071−1081
Structural, Spectroscopic, and Electrochemical Studies of the
Complexes [Ni2(µ-CNR)(CNR)2(µ-dppm)2]n+ (n ) 0, 1, 2): Unusual
Examples of Nickel(0)−Nickel(I) and Nickel(0)−Nickel(II) Mixed Valency
Gregory M. Ferrence,† Eugenio Simo´n-Manso,‡ Brian K. Breedlove,‡ Leah Meeuwenberg,§ and
,‡
Clifford P. Kubiak*
Department of Chemistry & Biochemistry, UniVersity of California San Diego,
La Jolla, California 92093-0358, and Department of Chemistry, Illinois State UniVersity,
Normal, Illinois 61790
Received August 30, 2003
Reaction of Ni(COD)2 (COD ) cyclooctadiene) with dppm (dppm ) bis(diphenylphosphino) methane) followed by
addition of alkyl or aryl isocyanides yields the class of nickel(0) dimers Ni2(µ-CNR)(CNR)2(µ-dppm)2 (R ) CH (1),
3
n-C H (2), CH C H (3), i-C H (4), C H (5), t-C H (6), p-IC H (7), 2,6-(CH ) C H (8)). The cyclic voltammograms
4
9
2
6
5
3
7
6
11
4
9
6
4
3 2 6 3
of the dimers exhibit two sequential single electron oxidations to the +1 and +2 forms. Specular reflectance infrared
spectroelectrochemical (IRSEC) measurements demonstrate reversible interconversions between the neutral Ni(0)
dimers and their +1 and +2 forms. Bulk samples of the +2 forms are prepared by chemical oxidation using
[FeCp2][PF ], while the +1 forms are prepared by the comproportionation of neutral and +2 forms. The neutral
6
complexes 6 and 8 were characterized by X-ray diffraction as symmetric, locally tetrahedral binuclear Ni(0) complexes.
The +2 forms of these complexes, 62+ and 82+, have asymmetric structures with one locally square planar and
one locally tetrahedral metal center, evidence for a Ni(II)−Ni(0) mixed valence state. The X-ray structural
characterization of 6+ is symmetrical and qualitatively similar to that of the neutral complex 6. The +1 forms all
exhibit intense near IR electronic absorptions that are assigned as intervalence charge transfer (IVCT) bands. On
the basis of structural, spectroscopic, and electrochemical data, the +1 forms of the complexes, 1+−8+, are assigned
as Robin−Day class III, fully delocalized Ni(+0.5)−Ni(+0.5) mixed valence complexes.
Introduction
of 1 to 12+ and several reactions of 1 with electrophiles were
reported.1-4 The bridging isocyanide of 1 can be alkylated
with alkyl halides to form [Ni2(µ-CN(R)Me)(CNMe)2(µ-
dppm)2]+.1,2 CO2 undergoes multiple bond metathesis with
the carbon-nitrogen triple bonds of the isonitrile ligand to
yield Ni2(µ-CO)(CO)2(µ-dppm)2 and methyl isocyanate.3,4
The bridging isonitrile of 1 is labile and is readily exchanged
The d10-d10, dppm bridged dinuclear nickel complex
Ni2(µ-CNCH3)(CNCH3)2(µ-dppm)2 (1) exhibits rich chem-
istry ranging from simple redox chemistry to transmetalation
and multiple bond metathesis.1-14 Previously, the oxidation
* Author to whom correspondence should be addressed. E-mail:
ckubiak@ucsd.edu.
† Illinois State University.
(7) Ratliff, K. S.; Broeker, G. K.; Fanwick, P. E.; Kubiak, C. P. Angew.
Chem., Int. Ed. Engl. 1990, 29, 395.
(8) Gong, J.; Huang, J.; Fanwick, P. E.; Kubiak, C. P. Angew. Chem.,
Int. Ed. Engl. 1990, 29, 396.
(9) Ni, J.; Fanwick, P. E.; Kubiak, C. P. Inorg. Chem. 1988, 27, 2017.
(10) Ratliff, K. S.; Fanwick, P. E.; Kubiak, C. P. Polyhedron 1990, 9, 2651.
(11) Kim, H. P.; Fanwick, P. E.; Kubiak, C. P. J. Organomet. Chem. 1988,
346, C39.
(12) Kubiak, C. P.; Ratliff, K. S. Isr. J. Chem. 1991, 31, 3.
(13) Kubiak, C. P. In ComprehensiVe Organometallic Chemistry II; Abel,
E. W., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon: Tarrytown,
1995; Vol. 9.
‡ University of California San Diego.
§ Visiting undergraduate from Kalamazoo College.
(1) DeLaet, D. L.; Fanwick, P. E.; Kubiak, C. P. Organometallics 1986,
5, 1807.
(2) Ratliff, K. S.; DeLaet, D. L.; Gao, J.; Fanwick, P. E.; Kubiak, C. P.
Inorg. Chem. 1990, 29, 4022.
(3) DeLaet, D. L.; del Rosario, R.; Fanwick, P. E.; Kubiak, C. P. J. Am.
Chem. Soc. 1987, 109, 754.
(4) DeLaet, D. L.; Fanwick, P. E.; Kubiak, C. P. J. Chem. Soc., Chem.
Commun. 1987, 1412.
(5) DeLaet, D. L.; Powell, D. R.; Kubiak, C. P. Organometallics 1985,
4, 954.
(14) Morgenstern, D. A.; Ferrence, G. M.; Washington, J.; Rosenhein, L.;
Henderson, J. I.; Heise, J. D.; Fanwick, P. E.; Kubiak, C. P. J. Am.
Chem. Soc. 1996, 118, 2198.
(6) Lemke, F. R.; DeLaet, D. L.; Gao, J.; Kubiak, C. P. J. Am. Chem.
Soc. 1988, 110, 6904.
10.1021/ic035021j CCC: $27.50 © 2004 American Chemical Society
Published on Web 01/07/2004
Inorganic Chemistry, Vol. 43, No. 3, 2004 1071

Ferrence et al.
Table 1. Data for [Ni2(µ-L)(L)2(µ-dppm)2]n+ (n ) 0, 1, 2) Dimers
31P, ppm
ν(CN), cm-1
n ) 0
n ) 2+
compound
L
n ) 0 n ) 2+
1
2
3
5
6
7
8
CH3NC
n-C4H9NC
C6H5CH2NC 18.6
C6H11NC
t-C4H9NC
p-IC6H4NC
xylylNC
18.3
17.9
35.2
35.8
36.2
36.0
10.2
36.2
2064, 1710 2196, 2085
2051, 1698 2181, 2088
2047, 1678 2184, 2097
2047, 1712 2169, 2069
2024, 1757 2169, 2078
1986, 1666 2135, 2020
2000,a 1820 2139, 2037
17.5
17.3
20.7
20.4
9.49
a Average.
with stronger π-acceptor ligands including aryl isocyanides
and NO+ to form Ni2(µ-CNAr)(CNMe)2(µ-dppm)2 (Ar )
aryl) and [Ni2(µ-NO)(CNMe)2(µ-dppm)2][PF6], respectively.2
1 readily reacts with other metal containing species to form
transmetalation products or larger clusters such as [NiAu-
(CNMe)2(µ-dppm)2]Cl and [{Ni2(µ-CNMe)(CNMe)2(µ-
dppm)}2Hg]2+, respectively.8-11
Prior to the studies presented here, no investigations of
the effects of altering the isocyanide ligands have been
reported. Similarly, the nature of the mixed valencies in the
+1 and +2 states had not been characterized. Here, we report
the synthesis of a series of nickel dimers, [Ni2(µ-CNR)-
(CNR)2(µ-dppm)2] (R ) CH3 (1), n-C4H9 (2), CH2C6H5 (3),
i-C3H7 (4), C6H11 (5), t-C4H9 (6), p-IC6H4 (7), 2,6-(CH3)2C6H3
(8)) with aryl and alkyl isocyanides. X-ray crystallographic,
electrochemical, and spectroscopic evidence for unusual
[Ni2]+ and [Ni2]2+ mixed valence states are presented.
