Synthesis, spectroscopic characterization, and crystal structure of the bimetallic complex [Ni2(μ-CO)(CO)2(μ-NH(PPh2)2)2]

Article abstract of DOI:10.1021/ic990773s

The dinuclear Ni(0) complex [Ni₂(μ-CO)(CO)₂(μ-dppa)₂] (1; dppa = bis(diphenylphosphino)amine) was synthesized by two routes in good yield. Complex 1 has a triclinic crystal system and P1 space group, with a = 13.009(1) A, b = 13.063(2) A, c = 14.664(2) A, α = 79.91(1)°, β = 79-96(1)°, γ = 71-32(1)°, and Z = 2. The structure of this compound exhibits two μ-coordinated dppa ligands in a cis,cis arrangement. Nickel atoms are at a 2.5824(7) A distance. Theoretical calculations predict a 0.39 bond order between metal atoms. The cyclic voltammograms show two quasi-reversible redox pairs, which correspond to the successive oxidation of the metal centers. The dinuclear complex described absorbs carbon monoxide, yielding a mixture of nickel carbonyl compounds.

Full text of DOI:10.1021/ic990773s

1650
Inorg. Chem. 2000, 39, 1650-1654
Synthesis, Spectroscopic Characterization, and Crystal Structure of the Bimetallic Complex
[Ni2(µ-CO)(CO)2(µ-NH(PPh2)2)2]
Eugenio Simo´n-Manso,*,† Mauricio Valderrama,*,‡ Vero´nica Arancibia, and
Yamil Simo´n-Manso
Departamento de Qu´ımica Inorga´nica, Facultad de Qu´ımica, Pontificia Universidad Cato´lica de Chile,
Casilla 306, Santiago, Chile
Daphne Boys
Departamento de F´ısica, Facultad de Ciencias F´ısicas y Matema´ticas, Universidad de Chile,
Casilla 487-3, Santiago, Chile
ReceiVed June 30, 1999
The dinuclear Ni(0) complex [Ni2(µ-CO)(CO)2(µ-dppa)2] (1; dppa ) bis(diphenylphosphino)amine) was synthesized
by two routes in good yield. Complex 1 has a triclinic crystal system and P1h space group, with a ) 13.009(1) Å,
b ) 13.063(2) Å, c ) 14.664(2) Å, R ) 79.91(1)°, â ) 79.96(1)°, γ ) 71.32(1)°, and Z ) 2. The structure of
this compound exhibits two µ-coordinated dppa ligands in a cis,cis arrangement. Nickel atoms are at a 2.5824(7)
Å distance. Theoretical calculations predict a 0.39 bond order between metal atoms. The cyclic voltammograms
show two quasi-reversible redox pairs, which correspond to the successive oxidation of the metal centers. The
dinuclear complex described absorbs carbon monoxide, yielding a mixture of nickel carbonyl compounds.
Introduction
Scheme 1
There is considerable interest in the synthesis of dinuclear
complexes containing short-bite bidentate ligands as Ph2PXPPh2
[X ) CH2(dppm), NH(dppa); dppm ) bis(diphenylphosphine)-
methane (CH2(PPh2)2); dppa ) bis(diphenylphosphino)amine
(NH(PPh2)2)].1-4 In particular, the use of dppm as a bridging
ligand has been studied extensively because it can form very
strong metal-phosphorus bonds and can lock together the two
metal atoms close to each other, promoting reactions that involve
the two metal centers.1 Dinuclear nickel complexes containing
two bridging dppm ligands show either a trans,trans (A-frame
complexes) or a cis,cis (cradle-type complexes) dppm confor-
mation (Scheme 1).
