Synthesis, structure, and electrochemistry of di- and zerovalent nickel, palladium, and platinum monomers and dimers derived from an enantiopure (S,S)-tetra(tertiary phosphine)

Article abstract of DOI:10.1021/ic700912q

The ligand (S,S)-1,1,4,7,10,10-hexaphenyl-1,4,7,10-tetraphosphadecane, (S,S)-tetraphos, reacts with hexa(aqua)-nickel(II) chloride in the presence of trimethylsilyl triflate (TMSOTf) in dichloromethane to give the yellow square-planar complex [Ni{(R,R)-te

Full text of DOI:10.1021/ic700912q

Inorg. Chem. 2007, 46, 8059−8070
Synthesis, Structure, and Electrochemistry of Di- and Zerovalent Nickel,
Palladium, and Platinum Monomers and Dimers Derived from an
Enantiopure (S,S)-Tetra(tertiary Phosphine)
Heather J. Kitto, A. David Rae, Richard D. Webster,‡ Anthony C. Willis, and S. Bruce Wild*
Research School of Chemistry, Australian National UniVersity, Canberra, Australian Capital
Territory 0200, Australia
Received May 11, 2007
The ligand (S,S)-1,1,4,7,10,10-hexaphenyl-1,4,7,10-tetraphosphadecane, (S,S)-tetraphos, reacts with hexa(aqua)-
nickel(II) chloride in the presence of trimethylsilyl triflate (TMSOTf) in dichloromethane to give the yellow square-
planar complex [Ni
acetonitrile adduct [Ni(NCMe)
romethane and acetonitrile at 20
{
(R,R)-tetraphos
}](OTf)2, which has been crystallographically characterized as the square-pyramidal,
{(R,R)-tetraphos
}
]OTf. Cyclic voltammograms of the nickel(II) complex in dichlo-
ox
°
C showed two reduction processes at negative potentials with oxidative (Ep
)
and reductive (Epred) peak separations similar to those observed for ferrocene/ferrocenium under identical conditions,
suggesting two one-electron steps. The cyclic voltammetric data for the divalent nickel complex in acetonitrile at
temperatures below
nickel(0) complexes. The divalent palladium and platinum complexes [M
tetraphos 2](OTf)4 have been prepared. The reduction potentials for the complexes [M
increase in the order nickel(II) < palladium(II) < platinum(II). The reaction of (S,S)-tetraphos with bis(cycloocta-
1,5-diene)nickel(0) in benzene affords orange [Ni (R,R)-tetraphos ], which slowly rearranges into the thermodynami-
cally more stable, yellow, double-stranded helicate [Ni2 (R,R)-tetraphos 2]; the crystal structures of both complexes
have been determined. The reactions of (S,S)-tetraphos with [M(PPh3)4] in toluene (M Pd) or benzene (M Pt)
furnish the double-stranded helicates [M2 (R,R)-tetraphos 2]; the palladium complex crystallizes from hot benzene
−20
°
C were interpreted according to reversible coordination of acetonitrile to the nickel(I) and
(R,R)-tetraphos ](PF6)2 and [M2 (R,R)-
(R,R)-tetraphos ](PF6)2
{
}
{
{
}
}
{
}
{
}
)
)
{
}
as the 2-benzene solvate and was structurally characterized by X-ray crystallography. In each of the three zerovalent
complexes, the coordinated (R,R)-tetraphos stereospecifically generates tetrahedral M(PP)2 stereocenters of M
configuration.
Introduction
double-stranded helicates, metallocatenanes, rotaxanes, mo-
lecular grids, and knots has been achieved.1 Our work in
this area has concerned the use of configurationally pure
tetra- and hexa-tertiary phosphines as agents for the self-
assembly of double-stranded di- and trinuclear metal heli-
cates. Thus, the C2-tetra(tertiary phosphine) (S,S)-tetraphos2
and the homologous hexa(tertiary phosphine) (R,S,S,R)-
hexaphos,3 in the presence of hexafluorophosphate, combine
with copper(I), silver(I), and gold(I) ions to spontaneously
Aggregation in coordination complexes is facilitated by
dynamic dative bonding between the donor atoms of the
ligands and the metal ions. Depending upon the bonding
modes and geometric constraints of the ligands, in particular
rigidity, and the preferred coordination geometries of the
metal ions, frequently tetrahedral, a variety of interesting
structural motifs can be generated under equilibrium condi-
tions by the judicious choice of metal and ligand. Thus, with
use of semirigid di- and oligo-2,2′-bipyridines and related
ligands in conjunction with kinetically labile metal ions
favoring tetrahedral coordination, the self-assembly of
(1) (a) Piguet, C.; Bernardinelli, G.; Hopfgartner, G. Chem. ReV. 1997,
97, 2005-2062. (b) Swiegers, G. F.; Malefetse, T. J. Chem. ReV. 2000,
100, 3483-3537. (c) Machado, V. G.; Baxter, P. N.; Lehn, J. M. J.
Braz. Chem. Soc. 2001, 12, 431-462.
(2) Airey, A. L.; Sweigers, G. F.; Willis, A. C.; Wild, S. B. Inorg. Chem.
1997, 36, 1588-1597.
(3) Bowyer, P. K.; Cook, V. C.; Gharib-Naseri, N.; Gugger, P. A., Rae,
A. D.; Swiegers, G. F.; Willis, A. C.; Zank, J.; Wild, S. B. Proc.
Natl. Acad. Sci. U.S.A. 2002, 99, 4877-4882.
* To whom correspondence should be addressed. E-mail: sbw@
rsc.anu.edu.au.
‡ Present address: Division of Chemistry and Biological Chemistry,
Nanyang Technological University, Singapore 637616
10.1021/ic700912q CCC: $37.00
Published on Web 08/15/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 19, 2007 8059