Figure 1. ORTEP plot of Ni2(µ-CN(t-C4H9))(CN(t-C4H9))2(µ-dppm)2 (6)
showing 50% thermal ellipsoids.
cm-1, and a weak broad band below 2000 cm-1 correspond-
ing to the bridging isocyanide ligand (Table 1). In solution,
the terminal CN bands collapse to a single broad band. The
terminal ν(CtN) bands are not significantly affected by the
nature of the alkyl or aryl group. Notable exceptions are
complexes 6 and 8 with sterically demanding tert-butyl-
and xylyl-isocyanide groups. The difference in the ν(CtN)
stretching frequencies for 6 and 8 suggests that the structures
are somewhat different from the others due to the presence
of the bulky isocyanides. This is borne out by the structural
studies described below.
The structures of 6 (tert-butyl) and 8 (2,6-xylyl) are shown
in Figures 1 and 2, and selected bond lengths and angles are
summarized in Table 4. Both complexes have overall
“cradle” or “W-frame” type structures, and each Ni(0) center
has an approximately tetrahedral coordination geometry with
two phosphorus atoms from two bridging dppm ligands, one
terminal isocyanide carbon atom, and one bridging iso-
cyanide carbon atom forming the vertices. The average bond
angles and bond distances about the nickel core in 6 and 8
are quite similar to those of 1, but the steric bulk of the R
group causes the Cbridge-N-C angle in 6 (159.0(2)°) and 8
(180°) to be quasi-linear and linear, respectively, compared
to 1 (129.9(6)°). The Cbridge-N-C bond angle of 129.9(6)°
and the reactivity of 1 are consistent with substantial sp2
hybridization of the bridging isocyanide ligand,1,2 whereas
the linear nature of the bridging isocyanides of 6 and 8 are
consistent with sp hybridization. The degree of sp2 vs sp
Results and Discussion
Synthesis and Structures of the Neutral Dimers Ni2(µ-
CNR)(CNR)2(µ-dppm)2. The nickel dimers Ni2(µ-CNR)-
(CNR)2(µ-dppm)2 (R ) CH3 (1), n-C4H9 (2), CH2C6H5 (3),
i-C3H7 (4), C6H11 (5), t-C4H9 (6), p-IC6H4 (7), 2,6-(CH3)2C6H3
(8)) were prepared in order to elucidate the electronic and
steric effects of the isocyanide ligand on the structure and
redox properties of this class of compounds. Each of the
formally zerovalent nickel dimers 1-8 was synthesized by
the room temperature addition of 3 equivalents of the
appropriate isocyanide to a THF solution containing Ni(COD)2
and dppm in a 1:1 ratio (eq 1).
(15) Wu, J.; Fanwick, P. E.; Kubiak, C. P. Organometallics 1987, 6, 1805-
1807.
(16) Manojlovic-Muir, L.; Muir, K. W.; Davis, W. M.; Mirza, H. A.;
Puddephatt, R. J. Inorg. Chem. 1992, 31, 904-909.
(17) Gong, J. K. Ph.D. Thesis, Purdue University, 1990.
(18) Gong, J. K.; Fanwick, P. E.; Kubiak, C. P. J. Chem. Soc., Chem.
Commun. 1990, 1190-1191.
As the reaction proceeds, the solution changes from orange
to red, and the products are obtained as orange to brick red
solids (λmax ) 340-345 nm). Each of these complexes
exhibits a single resonance at ca. 20 ( 2 ppm (Table 1) in
the 31P{1H} NMR, which is similar to the related complex
Ni2(µ-CO)(CO)2(µ-dppm)2.24,25 The small difference in chemi-
cal shift suggests that the nickel-phosphorus interaction is
similar in each of the dimers.
(19) Hinze, S.; Gong, J.; Fanwick, P. E.; Kubiak, C. P. J. Organomet. Chem.
1993, 458, C10-C11.
(20) Fontaine, X. L. R.; Higgins, S. J.; Shaw, B. L.; Thornton-Pett, M.;
Yichang, W. J. Chem. Soc., Dalton Trans. 1987, 1501-1507.
(21) Morgenstern, D. A.; Bonham, C. C.; Rothwell, A. P.; Wood, K. V.;
Kubiak, C. P. Polyhedron 1995, 14, 1129-1137.
(22) Ferrence, G. M.; Fanwick, P. E.; Kubiak, C. P. Chem. Commun. 1996,
1575-1576.
(23) Maekawa, M.; Munakata, M.; Kuroda-Sowa, T.; Hachiya, K. Inorg.
Chim. Acta 1995, 233, 1-4.
(24) Osborn, J. A.; Stanley, G. G.; Bird, P. H. J. Am. Chem. Soc. 1988,
110, 2117.
In the solid state, each of the neutral dimers has one weak
and one intense terminal ν(CtN) stretching band above 2000
(25) Gong, J.; Kubiak, C. P. Inorg. Chim. Acta 1989, 162, 19.
1072 Inorganic Chemistry, Vol. 43, No. 3, 2004

Mixed Valency in [Ni2(µ-CNR)(CNR)2(µ-dppm)2]n+
Figure 2. ORTEP plot of the complex Ni2(µ-CN(2,6-(CH3)2C6H3))-
(CN(2,6-(CH3)2C6H3))2(µ-dppm)2 (8) showing 50% thermal ellipsoids.
Figure 3. ORTEP plot of the complex [Ni2(µ-CN(t-C4H9))(CN(t-C4H9))2-
(µ-dppm)2][PF6]2 (62+) showing 50% thermal ellipsoids.
Table 2. Electrochemical Data for [Ni2(µ-L)(L)2(µ-dppm)2]n+ (n ) 0,
1, 2) Species
E1/2(2+/+) E1/2(+/0)
∆E
compound
L
(V vs SCE) (V vs SCE) (mV)
Kc
1
2
3
4
5
6
7
8
CH3NC
-0.64
-0.60
-0.52
-0.62
-0.57
-0.50
-0.25
-0.19
-1.00
-0.97
-0.85
-0.98
-0.99
-1.13
-0.53
-1.29
360 1.2 × 106
370 1.8 × 106
330 3.7 × 105
360 1.2 × 106
420 1.2 × 107
630 4.3 × 1010
280 5.3 × 104
1100 3.7 × 1018
n-C4H9NC
C6H5CH2NC
i-C3H7NC
C6H11NC
t-C4H9NC
p-IC6H4NC
xylylNC
character of the bridging ligand does appear to be correlated
with the extent of Ni(0)-to-bridging ligand π-back-donation
(vide infra).
Figure 4. ORTEP plot of the complex [Ni2(µ-CN(2,6-(CH3)2C6H3))-
(CN(2,6-(CH3)2C6H3))2(µ-dppm)2][PF6]2 (82+) showing 50% thermal el-
lipsoids.
The slightly longer Ni-Cbridge (6, 1.925(3)Å; 8, 1.927(2)
vs 1, 1.903(6)Å) and Ni-Ni (8, 2.6194(5) Å vs 1, 2.572(1)Å)
bond distances indicate net weaker metal-isocyanide inter-
actions in 6 and 8 compared to 1. The longer Ni-Cbridge bond
distances are also consistent with the observed higher
bridging ν(CtN) band energies of 6 (1757 cm-1) and 8
(1820 cm-1) compared to 1 (1710 cm-1) and with a decrease
in π-back-bonding. In the xylyl complex 8, there is a
pronounced twist of the xylyl ring (40°) with respect to the
plane which contains the Ni-Cbridge-Ni plane. This would
be expected to reduce the overlap between Ni dπ and ligand
π* orbitals of the bridging isocyanides and decrease π-back-
bonding significantly.
Synthesis and Structures of the Divalent Dimers [Ni2(µ-
CNR)(CNR)2(µ-dppm)2]2+. Addition of two equiv of ferro-
cenium hexafluorophosphate to an acetonitrile suspension
of 1-8 resulted in the formation of solutions containing a
mixture of ferrocene and the dicationic [Ni2]2+ product (eq
2). After ferrocene was extracted with benzene, the doubly
then fully characterized. The two terminal isocyanide ligands
give rise to IR absorption bands between 2130 and 2200
cm-1, and the third isocyanide exhibits a semibridging mode
ca. 100 cm-1 lower in energy.
The structures of 62+ and 82+ were determined by single-
crystal X-ray diffraction. The structure of [Ni2(µ-CN(t-
C4H9))(CN(t-C4H9))2(µ-dppm)2][PF6]2, 62+, and [Ni2(µ-CN-
(2,6-(CH3)2C6H3)(CN(2,6-(CH3)2C6H3)2(µ-dppm)2][PF6]2, 82+,
are shown in Figures 3 and 4. Selected bond distances and
angles are summarized in Table 4.