Few nickel A-frame complexes have been reported showing
similar coordination geometry for the two metal centers as in
[Ni2(µ-SO)Cl2(dppm)2] 5 or different stereochemistries as in
complex [Ni2(µ-CO)Cl2(dppm)2],6 which presents a Ni-Ni
bond. On the other hand, the unusual cis,cis bridging dppm
arrangement is reported for [Ni2(µ-CNMe)(CNMe)2(dppm)2]7
and [Ni2(µ-CO)(CO)2(dppm)2] complexes.8
In the synthesis of related dinuclear complexes the similar
bidentate ligand dppa has received less attention. Recently, the
synthesis of dinuclear gold,9 platinum,10 and palladium11
complexes with a trans,trans dppa arrangement has been
reported. The X-ray structures of complexes [Pd2(µ-dppa)2Cl2]
and [Pd2(µ-dppa)2LL′](BF4)2 (L ) L′ ) PPh3; L ) PPh3, L )
THF) have been described.11 In this paper we report the
synthesis, crystal characterization, electrochemical behavior, and
theoretical calculations of a novel dinuclear nickel(0) complex
[Ni2(µ-CO)(CO)2(µ-dppa)2] that presents a cis,cis bridging dppa
configuration. The relationship between electrochemical results
and theoretical calculations is discussed. Also, the reaction of
dinuclear nickel complex with carbon monoxide is studied.
Experimental Section
Materials and Physical Measurements. All manipulations were
carried out by Schlenk tube techniques under purified nitrogen. Reagent-
grade solvents were dried and distilled under nitrogen atmosphere prior
to use. Ni(CO)4 was purchased from a commercial source, and the
ligand NH(PPh2)2 was synthesized according to a literature procedure.12
* To whom correspondence should be addressed.
† E-mail: esimon@puc.cl. Fax: 56-2-6864744.
‡ E-mail: mvalder@puc.cl. Fax: 56-2-6864744.
(1) Puddephatt, R. J. Chem. Soc. ReV. 1983, 99-127.
(2) Laguna, A.; Laguna, M. J. Organomet. Chem. 1990, 394, 743-756.
(3) Bhattacharyya, P.; Woollins, J. D. Polyhedron 1995, 14, 3367-3388.
(4) Cotton, F. A. Chem. Eng. News 1998, March 30, 43-46.
(5) Gong, J. K.; Fanwick, P. E.; Kubiak, C. P. J. Chem. Soc., Chem.
Commun. 1990, 1190-1191.
(6) Manojlivic-Muir, L.; Muir, K. W.; Davis, W. M.; Mirza, H. A.;
Puddephatt, R. J. Inorg. Chem. 1992, 31, 904-909.
(7) DeLaet, D. L.; Fanwick, P. E.; Kubiak, C. P. Organometallics 1986,
5, 1807-1811.
(8) Osborn, J. A.; Stanley, G. G.; Bird, P. H. J. Am. Chem. Soc. 1988,
110, 2117-2122.
(9) Uso´n, R.; Laguna, A.; Laguna, M.; Fraile, M. N.; Jones, P. G.;
Sheldrick, G. M. J. Chem. Soc., Dalton Trans. 1986, 291-296.
(10) Browning, C. S.; Farrar, D. H. J. Chem. Soc., Dalton Trans. 1995,
521-530.
(11) Browning, C. S.; Farrar, D. H.; Frankel, D. C.; Vittal, J. J. Inorg.
Chim. Acta 1997, 254, 329-338.
(12) Wang, F. T.; Najdzionek, J.; Leneker, K. L.; Wasserman, H.; Braitsh,
D. M. Synth. React. Inorg. Metal-Org. Chem. 1978, 8, 119-125.
10.1021/ic990773s CCC: $19.00 © 2000 American Chemical Society
Published on Web 03/21/2000

[Ni2(µ-CO)(CO)2(µ-NH(PPh2)2)2]
Inorganic Chemistry, Vol. 39, No. 8, 2000 1651
Elemental analysis (C, H, and N) was performed with a Fisons EA
1108 microanalyzer. The FTIR spectrum was recorded on a Bruker
Vector-22 spectrophotometer using KBr pellets. H (200 MHz) and
Table 1. Crystallographic Data and Structure Refinement for
[Ni2(µ-CO)(CO)2(µ-dppa)2]
1
chemical formula C51H42N2Ni2O3P4 fw
972.17
P1h
31P(81 MHz) NMR spectra were recorded on a Bruker AC-200P
spectrometer, and the chemical shifts are reported in ppm relative to
Me4Si and 85% H3PO4 (positive shifts downfield) in D2O as internal
and external standards, respectively. Cyclic voltammetric (CV) mea-
surements were carried out with a potentiostat bank (model Wenking
ST-72) coupled to a voltage scan generator (model USG-72) and a
Graphtec recorder (model WX-1100). Bulk electrolyses were performed
with a voltage integrator bank (model Wenking EVI-80). The working
electrodes used in the CV and coulommetric measurements were a
platinum disk and a platinum mesh, respectively. The auxiliary electrode
was a platinum-coil electrode and the reference electrode a Ag/AgCl
(aqueous tetramethylammonium chloride) cracked glass bead electrode,
adjusted to 0.00 vs SCE. The reference electrode was located inside a
Luggin capillary in the cell assembly. Electronic spectra were recorded
in a Milton Roy Spectronic 3000 array spectrophotometer.
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
13.009(1)
13.063(2)
14.664(2)
79.91(1)
79.96(1)
71.32(1)
2305.4(5)
2
space group
temp (°C)
λ (Å)
25
0.710 73
1.400
1.000
F
calcd (Mg m-3
)
µ (Mo KR; mm-1
)
R(F)a [F > 4σ(F)] 0.0418
Rw(F2)b (all reflns) 0.1080
a R(F) ) ∑||Fo| - |Fc||/∑|Fo|. b Rw(F2) ) [∑[w(Fo - Fc2)2]/
2
∑w(Fo )2]1/2; w-1 ) [σ2(Fo ) + (0.0536P)2], where P ) (Fo2 + 2Fc2)/3.
2
2
A riding model was used for H atoms, placed at calculated positions,
with C-H and N-H distances of 0.96 and 0.90 Å, respectively, with
isotropic displacement parameters equal to 1.2 times the equivalent
isotropic displacement parameters of their parent atoms. Crystal data
and relevant refinement parameters are summarized in Table 1.
Theoretical and Computational Details. All calculations were done
at a semiempirical level using a PM3 Hamiltonian, parametrized for
transition metals, as implemented in the SPARTAN package.13 Full
geometry optimizations were done to obtain bond distances, bond
angles, etc. for the interpretation of experimental results. It was also
possible to calculate frequencies at this level of theory to ensure that
the structure was a true minima, without any imaginary frequency.
The Fukui function, which measures reactivity toward nucleophilic,
electrophilic, or radical agents in preferential sites,14 is defined as
Results and Discussion
The nickel tetracarbonyl complex Ni(CO)4 reacts with the
ligand bis(diphenylphosphino)amine [NH(PPh2)2, dppa] by
refluxing in benzene to give the dinuclear complex [Ni2(CO)3-
(dppa)2] (1) in good yield (73%). The synthesis of the related
complex [Ni2(µ-CO)(CO)2(µ-dppm)2] [dppm ) CH2(PPh2)2] has
been reported in the literature to be unsuccessful via this route.
Therefore, the dppm complex was described to be obtained by
decomposition at room temperature of the tripod compound [Ni-
∂F(br)
∂N
8
f(br) )
(1)
(CO)2]3HC(PPh2)3 or from the reaction of Ni(II) salts, dppm,
(
)
ν
and NaBH3CN.15 The latter method also gave good results for
the dppa ligand. Thus, the reaction of NiCl2‚6H2O with dppa
in methanol solution gave the intermediate red complex [Ni-
(dppa)2]Cl2, which could be reduced with NaBH4 at 0 °C, with
a running stream of carbon monoxide, to give complex 1.
Complex 1 was isolated as a stable microcrystalline yellow
solid; it was soluble in chloroform, benzene, tetrahydrofuran,
and dimethyl sulfoxide and insoluble in polar solvents (metha-
nol, water). These solutions slowly decompose in contact with
air. The solid-state infrared spectrum in a KBr pellet shows one
absorption band at 1789 cm-1 (very close to 1784 cm-1 for the
analogous compound with dppm8) and two absorption bands at
1976 and 1959 cm-1, assigned to bridge and terminal carbonyl
groups, respectively. The 31P NMR spectrum in deuterated
chloroform shows a singlet resonance at δ 72.1 ppm.
where F(r) is the electron density and N is the number of electrons in
the molecule. The derivative is taken at constant external potential.