The structure of 62+ is essentially isostructural with 12+.3
Generally, the Ni-L distances of 12+ and 62+ agree within
experimental error. A notable exception is the longer Ni(2)-
C(1) semibridging distance for 62+ (2.360(5) Å) compared
to 12+ (2.19(1) Å), which may reflect the added bulk of the
tert-butyl groups of 62+. Similarly the isocyanide CtN
distances do not differ significantly. All of the L-Ni-L
angles in 62+ are within 12° of 12+. C-N-C bond angles
are similar in both structures and are nearly linear. The
structure of 62+ is best described as resulting from the dative
interaction between a tetrahedral 18e-, Ni0L4 fragment (Ni(1)
in Figure 3) and a T-shaped, 14e-, NiIIL3 fragment (Ni(2)
in Figure 3).
oxidized dimers 12+-82+ were isolated as dark blue solids
(λmax ) 580-590 nm) by recrystallization from methylene
chloride/pentane. These +2 mixed valence complexes were
The overall differences between the structures of 62+ and
82+ are in the pattern of substitution of the approximately
tetrahedral nickel atom. In 62+, both of the isocyanides
Inorganic Chemistry, Vol. 43, No. 3, 2004 1073

Ferrence et al.
Table 3. Crystal Data and Data Collection Parameters for 6, 62+, 6+‚CH2Cl2, 8, and 82+‚THF
compound
6
6+‚CH2Cl2
62+
8
82+‚THF
formula
C65H71N3Ni2P4
1170.57
C2/c (No. 15)
monoclinic
23.657(5)
11.322(2)
23.208(5)
90
109.16(3)
90
5872(2)
4
C66H73Cl2F6N3Ni2P5
1365.52
P1 (No. 1)
triclinic
10.6578(7)
13.3188(9)
13.5122(6)
103.439(3)
101.316(3)
110.051(3)
1670.6(4)
1
C66H74Cl2F12N3Ni2P6
1511.42
C2/c (No. 15)
monoclinic
22.05(1)
16.34(1)
40.28(3)
90
102.58(5)
90
14164(15)
8
C77H71N3Ni2P4
1281.68
C81H79F12N3Ni2OP6
1630.71
P2(1)/n
monoclinic
13.8173(16)
23.585(3)
23.519(3)
90
100.956(2)
90
7524.7(15)
4
formula weight
space group
crystal system
a, Å
b, Å
c, Å
C2/c (No. 15)
monoclinic
21.4008(16)
12.9100(10)
23.6900(18)
90
106.6950(10)
90
6269.3(8)
4
R, deg
â, deg
γ, deg
V, Å3
Z
d
calc, g cm-3
1.324
1.357
1.418
1.358
1.439
crystal dimens, mm
temp, K
0.25 × 0.22 × 0.12
0.44 × 0.40 × 0.38
2.00 × 0.60 × 0.35
0.80 × 0.20 × 0.12
0.4 × 0.2 × 0.10
100(2)
193.
186.
100(2)
100(2)
F000
2460
709.0
6232
2688
3372
GOF
1.074
1.030
1.049
0.950
0.978
final R indices [I > 2σ(I)] R1
0.0324
0.042
0.0493
0.0364
0.0621
Table 4. Selected Bond Lengths (Å) and Angles (deg) for Compounds 6, 8, 62+, 82+, and 6+
62+
6
8
82+
6+
Ni(1)-Ni(1A)
Ni(1)-C(1)
Ni(1)-C(2)
C(2)-N(2)
N(1)-C(1)
Ni(1)-P(2)
Ni(1)-P(1)
2.5555(10) Ni(1)-Ni(1A)
1.823(2) Ni(1)-C(2)
1.925(3) Ni(1)-C(5)
1.202(5) Ni(1)-P(2)
1.165(3) N(1)-C(2)
2.1849(7) N(2)-C(5)
2.1911(7) N(1)-C(3E)
N(2)-C(6)
2.6194(5) Ni(1)-Ni(2)
1.8157(19) Ni(1)-C(1)
1.927(2) Ni(1)-C(2)
2.2046(5) Ni(2)-P(2)
1.155(3) Ni(1)-P(4)
1.206(4) Ni(2)-C(1)
1.3905(19) Ni(1)-P(1)
1.359(4) N(1)-C(1)
N(2)-C(2)
2.3931(13) Ni(1)-Ni(2)
1.847(5) Ni(1)-C(2)
1.850(5) Ni(1)-C(1)
2.234(2) Ni(1)-P(1)
2.233(2) Ni(1)-P(3)
2.360(5) Ni(2)-C(1)
2.234(2) Ni(2)-C(3)
1.157(6) Ni(2)-P(2)
1.146(6) Ni(2)-P(4)
1.854(5) N(2)-C(2)
1.137(6) N(2)-C(12)
N(1)-C(1)
2.4813(10) Ni(1)-Ni(2)
1.847(6) Ni(1)-C(10)
2.436(5) Ni(2)-C(30)
2.2497(16) Ni(1)-C(30)
2.2568(16) Ni(2)-P(4)
1.843(6) Ni(1)-P(3)
1.857(6) N(10)-C(10)
2.2339(16) N(20)-C(20)
2.2356(16) Ni(2)-C(20)
1.160(7) N(30)-C(30)
1.415(7)
2.5879(8)
1.859(6)
1.944(6)
2.009(6)
2.2177(15)
2.2141(15)
1.170(8)
1.155(7)
1.881(6)
1.147(7)
Ni(2)-C(3)
N(3)-C(3)
1.170(7)
N(1)-C(4)
1.421(7)
C(1)-Ni(1)-C(2) 100.72(10) C(2)-Ni(1)-C(5) 94.82(8)
C(1)-Ni(1)-C(2) 147.2(2)
P(1)-Ni(1)-P(3) 166.60(6)
C(10)-Ni(1)-C(30) 102.7(3)
P(2)-Ni(1)-P(1) 108.03(3)
C(1)-N(1)-C(34) 165.9(2)
C(2)-N(2)-C(27) 159.0(2)
N(1)-C(1)-Ni(1) 175.1(2)
C(2)-Ni(1)-P(1) 108.48(6)
C(2)-Ni(1)-P(2) 106.71(6)
P(1)-Ni(1)-P(2) 106.22(2)
C(2)-N(1)-C(3E) 175.3(2)
C(5)-N(2)-C(6) 180
C(1)-Ni(1)-P(4) 97.66(14) C(2)-Ni(1)-P(3) 92.78(17) P(3)-Ni(1)-P(1)
C(2)-Ni(1)-P(4) 91.63(14) C(2)-Ni(1)-P(1) 88.99(17) P(4)-Ni(2)-P(2)
124.33(6)
122.43(6)
C(1)-Ni(1)-P(1) 91.00(14) P(2)-Ni(2)-P(4) 115.59(6)
C(20)-Ni(2)-C(30) 103.8(2)
C(30)-Ni(2)-P(4) 112.98(17)
C(10)-N(10)-C(11) 171.3(7)
C(20)-N(20)-C(21) 172.6(6)
C(30)-N(30)-C(31) 175.4(7)
N(30)-C(30)-Ni(1) 134.5(5)
C(2)-Ni(1)-P(1) 104.5(2)
P(4)-Ni(1)-P(1) 134.79(6)
P(4)-Ni(1)-Ni(2) 112.13(6)
P(1)-Ni(1)-Ni(2) 111.95(5)
C(3)-Ni(2)-Ni(1) 178.27(14)
C(3)-Ni(2)-P(3) 94.39(14)
P(2)-Ni(2)-P(3) 170.39(5)
C(1)-Ni(2)-C(3) 93.5(2)
C(1)-N(1)-C(4) 170.9(5)
coordinated to Ni(1) lean toward the other nickel atom in a
semibridging manner. In 82+, only one of the isocyanides is
semibridging. The other is oriented well away from Ni(2).
This has the effect of producing a coordination geometry
around Ni(1) in 82+ quite similar to that of the neutral
“cradle” or “W-frame” type structure while the coordination
geometry around Ni(2) becomes markedly more rectilinear
The oxidation of the dimers from [Ni2]0 to [Ni2]2+ leads
to conversion of the cis-cis W-frame structure of [Ni2]0 into
trans-cis structure of [Ni2]2+, breaking the symmetry between
phosphorus atoms of the dppm ligand. The solution 31P NMR
spectra of all [1-8]2+ complexes show a broad singlet at
room temperature and an even sharper singlet below -90
°C (see Supporting Information). Since the ground-state
structures for complexes 12+,15 62+, and 82+ (Figures 3 and
4) have two very different phosphorus environments, it is
clear that in solution the complexes must be fluxional even
at very low temperatures (Scheme 1). The simplest intra-
molecular mechanism which renders the phosphorus envi-
ronment equivalent is the stepwise rotation of the three
isocyanide ligands about the two nickel atoms, creating an
effective symmetry plane perpendicular to the Ni-Ni axis.