The finite-difference approximation for evaluating the Fukui function
was used.
Synthesis of [Ni2(µ-CO)(CO)2(µ-dppa)2]. The complex was pre-
pared according to the two methods described below.
(i) To a solution of the ligand NH(PPh2)2 (1.0 g; 2.6 mmol) in 10
mL of benzene was added Ni(CO)4 (443 mg, 2.6 mmol) dissolved in
10 mL of benzene. The solution obtained was boiled under reflux for
5 min. During this time the color of the solution changed from pale-
yellow to orange and a yellow solid precipitated. The complex was
filtered off, washed with pentane, and dried under vacuum. Yield, 923
mg (73%). Anal. Calcd for C51H42N2Ni2O3P4: C, 63.01; H, 4.35; N,
2.88. Found: C, 62.08; H, 4.37; N, 2.93%. IR (KBr): ν(CO) 1976,
1959, and 1789 cm-1 31P NMR (298 K, CDCl3-d1): δ 72.1 (s).
.
The first synthetic procedure gives better yields. It consists
of a clean reaction that forms a single product (complex 1) and
carbon monoxide, as confirmed by 31P NMR spectra of the crude
product of the reaction. The second method is a particularly
convenient route to obtain the cradle compound and avoids the
use of Ni(CO)4. However, there are difficulties in separating
the dinuclear complex from colloidal Ni(0), and in the subse-
quent elimination of other salts by methanol washing procedures.
To confirm the assumed formula and to determine the detailed
geometry, an X-ray structural determination of complex 1 was
undertaken. Figure 1 shows an ORTEP representation of the
complex with the atom-numbering scheme. Selected bond
lengths and angles (calculated and experimental) are collected
in Table 2.
The molecular structure consists of two Ni-CO fragments
linked by two bridging dppa ligands and a bridging carbonyl
ligand. The Ni-Ni distance, 2.5824(7) Å, is consistent with a
Ni-Ni single bond8 (Ni-Ni distance in the crystal lattice of
the metal is 2.487 Å at 18° 16). Numerous measurements of
metal-metal bonds in analogous zerovalent compounds of iron
(ii) To a solution of NiCl2‚6H2O (0.5 g; 2.1 mmol) in methanol (20
mL) the ligand dppa (1.0 g; 2.1 mmol) was added. The resulting red-
purple solution was saturated with carbon monoxide (1 atm) at low
temperature (0 °C). A large excess of sodium tetrahydroborate (6 mmol)
in methanol solution was added over a 5 min period with a constant
slow flow of carbon monoxide. The black solution obtained was left
at 0 °C for 8 h (1 night). The yellow solid formed was washed several
times with methanol to eliminate the black suspension and then
extracted with tetrahydrofuran. The complex was crystallized by
addition of diethyl ether. Yield, 632 mg (50%).
X-ray Data Collection. An orange crystal, obtained from slow
diffusion of n-hexane into a dimethyl sulfoxide solution of complex
[Ni2(µ-CO)(CO)2(µ-dppa)2], having approximate dimensions of 0.36
mm × 0.26 mm × 0.1 mm, was mounted on a glass fiber. Intensity
data were collected on a Siemens R3m/V diffractometer using graphite-
monochromated Mo KR radiation in θ/2θ scan mode. Cell parameters
were determined from a least-squares fit of 32 reflections with 8 e 2θ
e 30. Semiempirical corrections, via ψ-scans, were applied to intensities
for absorption.
(13) SPARTAN DEC, version 5.0 3X 1; Wavefunction Inc.: Irvine, CA,
1997.
(14) Parr, R. G.; Yang, W. Density Functional of Atoms and Molecules;
Oxford Press: New York, 1989.
(15) Holah, D. G.; Hughes, A. N.; Mirza, H. A.; Thompson, J. D. Inorg.
Chim. Acta 1987, 126, L7-L8.

1652 Inorganic Chemistry, Vol. 39, No. 8, 2000
Simo´n-Manso et al.
the mean plane] and are inclined toward each other with a
dihedral angle of 77.63(3)°.
The Ni-P bond lengths [range 2.200(1)-2.215(1) Å] are
slightly longer than those in the similar dppm complexes [Ni2-
(µ-CO)(CO)2(dppm)2] [Ni-P: average of 2.120(2) Å]8 and are
similar to those found in the complex [Ni2(µ-CNMe) (CNMe)2-
(dppm)2]7 [Ni-P: average of 2.205(2) Å]. On the other hand,
Ni-C(1) [1.899(3) and 1.918(3) Å] and Ni-C(2,3) [1.747(4)
and 1.758(4) Å] are in the range of other similar carbonyl
complexes.8,18 In the bridging dppa ligands, the values of the
P-N distances [average of 1.699(3) Å] and P-N-P angles
[118.2(1) and 119.4(1)°] compare favorably with those found
in the binuclear palladium(II) complex [Pd2Cl2(µ-dppa)2] [P-N,
average of 1.683(6) Å; P-N-P, 116.3(4) and 117.1(3)°].11
Theoretical Calculation. Geometry optimization to the PM3
level gives results very close to the experimental X-ray
geometry. Table 2 shows selected distances and angles for
comparison of the experimental and calculated values. The
coincidence between experimental and theoretical geometrical
parameters is surprising for such a complex structure. It is
interesting to note a slight asymmetric structure surrounding
the nickel centers, as observed in the experimental and calculated
results. The theoretical calculations predict a certain degree of
bonding between metal centers, the bond order being ap-
proximately 0.39. A reduction of the carbon monoxide C-O
bond, with respect to the free ligand, was observed. The µ-bridge
carbon monoxide has a bond order of 1.87, while the CO
terminals have 2.14. The electrostatic charges give a negative
net charge on the nickel centers of -1.26 and -1.31 e,
respectively, while phosphorus atoms have positive charges of
1.1 e.
The HOMO frontier molecular orbital reveals a strong
contribution of the metal d orbitals bonding to the µ-bridge
carbon monoxide and the phosphorus atoms simultaneously.
Thus, the d electrons are in dπ delocalized orbitals between
both metals, across the conjugated bridge ligand. This orbital
is bonding with respect to Ni-C (µ-CO) bond, but it is Ni-Ni
antibonding, having a nodal plane passing through the µ-CO
bridge. The plane is perpendicular to the Ni-Ni axis and bisects
the molecule in two symmetrical fragments. The mentioned
properties of the HOMO orbital predict a moderate interaction
between metal centers.
Figure 2 shows the Fukui function, f(br), on the electron
density surface (0.002 e/au3) for electrophilic attack. The
molecule was oriented in the same way as in Figure 1, where
carbon monoxide ligands are pointing out of the paper. The
Fukui function shows two symmetrical maxima between Ni-
CO (µ-bridge) bonds (blue zones). Electrophilic agents, such
as free carbon monoxide, prefer to attack these regions. It might
lead to the formation of a new metal-ligand bond, promoting
the reversible absorption of carbon monoxide to form compound
2, as represented in Scheme 2. It is interesting to note two less
intense maxima on both sides of the oxygen of the µ-bridge
carbon monoxide. Isocyanide analogues show reactivity in the
nitrogen atom occupying the same position as this oxygen.7
Cyclic Voltammetry. Figure 3 shows the cyclic voltammo-
grams of complex 1 in THF solution. The positive scans show
two quasi-reversible peaks at 0.24 and 0.51 V vs SCE (peaks a
and b). Small reductions at 0.31, 0.04, -0.26, and -0.57 V vs
SCE, respectively peaks c, d, e, and f, correspond to products
that exist on the surface of the electrode and that were generated
upon oxidation. The scan toward negative potentials shows two
Figure 1. ORTEP plot of [Ni2(µ-CO)(CO)2(µ-dppa)2] complex with
50% thermal ellipsoids (H atoms omitted for clarity).