A close interaction between the C atom of the terminal ligand
L1 (Scheme 1) and Ni(2) (Ni(2)-C(L1) ) 2.789(5) Å)
allows for direct interconversion of the bridging and terminal
ligands.
Further information about the mechanism for exchange is
1
obtained from the H NMR spectra. At +35 °C, the CH2P2
unit of the dppm ligands and the t-C4H9 protons of the
bridging and terminal isocyanide ligands give broad signals
centered at [δ CHaHb, 2.95; δ Me, 2.2], and upon cooling
the solution to -25 °C, the signal corresponding to the CH2P2
protons splits into an “AB” pattern [δHa, 3.2; δHb, 3.35
2J(HaHb) ) 14 Hz] and the signals corresponding to terminal
and bridging isocyanides split into two broad singlets [δ
CH3bridg, 2.8; δ CH3term, 2.6]. These data show that at low
temperature (-25 °C) the fluxionality creates an effective
plane of symmetry perpendicular to the Ni-Ni axis, thus
1074 Inorganic Chemistry, Vol. 43, No. 3, 2004

Mixed Valency in [Ni2(µ-CNR)(CNR)2(µ-dppm)2]n+
Scheme 1. Direct Interconversion of Terminal and Bridge Ligands by
Rotation
Figure 5. Cyclic voltammogram of the complex [Ni2(µ-CN(t-C4H9))-
(CN(t-C4H9))2(µ-dppm)2][PF6]2 (62+).
making the two nickel atoms and the four phosphorus atoms
equivalent. It does not, however, create a plane of symmetry
within the Ni2P4C2 skeleton (Scheme 1). At higher temper-
ature (+35 °C) the signals corresponding to CHaHb protons
and the terminal and bridge isocyanide protons collapse,
suggesting a fast exchange between terminal and bridging
isocyanides and creating an effective plane of symmetry in
the Ni2P4C2 skeleton (Scheme 1). An analogous mechanism
was proposed for the complexes [Ni2(µ-CNMe)(CNMe)2(µ-
dppm)2][PF6]2 and Ni2(µ-CO)Cl2(µ-dppm)2.16
5
Infrared Spectroelectrochemistry of the Nickel Dimers.
All of the dimers have two well-separated reversible single
electron redox couples evident in their cyclic voltammograms
(CV). The CV for 6 is presented in Figure 5. Table 2
summarizes the half-wave potentials and the observed
splittings between the E1/2(+/0) and E1/2(2+/1+) for 1-8.
Compounds 1-5 show similar redox behavior, while 6-8
have larger separations between the two oxidation waves.
The conversion between the neutral and +2 oxidation
states involves a [Ni2]+ species. To assess the structural
trends in the [Ni2]0, [Ni2]+, and [Ni2]2+ states, we employed
infrared spectroelectrochemistry (IRSEC). Figure 6 shows
the isocyanide ν(CN) region of Ni2(µ-CN(t-C4H9))(CN(t-
C4H9))2(µ-dppm)2 (6) over the course of one SEC experiment.
After the cell was charged with the neutral dimer, the
potential was stepped to ca. -0.8 V vs SCE, halfway between
the two redox potentials of 6. The bands corresponding to
the neutral dimer 6 at 2035 and 1745 cm-1 disappear as new
bands appear at 2109 and 1976 cm-1 (Figure 6a). The
potential is then stepped sufficiently positive (E > -0.5 V
vs SEC) to generate the di-oxidized species 62+. The bands
at 2109 and 1976 cm-1 decrease in intensity as the bands
corresponding to 62+ appear (Figure 6b). Thus, the bands at
2109 and 1976 cm-1 correspond to 6+. The +1 and neutral
dimers are regenerated by stepping back to a sufficiently
negative potential (E < -1.1 V vs SCE). Similar SEC
experiments were carried out on the nickel dimers 1, 2, 3,
5, 6, and 8, and the IRSEC response for the oxidation of 1
is shown in Figure 7a. Here, bands at 1716 and 2072 cm-1
decrease as the band at 2141 cm-1 increases. Upon going to
more positive potentials, Figure 7b, the band at 2141 cm-1
Figure 6. Infrared spectra from IRSEC of [Ni2(µ-CN(t-C4H9))(CN(t-
C4H9))2(µ-dppm)2][PF6]2, 6: (a) oxidation of 6 to 6+; (b) oxidation of 6+
to 62+
.
decreases as the band at 2194 cm-1 grows. Similar results
were obtained for each of the dimers studied, and clean
isosbestic points are observed for the terminal ν(CtN) bands,
indicating that conversion between oxidation states is direct
and without the formation of any intermediates.
The one electron oxidized dimers [Ni2(µ-CNR)(CNR)2-
(µ-dppm)2]+ formally contain a [Ni2]+ core, suggesting the
possibility of a Ni0, NiI formal oxidation state. The character
of this mixed valency is considered more carefully in the
Inorganic Chemistry, Vol. 43, No. 3, 2004 1075

Ferrence et al.
next section. The terminal ν(CtN) infrared stretching
frequencies show a similar shift in energy of 65 ( 10 cm-1
upon removal of each subsequent electron. This is consistent
with a decrease in π-back-bonding between the nickel and
isocyanide carbon in progressively higher oxidation states.
The bridging ν(CtN) stretching frequencies are affected to
a much larger degree, and generally changes in bridging
ν(CtN) stretching frequency are significantly greater upon
oxidation from [Ni2]0 to [Ni2]+ as compared to oxidation from
[Ni2]+ to [Ni2]2+. These results give further evidence for the
HOMO of 1 being partially metal and substantially bridging
CtN based.2
Preparation of One Electron Oxidized [Ni2(µ-CNR)-
(CNR)2(µ-dppm)2]+ Dimers and X-ray Structural Char-
acterization of [Ni2(µ-CN(t-C4H9))(CN(t-C4H9))2(µ-dppm)2]-
[PF6]. It is possible to prepare the one electron oxidized
dimers 1+-8+ by the comproportionation of the neutral and
divalent states, eq 3. The ν(CtN) stretching frequencies
(KBr pellets) are similar to the corresponding values observed
in the SEC experiments.
The structure of 6+ is shown in Figure 8. The shift in
bridging ν(CtN) stretching frequencies upon oxidation from
6 to 6+ is substantial (200 cm-1). Indeed, a ν(CtN)
frequency of 1976 cm-1 is more typical of a semibridging
isocyanide, like that in 62+, than the symmetrically bridging
isocyanide found in 6. However, the structure of 6+ is
remarkably similar to that of 6, including the presence of a
symmetrical bridging isocyanide ligand. The bond angles and
distances of 6+, Table 4, are also within experimental error
of those of 6!
Figure 7. Infrared spectra from IRSEC of [Ni2(µ-CNCH3)(CNCH3)2(µ-
dppm)2][PF6]2, 1: (a) oxidation of 1 to 1+; (b) oxidation of 1+ to 12+
.
The similarities in the structures of 6 and 6+ suggest that
the charge is evenly distributed over both metal centers, and
possibly the ligands. Previous molecular orbital studies
involving the model complex [Ni2(µ-CNH)(CNH)2(µ-
PH2CH2PH2)2] showed that the HOMO involves the π*
orbital of the bridging isocyanide and Ni2 dπ* orbitals.2 The
HOMO involves a metal-to-ligand interaction of a type that
can be considered the two metal-bridging ligand version
of the usual Dewar-Chatt-Duncanson back-bonding model,
see Figure 11, for example. Thus, removal of an electron
should result in a significant reduction of π-back-bonding
to the bridging isocyanide and a shortening of the CN triple
bond. Only a slight shortening actually occurs on going from
6 (1.202(5) Å) to 6+ (1.147(9) Å). The symmetric structure
suggests a fully delocalized mixed valence description.
Spectroscopic evidence for a Ni(+0.5)-Ni(+0.5) fully
delocalized [Ni2]+ core is discussed later.