Table 2. Selected Experimental and Calculated Interatomic
Distances (Å) and Angles (deg) for Complex
[Ni2(µ-CO)(CO)2(µ-dppa)2]
Distance (Å)
exptl
calcd
Ni(1)-Ni(2)
Ni(1)-P(1)
Ni(1)-P(2)
Ni(1)-C(1)
Ni(1)-C(2)
Ni(2)-P(3)
Ni(2)-P(4)
Ni(2)-C(1)
Ni(2)-C(3)
P(2)-N(2)
P(1)-N(1)
P(3)-N(1)
P(4)-N(2)
C(1)-O(1)
C(2)-O(2)
C(3)-O(3)
2.5824(7)
2.2103(9)
2.1998(9)
1.918(3)
1.747(4)
2.2097(9)
2.2147(9)
1.899(3)
1.758(4)
1.697(2)
1.692(3)
1.695(2)
1.701(3)
1.168(4)
1.144(4)
1.148(4)
2.5975
2.2766
2.2781
1.9308
1.7756
2.2795
2.2776
1.9307
1.7760
1.8027
1.8059
1.8021
1.8059
1.1946
1.1623
1.1623
Angle (deg)
exptl
calcd
P(2)-Ni(1)-P(1)
P(3)-Ni(2)-P(4)
C(2)-Ni(1)-C(1)
C(3)-Ni(2)-C(1)
Ni(2)-C(1)-Ni(1)
P(1)-N(1)-P(3)
P(2)-N(2)-P(4)
Ni(1)-Ni(2)-C(3)
Ni(2)-Ni(1)-C(2)
109.20(3)
109.64(3)
99.7(2)
99.8(2)
85.1(1)
118.1(1)
119.4(1)
147.2(1)
146.4(2)
112.25
112.27
98.90
99.05
85.54
114.91
114.90
146.49
146.34
and cobalt indicate that the metal-metal distances found in these
compounds lie within the range of distances expected for Ni-
(0)-Ni(0) bonds.17 This result was also confirmed by theoretical
calculations, as described below. The structure displays unusual
cis,cis bridged diphosphine ligands with the nickel centers
showing a distorted tetrahedral coordination [average angles:
Ni(1), 109.07° and Ni(2), 109.14°, ignoring the Ni-Ni bond].
The five-member rings Ni2P2N are nonplanar [deviations: Ni-
(1) 0.158(0), Ni(2) -0.116(0), P(2) -0.194(1), P(4) 0.047(1),
N(2) 0.105(1) Å and Ni(1) 0.105(0), Ni(2) -0.172(0), P(1)
-0.007(1), P(3) 0.243(1), N(1) -0.170(1) Å with respect to
(16) Pearson, B. W. Lattice Spacing and Structures of Metals and Alloys;
Pergamon Press: London, 1958.
(17) Wells, A. F. Structural Inorganic Chemistry, 5th ed.; Oxford University
Press: Oxford, U.K., 1986.
(18) Einspahr, H.; Donohue, J. Inorg. Chem. 1974, 13, 1839-1843.

[Ni2(µ-CO)(CO)2(µ-NH(PPh2)2)2]
Inorganic Chemistry, Vol. 39, No. 8, 2000 1653
changes with scan rate, but it remains less than unity even at
very high scan values (0.50 V s-1). These results suggest that
the oxidation of the metal centers is coupled with a chemical
reaction.20 Cyclic voltammograms in DMSO solution show
similar peak patterns and potentials; however, the ipc/ipa relation-
ship is smaller than in THF, suggesting that in this solvent the
oxidation product is more unstable. The oxidation peaks a and
b would correspond to the consecutive oxidation of each metallic
center according to the reaction
[Ni2(CO)3(µ-dppa)2]0 {-e, 0.24 V
}
+e, 0.04 V
[Ni2(CO)3(µ-dppa)2]1+
{
-e, 0.51 V} [Ni2(CO)3(µ-dppa)2]2+
+e, 0.31 V
The peak separation (∆E ) 0.27 V) can be explained if it is
assumed that the oxidation of the first metal center produces a
mixed valence compound that, according to the classification
of Robin and Day,21 would correspond to a type II compound.
The most obvious manifestation of interaction between two
metal centers is the separation of the two metal-centered redox
potentials. Oxidation of one metal center results in a change of
electron density, which is communicated to the other across the
bridging; the second metal ion is affected by the additional
positive charge and is therefore more difficult to oxidize than
the first one.19 The cyclic voltammetry experimental evidence
of interaction between the metal centers relates well to the
theoretical calculations, where a metal-metal bond order of 0.39
was calculated and where the HOMO orbital shows partial
delocalization over the metal centers and bridging ligand. The
oxidation process affects the electron density on this orbital,
whereas the presence of a nodal plane diminishes the direct
communication between both metals. Clearly, a simple one-
electron orbital picture will not account for the complexity of
this process, but it may support the idea that the mixed valence
compound, obtained by one-electron oxidation of 1, corresponds
to a type II classification, a case between complete delocalization
and nondelocalization.
Figure 2. Fukui function plot for electrophilic attack on [Ni2(µ-CO)-
(CO)2(µ-dppa)2]. The blue color represents a maximum value for the
function. Carbon monoxide ligands are pointing out of paper as in
Figure 1.
Scheme 2
The electron stoichiometry of the first oxidation process was
determined by controlled potential electrolysis. The electro-
chemical cell was connected to a flow quartz cell to register
the UV-vis spectra while the mixed-valence compound was
formed. Thus, at the beginning of the oxidation an intense band
appeared at 544 nm (ꢀ ) 50 000 M-1 cm-1), characteristic of
a Ni-Ni charge transfer.22 The magnitude of the interaction is
often expressed in terms of a coupling parameter J in units of
cm-1, which may be expressed as19
2.05 × 10-2[ꢀν1/2E]1/2
J )
(2)
r
where ꢀ is the extinction coefficient for the intervalence charge
transfer (IVCT) band in dm3 mol-1 cm-1, ν1/2 is the width at
half-height of the IVCT band in cm-1, E is the energy of the
IVCT band maximum in cm-1, and r is the metal-metal
separation in Å. If the metal-metal distance does not change
during the first oxidation, the coupling parameter value is J )
69 432 cm-1. On the other hand the ∆E value (0.270 V)
corresponds to a comproportionation constant of Kc ) 3.7 ×
104 (quasi-reversible process).
Figure 3. Cyclic voltammograms of a 3.6 mM solution of [Ni2(µ-
CO)(CO)2(µ-dppa)2] complex in THF. Scan rate is (- - -) 0.5 and (-)
0.20 V s-1. TBATFB (10-3 M) is the supporting electrolyte.
irreversible reactions at -1.04 and -1.56 V vs SCE (peaks g
and h), which correspond to reductions of the ligand with
decomposition of the complex.
In the voltammograms an accurate measurement of the
cathodic and anodic currents is difficult. Despite this, it can be
seen that the ipc/ipa relationship is less than unity. This relation
(19) Ward, M. D. Chem. Soc. ReV. 1995, 24, 121-134.
(20) Bard, A. J.; Faulkner, L. J. Electrochemical Methods; Wiley: New
York, 1980; Chapter 11.
(21) Robin, M. B.; Day, P. AdV. Inorg. Chem. Radiochem. 1967, 10, 247.
(22) Lever, A. B. P. Inorganic Electronic Spectroscopy, 2nd ed.; Elsevier:
New York, 1984; Series 33.

1654 Inorganic Chemistry, Vol. 39, No. 8, 2000
Simo´n-Manso et al.
superposition of the three compounds, with a weak band at 1789
cm-1 corresponding to compound 1. Attempts to wash out the
dinuclear complex from the solid with THF gave the same IR
pattern. This result demonstrates that even in the solid state 2
and 3 lose CO to give 1.