Figure 8. ORTEP plot of the complex [Ni2(µ-CN(t-C4H9))(CN(t-C4H9))2-
(µ-dppm)2][PF6] (6+) showing 50% thermal elllipsoids.
of E1/2, ∆E, and the comproportionation equilibrium constant,
KC, for [Ni2(µ-CNR)(CNR)2(µ-dppm)2], 1-8, are sum-
marized in Table 2. The aliphatic substituted isocyanide
dimers 1-5 exhibit similar E1/2(2+/+) and E1/2(+/0) values,
and thus similar ∆E values that correspond to KC on the order
of 105 to 107. No particular correlations between electronic
and steric factors for these ligands appear to exist. For 7, R
Stability of the One Electron Oxidized Nickel Dimer,
[Ni2(µ-CNR)(CNR)2(µ-dppm)2]+. While the nature of the
isocyanide ligand generally has little effect upon the struc-
tures of the neutral series of complexes, a profound effect
on the electrochemistry of the dimers is observed. The values
1076 Inorganic Chemistry, Vol. 43, No. 3, 2004

Mixed Valency in [Ni2(µ-CNR)(CNR)2(µ-dppm)2]n+
Figure 9. The comproportionation reaction of the two different oxidation
states ([Ni2]0 + [Ni2]2+) for complex Ni2(µ-CN(n-C4H9))(CN(n-C4H9))2-
(µ-dppm)2 (2), monitored by UV-vis spectroscopy.
) p-IC6H4, single electron oxidation is significantly more
positive than in dimers 1-6, E1/2(0/+) ) -0.25 V vs SCE.
Presumably, the aromatic ring helps to delocalize the elec-
tron density π-back-donated to the isocyanide more ef-
fectively, thus stabilizing the Ni(0) centers with respect to
oxidation. Once formed, however, the one electron oxidized
dimer readily loses the second electron to form the dication,
E1/2(+/2+) ) -0.25 V vs SCE, thus the relatively small
KC, 5.3 × 104. Interestingly, the tert-butyl isocyanide dimer
6 and the xylyl isocyanide dimer 8 show markedly different
electrochemistry and large increases in KC, 1010 and 1018,
respectively. The first oxidations of these bulky dimers are
130-290 mV more cathodic than the other alkyl isocyanide
based dimers. This indicates neutral states of decreased
stability. Comparison of X-ray diffraction data of the 6 and
8 complexes to those of 1 indicates one substantial structural
variation: the quasi-linear bridging isocyanide of 6 and
completely linear isocyanide of 8. The origin of the decreased
stability of 6 and 8 toward oxidation appears to be simply
that the bridging isocyanides which are forced sterically to
be linear remain nominally sp hybridized. The bent (C-N-
C, 129.9(6)°) bridging isocyanide of 1 can achieve nominal
sp2 hybridization through more effective back-donation. The
“frustrated” back-donation to the bridging isocyanide ligands
of 6 and 8 destabilizes these complexes with respect to
oxidation. While the first oxidation of 6 is more cathodic
than 1, the second oxidation of 6 is comparable to the other
aliphatic dimers. Hence 6+ is stable over a large redox
potential range, KC ) 1010. The second oxidation of 8 is the
most anodic of any of the compounds studied. This results
in a very large KC, 1018. This is attributed to the ability of
the aromatic rings of the xylyl isocyanides to stabilize the
Ni(+0.5)-Ni(+0.5) mixed valence (+1) state.
Figure 10. The experimental and simulated UV-vis spectra for the
complexes [Ni2(µ-CNR)(CNR)2(µ-dppm)2][PF6]: (a) R ) n-C4H9 (2+); (b)
R ) t-Bu (6+); (c) R ) 2,6-(CH3)2C6H3 (8+).
UV-Vis Spectroscopy of the Nickel Dimers: Inter-
valence Charge Transfer. To further characterize the
electronic structures of the unusual [Ni2]+ oxidation state of
the nickel dimers, we investigated the UV-vis spectroscopy
of n-C4H9 (sterically nonhindered, alkyl), t-C4H9 (sterically
hindered, alkyl), and xylyl (sterically hindered, aromatic) as
representatives of the group. The UV-visible spectral data
for the complexes [Ni2(µ-CNR)(CNR)2(µ-dppm)2]+ are col-
Figure 11. Orbital overlap diagram for the Ni2 dπ*/ bridging ligand π*
combination that describes the HOMO in 1-8.
lected in Table 5. Figure 9 shows the comproportionation
reaction (eq 3) for complex Ni2(µ-CN(n-C4H9))(CN(n-
C4H9))2(µ-dppm)2 (2), monitored by UV-visible spectros-
copy. In each of the examples, comproportionation reactions
were accompanied by clean isosbestic points, indicating that
Inorganic Chemistry, Vol. 43, No. 3, 2004 1077

Ferrence et al.
Table 5. Summary of Electronic Spectral Data for the Two IVCT
Bands of the Mixed Valence [ ]+ State of the Complexes
[Ni2(µ-CNR)(CNR)2(µ-dppm)2][PF6]
Conclusions
A series of Ni dimers, [Ni2(µ-CNR)(CNR)2(µ-dppm)2],
were synthesized by reacting Ni(COD)2, dppm, and the
appropriate isocyanide. As the steric bulk of the bridging
isocyanide becomes progressively greater, the bridging ligand
is prevented from adopting a bent geometry. This is reflected
in Cbridge-N-C angles of 159.0(2)° in the tert-butyl deriva-
tive (6) and 180° in the xylyl derivative (8), compared to
129.9(6)° in the methyl isocyanide complex (1). Electro-
chemical studies indicate that increased degrees of bending
of the bridging isocyanide ligand stabilize the neutral Ni(0)-
Ni(0) state with respect to oxidation. This is interpreted in
terms of more effective Ni(0) to bridging ligand π* back-
donation when the isocyanide ligand can bend to near 120°.
Cyclic voltammetry combined with IRSEC experiments
showed that three stable oxidation states exist for the dimers.
Both the +2 and +1 states are accessible and can be isolated
in pure form. The +2 state is assigned to a Ni(0)-Ni(II)
mixed valency, based on X-ray crystal structures of 62+ and
82+ that show a locally tetrahedral coordination geometry
about one nickel center and a locally T-shaped coordination
geometry about the other. A dative nickel-nickel bond is
presumed in these complexes. NMR studies reveal that the
fluctuation between the localized Ni(0)-Ni(II) valencies is
occurring on a time scale which is fast, even below -90°
C. The +1 state is assigned to a formally Ni(+0.5)-Ni(+0.5)
fully delocalized, Robin-Day class III classification. The
structure of 6+ is found to be completely symmetric, and
is similar overall to the structure of 6. The +1 states are
found to be enormously stable. Comproportionation equi-
librium constants express the relative stability of the +1
state with respect to the neutral and +2 states; and these are
found to be as high as 1010 for 6 and 1018 for 8. The +1
compounds all show multiple intervalence charge transfer
bands in the near IR. These show no solvent dependence
and most likely arise from a ground state split by spin-
orbit coupling.
νmax
νmax
43
43
R
(cm-1
)
ꢀνmax ∆ν1/2 HAB
(cm-1
) ꢀνmax ∆ν1/2 HAB
n-C4H9 (2) 10101 658 3787 1260.6 12820 424 3295 1063.4
t-C4H9 (6) 10266 790 3926 1413.5 12787 486 3273 1129.8
xylyl (8)
9090 1314 3327 1598.8 11750 796 3057 1359.4
conversion between oxidation states is direct and without
the formation of any intermediates, as was also observed in
the IRSEC experiments. Figure 10 shows the broad electronic
absorptions that appeared in the near IR region [6000-12000
cm-1]. Assuming a Gaussian shape for a single IVCT band,
the experimental spectra were simulated. The νmax, extinction
coefficient, ν1/2, and derived electronic coupling constant
(HAB) obtained from the spectra are summarized in Table 5.
All three complexes exhibit multiple IVCT bands in this
region. The n-C4H9 [10101 (658), 12820 (424) cm-1 (M-1
cm-1)] and t-C4H9 isocyanide complex [10266 (790), 12787
(486) cm-1 (M-1 cm-1)] bands appear at higher energy and
at approximately half the intensity compared to the xylyl
complex [9090 (1314), 11750 (796) cm-1 (M-1 cm-1)]. The
energy difference between the two IT transitions shifts from
2521 cm-1 for t-C4H9 to 2719 cm-1 for n-C4H9. The IVCT
band shape was examined in three different solvents (CH3CN,
THF, CH2Cl2) and showed no solvent dependence. There
are several well studied complexes which exhibit multiple
IVCT bands4226-43 in Ru(II)-Os(III) and Ru(II)-Ru(III)
systems. The multiplicity of the IVCT band was explained
to arise from a ground state split by spin-orbit coupling.