The ready and reversible uptake of CO by 1 adds another
example to the small list of dinuclear complexes and clusters
systems that are known to exhibit such behavior.8,23,24
Conclusions
The bis(diphenylphosphine)amine (dppa) ligand reacts with
Ni(CO)4 or NiCl26H2O to give the dinuclear complex [Ni2(µ-
CO)(CO)2(µ-dppa)2] (1). The X-ray crystal structure of 1
confirms a cradle-type structure similar to that of bis(diphen-
ylphosphine)methane (dppm) derivatives.
The presence of interaction between the two metal centers is
demonstrated by electrochemical and UV-vis spectrochemical
results. The difference between the two consecutive potentials
has a value of 0.27 V. The one-electron oxidation leads to a
possible formation of a mixed valence complex with a coupling
parameter of J ) 69 432 cm-1. These results are in good
agreement with theoretical calculations, which predict a metal-
metal bond order of 0.39, and the HOMO is bonding with
respect to Ni-C (µ-CO) but is Ni-Ni antibonding.
Complex 1 reversibly reacts with carbon monoxide to give a
mixture of nickel carbonyl compounds as established by 31P
NMR and IR spectroscopy. From a theoretical point of view,
these results can be explained by an electrophilic attack on a
Ni-µ(CO) bond, as was predicted by Fukui function calcula-
tions.
Figure 4. Variable temperature 31P NMR spectra for a solution of
[Ni2(µ-CO)(CO)2(µ-dppa)2] in chloroform saturated with carbon mon-
oxide.
After arriving at a maximum, the absorption decreases and
the reduction peaks at 0.31 and 0.04 V are not observed,
indicating that the oxidation is coupled to a chemical reaction.
Complex 1 readily reacts with carbon monoxide to yield a
mixture of nickel carbonyl compounds at equilibrium (com-
plexes 1-3 of Scheme 2). This reaction is reversible, and the
displacement of the equilibrium was studied by monitoring the
reaction by 31P NMR at variable temperature. Thus, a CDCl3
solution of a well-crystallized sample of compound 1 was placed
into a NMR tube and saturated with carbon monoxide (25 psi
at -30 °C).
Acknowledgment. This work was supported by Projects
Fondecyt 2980031 and Fondecyt (Lineas Complementarias) No.
8980007. E.S.M acknowledges DIPUC (Pontificia Universidad
Cato´lica de Chile) for a doctoral fellowship. We also thank
Fernando Zuloaga and Barbara Loeb for helping with the
theoretical calculations and revision of the manuscript.
Figure 4 shows the spectra recorded at different temperatures.
The 26 °C spectrum shows a weak resonance at δ 72.1 ppm
corresponding to the starting complex 1, a singlet at 67.2 (2),
and two doublets at 75.7 and 34.9 (3) [2J(PP) ) 150 Hz].
Warming the solution to 55 °C causes the decrease of the
doublet signals and the singlet (δ 67 ppm) signal while the signal
assigned to complex 1 increases in intensity. Cooling the
solution to the starting temperature causes the equilibrium to
shift back to species 2 and 3, like the first recorded at this
temperature. This equilibrium is reversible, and we can cycle
the temperatures to regenerate the 31P spectra shown. These
temperature effects partially reflect the decreasing solubility of
CO with the increase of temperature and, hence, displacement
of the equilibrium 3 f 2 f 1. These results are in good
agreement with other equilibria reported in the literature.8
Attempts to isolate the mononuclear complex Ni(CO)3(η1-
dppa) from a CO-saturated solution at low temperature (-30
°C) yield a white solid. The IR spectrum appeared as a
Supporting Information Available: Tables of crystal data (atomic
coordinates and equivalent isotropic displacement parameters, bond
lengths and angles, anisotropic displacement parameters, hydrogen
coordinates, observed and calculated structure factors) and data from
theoretical calculation (bond order matrix, electrostatic charges, dis-
tances and angles, etc.). Data in CIF format available. This material is
available free of charge via the Internet at http://pubs.acs.org.
IC990773S
(23) McLennan, A. J.; Puddephatt, R. J. Organometallics 1986, 5, 811-
814.
(24) Lloyd, B. R.; Bradford, A.; Puddephatt, R. J. Organometallics 1987,
6, 424-427.