The origins of the split IVCT bands in the [Ni2]+ species
may well be similar, as the SOMO is expected to be the Ni2
dπ*/bridging ligand π* combination that could develop
significant spin-orbit coupling, Figure 11.
(26) DeLaet, D. L. Ph.D. Thesis, Purdue University, 1987.
(27) Schunn, R. A.; Ittel, S. D.; Cushing, M. A. In Inorganic Syntheses;
Angelici, R. J., Ed; Wiley: New York: 1990; Vol. 28, pp 94-98.
(28) Guerrieri, F.; Salerno, G. J. Organomet. Chem. 1976, 114, 339.
(29) Casanova, J.; Schuster, R. E.; Werner, N. D. J. Chem. Soc. 1963,
4280.
(30) Weber, W. P.; Gokel, G. W. Tetrahedron Lett. 1972, 17, 1637.
(31) Wittrig, R. E.; Kubiak, C. P. J. Electroanal. Chem. 1995, 393, 75.
(32) McArdle, P. C. J. Appl. Crystallogr. 1996, 239, 306.
(33) Otwinowski, Z.; Minor, W. Methods Enzymol. 1996, 276, 307.
(34) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Moliterni,
A. G. G.; Burla, M. C.; Polidori, G.; Camalli, M.; Spagna, R. In
preparation.
(35) International Tables for Crystallography; Kluwer Academic Publish-
ers: Dordrecht, The Netherlands, 1992; Vol. C, Tables 4.2.6.8 and
6.1.1.4.
(36) Sheldrick, G. M. SHELXS97. A Program for Crystal Structure
Refinement; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(37) Johnson, C. K. ORTEPII; Report ORNL-5138; Oak Ridge National
Laboratory: Oak Ridge, TN, 1976.
Experimental Section
General Information. All manipulations were performed under
an N2 atmosphere using an inert atmosphere glovebox or Schlenk
techniques. Solvents were reagent or HPLC grade and dried over
the appropriate drying agents. 2,6-Dimethylphenyl isocyanide was
purchased from Fluka. Dppm, n-butyl isocyanide, tert-butyl iso-
cyanide, cyclohexyl isocyanide, and benzyl isocyanide were
purchased from Aldrich. All materials were used as received.
Ni(COD)225,26 and methyl isocyanide27 were prepared by literature
methods. p-IC6H4NC was prepared by the carbene method from
the corresponding amine.28 1H and 31P NMR spectra were obtained
using Varian XL-200 or General Electric QE-300 spectrometers.
31P chemical shifts were referenced with respect to external 85%
H3PO4. All NMR spectra were obtained in CD2Cl2 unless otherwise
noted. Cyclic voltammetry was performed in acetonitrile or THF
containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAH)
using a Princeton Applied Research model 173 potentiostat/
galvanostat and model 175 universal programmer. A gold working
electrode, a platinum wire counter electrode, and a ferrocene/
ferrocenium reference electrode were employed. 31P NMR and solid
(38) Spek, A. L. PLUTON. Molecular Graphics Program; University of
Utrecht: Utrecht, The Netherlands, 1991.
(39) Hall, S. R., duBoulay, D., Eds. Xtal•GX Package; University of
Western Australia: Australia, 1995.
(40) Beurskens, P. T.; Admirall, G.; Beurskens, G.; Bosman, W. P.; Garcia-
Granda, S.; Gould, R. O.; Smits, J. M. M.; Smykalla, C. The DIRDIF92
Program System; Technical Report; Crystallography Laboratory,
University of Nijmegen: Nijmegen, The Netherlands, 1992.
(41) Flack, H. D. Acta Crystallogr. 1983, A39, 876.
(42) Haga, M.; Matsumura-Inoue, T.; Yamabe, S. Inorg. Chem. 1987, 26,
4148-4154 and references therein.
(43) Hush, N. S. Prog. Inorg. Chem. 1967, 8, 391.
1078 Inorganic Chemistry, Vol. 43, No. 3, 2004

Mixed Valency in [Ni2(µ-CNR)(CNR)2(µ-dppm)2]n+
state FT-IR data for the neutral and dication dimers are collected
in Table 1. Electrochemical data for the dimers are collected in
Table 2.
General Procedure for Preparation of [Ni2(µ-L)(L)2(µ-dppm)2]-
[PF6]2. Two equivalents of [FeCp2][PF6] was added to a stirred
CH3CN (ca.30 mL) solution of Ni2(µ-L)(L)2(µ-dppm)2 (ca. 0.5 g).
The dimer was initially suspended in the CH3CN. As the reaction
proceeded over 30 min, it reacted with the [FeCp2][PF6], and the
solution turned green as the soluble dicationic product was formed.
The resulting solution mixture evaporated to dryness, and the FeCp2
was removed by extraction with benzene. The product was dissolved
in CH2Cl2, precipitated with pentane, and filtered to obtain pure
[Ni2(µ-L)(L)2(µ-dppm)2][PF6]2.
L ) CNCH3 (12+). Following a method similar to the preparation
of [Ni2(µ-L)(L)2(µ-dppm)2][PF6]2 and previously reported, 1 (0.51
g, 0.51 mmol) and [FeCp2][PF6] (0.33 g, 1.0 mmol) gave 12+. Anal.
Calcd for C64H69N3Ni2P6F12: C, 53.25; H, 4.82. Found: C, 53.27;
H, 4.84.3 1H NMR (CD3CN): δ 7.2-7.8 (m, 40H, (C6H5)P), 3.4-
3.7 (m, 4 H, PCH2P), 2.7 (m, 9H, CH3).
General Procedure for Preparation of Ni2(µ-L)(L)2(µ-dppm)2.
A method adapted from the preparation developed by Gong was
followed.17 To a stirred THF (ca. 50 mL THF per 1 g Ni(COD)2)
solution containing two equiv of Ni(COD)2 and two equiv of dppm
was added three equiv of the appropriate isocyanide (L), giving an
orange-red solution. The solution was stirred for 1 h, during which
time a red solid began to precipitate. Hexanes (ca. 150 mL) were
added to precipitate an orange solid. The mixture was filtered
through a frit, and the isolated solid was rinsed with hexanes and
then CH3CN to remove an orange-brown impurity. The solid red
product Ni2(µ-L)(L)2(µ-dppm)2 was then dried in vacuo. The
product was recrystallized from THF/hexanes or toluene/hexanes
as needed.
L ) CNCH3 (1). Following a method similar to the preparation
of Ni2(µ-L)(L)2(µ-dppm)2, Ni(COD)2 (2.1 g, 7.6 mmol), dppm (2.9
g, 7.6 mmol), and methyl isocyanide (0.65 mL, 0.45 g, 10.9 mmol)
gave 1. Yield: 3.2 g (84%). 1H NMR (C6D6): δ 7.2-8.2 (m, 40H,
(C6H5)P), 4.0 (m, 4 H, PCH2P), 2.8 (s, 6H, CH3), 1.8 (m, 3H, CH3).
L ) CN(n-C4H9) (22+). Following a method similar to the
preparation of [Ni2(µ-L)(L)2(µ-dppm)2][PF6]2, 2 (0.51 g, 0.45 mmol)
and [FeCp2][PF6] (0.29 g, 0.88 mmol) gave 22+ 1H NMR
.
(CD3CN): δ 7.2-7.8 (m, 40H, (C6H5)P), 3.5-3.7 (m, 4 H, PCH2P),
0.4-1.1, 1.9, 2.1, 3.0 (m, m, s, s, 27H, CH2, CH2).
L ) CN(n-C4H9) (2). Following a method similar to the
preparation of Ni2(µ-L)(L)2(µ-dppm)2, Ni(COD)2 (1.0 g, 3.6 mmol),
dppm (1.5 g, 3.9 mmol), and n-butyl isocyanide (0.57 mL, 0.45 g,
L ) CNCH2C6H5 (32+). Following a method similar to the
preparation of [Ni2(µ-L)(L)2(µ-dppm)2][PF6]2, 3 (0.50 g, 0.40 mmol)
and [FeCp2][PF6] (0.27 g, 0.82 mmol) gave 32+ 1H NMR
.
1
5.5 mmol) gave 2. Yield: 1.2 g (60%). H NMR (C6D6): δ 7.1-
(CD3CN): δ 6.5-7.8 (m, 55H, (C6H5)P, CH2C6H5), 3.3-3.7 (m,
4 H, PCH2P), 4.2 (s, 6H,CH2).
L ) CN(i-C3H7) (42+). Following a method similar to the
8.3 (m, 40H, (C6H5)P), 4.0 (m, 4 H, PCH2P), 3.2 (m, 6H, CH3),
2.3-2.7 (m, 3H, CH3), 0.9-1.9 (m, 18H, CH2). Anal. Calcd for
C65H71N3Ni2P4: C, 68.75; H, 6.30. Found: C, 68.82; H, 6.34.
preparation of [Ni2(µ-L)(L)2(µ-dppm)2][PF6]2, 4 (0.51 g, 0.47 mmol)
L ) CNCH2C6H5 (3). Following a method similar to the
preparation of Ni2(µ-L)(L)2(µ-dppm)2, Ni(COD)2 (1.0 g, 3.6 mmol),
dppm (1.5 g, 3.9 mmol), and benzyl isocyanide (0.69 g, 5.9 mmol)
and [FeCp2][PF6] (0.30 g, 0.91 mmol) gave 42+
.
L ) CNC6H11 (52+). Following a method similar to the
preparation of [Ni2(µ-L)(L)2(µ-dppm)2][PF6]2, 5 (0.51 g, 0.42 mmol)
and [FeCp2][PF6] (0.27 g, 0.82 mmol) gave 52+. Anal. Calcd for
C71H77N3Ni2P6F12: C, 56.72; H, 5.16. Found: C, 56.79; H, 5.29.
1H NMR (CD3CN): δ 7.2-7.8 (m, 40H, (C6H5)P), 3.0-3.7 (2m,
4 H, PCH2P), 0.5-2.4 (m, 33H, CH, CH2).
1
gave 3. Yield: 1.74 g (78%). H NMR (C6D6): δ 7.1-8.3 (m,
55H, (C6H5)P, C6H5CH2), 4.0 (m, 4 H, PCH2P), 1.2-1.8 (m, 6H,
CH2). Anal. Calcd for C74H65N3Ni2P4: C, 71.82; H, 5.29. Found:
C, 71.47; H, 5.29.
L ) CN(t-C4H9) (62+). Following a method similar to the
preparation of [Ni2(µ-L)(L)2(µ-dppm)2][PF6]2, 6 (0.50 g, 0.44 mmol)
L ) CN(i-C3H7) (4). Following a method similar to the
preparation of Ni2(µ-L)(L)2(µ-dppm)2, Ni(COD)2 (1.0 g, 3.6 mmol),
dppm (1.5 g, 3.9 mmol), and cyclohexyl isocyanide (0.50 mL, 0.38
g, 5.5 mmol) gave 4. Yield: 1.77 g (89%).
and [FeCp2][PF6] (0.29 g, 0.88 mmol) gave 62+ 1H NMR
.
(CD3CN): δ 6.9-8.0 (m, 40H, (C6H5)P), 3.4-3.8 (m, 4 H, PCH2P),
0.8 (br s, 27H, CH3).
L ) CN(p-IC6H4) (72+). Following a method similar to the
L ) CNC6H11 (5). Following a method similar to the preparation
of Ni2(µ-L)(L)2(µ-dppm)2, Ni(COD)2 (1.0 g, 3.6 mmol), dppm (1.5
g, 3.9 mmol), and cyclohexyl isocyanide (0.95 mL, 0.83 g, 7.6
mmol) gave 5. Yield: 1.13 g (52%). 1H NMR (C6D6): δ 7.1-8.3
(m, 40H, (C6H5)P), 4.0 (m, 4 H, PCH2P), 1.2-3.7 (m of m, 33H,
CH2, CH3).
preparation of [Ni2(µ-L)(L)2(µ-dppm)2][PF6]2, 7 (0.50 g, 0.32 mmol)
and [FeCp2][PF6] (0.18 g, 0.54 mmol) gave 72+
.
L ) CN(2,6-(CH3)2C6H3) (82+). Following a method similar to
the preparation of [Ni2(µ-L)(L)2(µ-dppm)2][PF6]2, 8 (0.50 g, 0.30
mmol) and [FeCp2][PF6] (0.18 g, 0.54 mmol) gave 82+
.
31P{1H}
L ) CN(t-C4H9) (6). Following a method similar to the
preparation of Ni2(µ-L)(L)2(µ-dppm)2, Ni(COD)2 (1.0 g, 3.6 mmol),
dppm (1.5 g, 3.9 mmol), and tert-butyl isocyanide (0.62 mL, 0.46
(THF): δ 9.49(br, s).
Infrared Spectroelectrochemistry. The design of the reflectance
IR spectroelectrochemical cell used in the nickel dimer study was
reported previously by our laboratory.31 Infrared spectral changes
accompanying thin-layer bulk electrolyses were measured using a
flow-through spectroelectrochemical cell. All spectroelectrochemical
experiments were carried out using 5 mM THF solutions of Ni2(µ-
L)(L)2(µ-dppm)2 with 0.1 M TBAH as the supporting electrolyte.
All solutions were prepared in a drybox and were degassed
completely before injection into the spectroelectrochemical cell.
Blank THF solutions of 0.1 M TBAH were used for the FT-IR
difference spectra. A PAR model 175 universal programmer with
a PAR model 176 current follower were used to effect and monitor
thin layer bulk electrolyses. The IR spectra were acquired using a
Mattson Research series FTIR with an external sampling port and
a MCT (mercury-cadmium-telluride) detector.
1
g, 5.5 mmol) gave 6. Yield: 1.6 g (78%). H NMR (C6D6): δ
7.0-8.5 (m, 40H, (C6H5)P), 4.0 (m, 4 H, PCH2P), 1.0-2.5 (m,
27H, CH2, CH3). Anal. Calcd for C65H71N3Ni2P4: C, 68.75; H, 6.30.
Found: C, 68.72; H, 6.32.
L ) CN(p-IC6H4) (7). Following a method similar to the
preparation of Ni2(µ-L)(L)2(µ-dppm)2, Ni(COD)2 (1.0 g, 3.6 mmol),
dppm (1.5 g, 3.9 mmol), and p-iodophenyl isocyanide (1.4 g, 6.1
mmol) gave 7. Yield: 1.9 g (69%).
L ) CN(2,6-(CH3)2C6H3) (8). Following a method similar to
the preparation of Ni2(µ-L)(L)2(µ-dppm)2, Ni(COD)2 (1.00 g, 3.6
mmol), dppm (1.5 g, 3.9 mmol), and xylyl isocyanide (0.72 g, 5.5
mmol) gave 8. Yield: 1.8 g (78%). 31P{1H} (THF): δ 20.4 (br, s).
Anal. Calcd for C77H71N3Ni2P4: C, 72.27; H, 5.59. Found: C,
72.09; H, 5.65.
Inorganic Chemistry, Vol. 43, No. 3, 2004 1079

Ferrence et al.
Preparation of [Ni2(µ-CNR)(CNR)2(µ-dppm)2][PF6] by Com-
proportionation. The preparation of 1+ is representative for the
preparation of each of the monocation nickel dimers. 1 (0.1 g, 0.10
mmol) was dissolved in ca. 20 mL of a 1:1 mixture of THF and
CH3CN. To this stirred solution was added 12+ (0.13 g, 0.10 mmol).
The solution turned yellow/brown. After stirring for 0.5 h, 1+ was
isolated by evaporation of the reaction solution to dryness. KBr
IR: 1+ (CNCH3), 2150, 1983 cm-1; 2+ (CN(n-C4H9)), 2117, 1995
cm-1; 3+ (CNCH2C6H5), 2117, 1980 cm-1; 5+ (CNC6H11), 2109,
1987 cm-1, 6+ (CN(t-C4H9)), 2105, 1970 cm-1; 7+ (p-IC6H4NC),
2074, 1899 cm-1. Elemental analysis was not obtained for the
monooxidized dimers as it is difficult to obtain this oxidation state
in bulk in the absence of either the dioxidized or neutral dimers.
General Procedures for UV-Vis Spectroscopy. All UV-
visible experiments were performed using a UV-vis-NIR scanning
spectrophotometer (UV-3101PC) and quartz cells with 1 cm path
length sealed with a septum. To avoid dilution during compropor-
tionation reactions, equimolar solutions were used. The following
procedure is standard for all complexes. The [Ni2(µ-L)(L)2(µ-
dppm)2]+[PF6] complex was obtained by comproportionation reac-
tion of the neutral and +2 species. To 3 mL (0.1 mM) of the neutral
complex [Ni2(µ-L)(L)2(µ-dppm)2]0 (L ) n-BuNC, t-BuNC, 2,6-
(CH3)2C6H3NC) was injected 3 mL (in 0.5 mL portions) of the
[Ni2(µ-L)(L)2(µ-dppm)2]2+[PF6]2 (0.1 mM), and the UV-vis spectra
were recorded.
X-ray Crystallography of Ni2(µ-CN(t-C4H9))(CN(t-C4H9))2-
(µ-dppm)2, 6. An orange plate of C65H71Ni2N3P4 having ap-
proximate dimensions of 0.25 × 0.22 × 0.12 mm was mounted on
a glass fiber in a random orientation. See Table 3 for a summary
of crystal data and X-ray collection information. Selected bond
lengths and angles are summarized in Table 4. Preliminary
examination and data collection were performed with Mo KR
radiation (λ ) 0.71073 Å) on a Nonius Kappa CCD. Cell constants
and an orientation matrix for data collection were obtained from
least-squares refinement, using the setting angles of 24791 reflec-
tions in the range 2° < θ < 30°. The monoclinic cell parameters
and calculated volume are a ) 23.657(5) Å, b ) 11.322(2) Å, c )
23.208(5) Å, â ) 109.16(3)°, and V ) 5872(2) Å3. For Z ) 4 and
fw ) 1170.57 the calculated density is 1.324 g/cm3. The space
group was determined by the program ABSEN.32 From the
systematic presences of: h0l l ) 2n and from subsequent least-
squares refinement, the space group was determined to be mono-
clinic C2/c (No. 15).
in calculating R. The final cycle of refinement included 355
variable parameters and converged (largest parameter shift was
0.03 × esd) with unweighted and weighted agreement factors of
2
R1 ) ∑|Fo - Fc|/∑Fo ) 0.0324 and R2 ) (∑w(Fo - Fc2)2/
2 2 1/2
∑w(Fo ) ) ) 0.0870.
The standard deviation of an observation of unit weight was 0.96.
The highest peak in the final difference Fourier had a height of
0.481 e/Å3. The minimum negative peak had a height of -0.291
e/A3. Refinement was performed on an AlphaServer 2100 using
SHELX-97.36 Crystallographic drawings were done using programs
ORTEP,37 PLUTON,38 and/or Xtal•GX.39
X-ray Crystallography of [Ni2(µ-CN(t-C4H9))(CN(t-C4H9))2-
(µ-dppm)2][PF6]2, 62+. The key parameters are summarized in
Table 3, and bond lengths and angles are summarized in Table 4.
The refinement method was a full-matrix least-squares on F2. The
scan speed was 10 to 20°/minute with a 0.6°, Wyckoff range. The
background range was 0.6° for 25% of the time, each side.
X-ray Crystallography of [Ni2(µ-CN(t-C4H9))(CN(t-C4H9))2-
(µ-dppm)2][PF6]‚CH2Cl2, 6+‚CH2Cl2. A dark red chunk of C66H73-
Cl2F6N3Ni2P5 having approximate dimensions of 0.44 × 0.40 ×
0.38 mm was mounted on a glass fiber in a random orientation.
See Table 3 for a summary of crystal data and X-ray collection
information. Selected bond lengths and angles are summarized in
Table 4. Preliminary examination and data collection were per-
formed with Mo KR radiation (λ ) 0.71073 Å) on a Nonius
KappaCCD. Cell constants and an orientation matrix for data
collection were obtained from least-squares refinement, using the
setting angles of 13500 reflections in the range 4° < θ < 27°. The
triclinic cell parameters and calculated volume are a ) 10.6578(7)
Å, b ) 13.3188(9) Å, c ) 13.5122(6) Å, R ) 103.439(3)°, â )
101.316(3)°, γ ) 110.051(3)°, and V ) 1670.6(4) Å3. For Z ) 1
and fw ) 1365.52 the calculated density is 1.36 g/cm3. The space
group was determined by the program ABSEN.32 There were no
systematic absences, and the space group was determined to be P1
(No. 1).
The data were collected at a temperature of 193 ( 1 K and to a
maximum 2θ of 55.0°. A total of 13500 reflections were collected,
of which 8374 were unique. Lorentz and polarization corrections
were applied to the data. The linear absorption coefficient is 8.2
/cm for Mo KR radiation. An empirical absorption correction using
SCALEPACK was applied.33 Transmission coefficients ranged from
0.371 to 0.735 with an average value of 0.628. Intensities of
equivalent reflections were averaged. The agreement factor for the
averaging was 4.2% based on intensity. The structure was solved
using the structure solution program PATTY in DIRDIF92.40 The
remaining atoms were located in succeeding difference Fourier
syntheses. Hydrogen atoms were included in the refinement but
restrained to ride on the atom to which they are bonded. The
structure was refined in full-matrix least-squares where the function
The data were collected at a temperature of 100 ( 0.1 K and to
a maximum 2θ of 55.0°. A total of 17682 reflections were collected,
of which 4219 were unique [Rint ) 0.0571]. Lorentz and polarization
corrections were applied to the data. The linear absorption coef-
ficient is 7.95 mm-1/cm for Mo KR radiation. An empirical
absorption correction using SCALEPACK was applied.33 Trans-
mission coefficients ranged from 0.455 to 0.912 with an average
value of 0.750. Intensities of equivalent reflections were averaged.
The agreement factor for the averaging was 12.9% based on
intensity. The structure was solved by direct methods using SIR97.34
The remaining atoms were located in succeeding difference Fourier
syntheses. Hydrogen atoms were included in the refinement but
restrained to ride on the atom to which they are bonded. The
structure was refined in full-matrix least-squares where the function
2
2 2
minimized was ∑w(|Fo| - |Fc| ) and the weight w is defined as
2
w ) 1/[σ2(Fo ) + (0.0481P)2 + 1.9721P] where P ) (Fo2 + 2Fc2)/
3.
Scattering factors were taken from the International Tables for
Crystallography.35 8374 reflections were used in the refinements.
2
2
However, only reflections with Fo > 2σ(Fo ) were used in
calculating R. The final cycle of refinement included 766 variable
parameters and converged (largest parameter shift was 0.05 times
its esd) with unweighted and weighted agreement factors of R1 )
2
2 2
minimized was ∑w(|Fo| - |Fc| ) and the weight w is defined as
2
w ) 1/[σ2(Fo ) + (0.0844P)2 + 0.0000P] where P ) (Fo2 + 2Fc2)/
2
2 2 1/2
∑|Fo - Fc|/∑Fo ) 0.047 and R2 ) (∑w(Fo - Fc2)2/∑w(Fo ) )
3.
) 0.115.
Scattering factors were taken from the International Tables
for Crystallography.35 7897 reflections were used in the refine-
ments. However, only reflections with Fo > 2σ(Fo ) were used
The standard deviation of an observation of unit weight was 1.03.
The highest peak in the final difference Fourier had a height of
0.81 e/A3. The minimum negative peak had a height of -0.55 e/A3.
2
2
1080 Inorganic Chemistry, Vol. 43, No. 3, 2004

Mixed Valency in [Ni2(µ-CNR)(CNR)2(µ-dppm)2]n+
The factor for the determination of the absolute structure refined
to 0.01.41 Refinement was performed on a AlphaServer 2100 using
SHELX-97.36 Crystallographic drawings were done using programs
ORTEP37 and PLUTON.38
opportunity to conduct her senior individualized project at
Purdue University.
Supporting Information Available: Crystallographic data in
1
CIF format. Variable temperature H and 31P NMR spectra for
Acknowledgment. The authors gratefully acknowledge
the National Science Foundation (CHE-9319173 and CHE-
0315593) for support. G.M.F. wishes to thank the Depart-
ment of Education for a predoctoral fellowship. L.M. wishes
to thank Kalamazoo College for providing her with the
complex 62+ and solution ν(CN) stretching frequencies of 1-3, 5,
6, and 8. This material is available free of charge via the Internet
at http://pubs.acs.org.
IC035021J
Inorganic Chemistry, Vol. 43, No. 3, 2004 1081