Kitto et al.
self-assemble double-stranded helicates of the type [M2-
{(R,R)-tetraphos}2](PF6)2 (M ) Ag, Au) and [Cu3{(S,R,R,S)-
hexaphos}2](PF6)3, in which the configurations of the internal
phosphorus stereocenters4 diastereoselectively generate he-
licates of M or P configuration.6 Moreover, dependent upon
the relationship between the configurations of the internal
donor stereocenters and the twists of the chiral ten-membered
rings containing the two metal ions, δ or λ, the hexafluo-
rophosphate salts of the helicates crystallize as DNA-like
double helices or protein-like parallel helices. It was also
found that the meso ligand (R*,S*)-tetraphos combines with
silver(I) and gold(I) ions in the presence of hexafluorophos-
phate to diastereoselectively self-assemble helicates in which
there is head-to-head recognition of the pairs of R and S
phosphorus stereocenters in the helicate ions.8
Apart from the isolation and structural characterization of
(M)-[Pt2{(R,R)-tetraphos}2](CF3SO3)4‚4.5H2O,9 work on the
Group 10 metals, in which one strand of tetraphos is fully
coordinated to one metal or two strands to two metals, has
hitherto involved the use of the racemic ligand, namely,
(R*,R*)-(()-tetraphos, or the meso diastereomer (R*,S*)-
tetraphos.10 Of particular relevance to this work, is the
isolation and solution molecular weight determination of a
complex having the formulation [Pt2(tetraphos)2], although
the diastereomer of the ligand used to prepare the complex
was not specified.11 Here we report our results concerning
the synthesis, structural characterization, and electrochemistry
of single- and double-stranded complexes of di- and zerova-
lent Group 10 metals derived from homochiral (S,S)-
tetraphos.
inspection of the crossed-bar diagrams shown in Figure 1a.
In the figure, it can be seen that when the metal has the M
configuration, the central 1,2-ethano group can effectively
bridge the two inner phosphorus stereocenters of R config-
uration.4 When the metal stereocenter has the P configuration,
however, the estimated distance apart of the two methylene
carbon atoms that would constitute the bridge is 6.41 Å.
Accordingly, (S,S)-tetraphos combines with tetra(acetonitri-
le)copper(I) hexafluorophosphate to give (MCu)-[Cu{R,R)-
tetraphos]PF6, which has been identified by an X-ray crystal
structure determination on the 1-ethanol solVate.2 The angle
subtended by the two terminal phosphorus atoms at copper
in the structure is 135.91(6)°. The strain implicit in this large
angle is alleviated when the (S,S)-tetraphos combines with
the larger silver(I) and gold(I) ions, where the double-
stranded helicates (MM,MM)-[M2{(R,R)-tetraphos}2](PF6)2 are
formed. The ligand (S,S)-diphars, an analogue of (S,S)-
tetraphos, in which the terminal phosphorus atoms have been
replaced by arsenic atoms, reacts with tetra(acetonitrile)-
copper(I) hexafluorophosphate to give the double-stranded
helicate (MCu,MCu)-[Cu2{(R,R)-diphars}2](PF6)2 because of
the further destabilization of the monocopper complex
brought about by the relatively long copper-arsenic bonds,
namely, Cu-As ) 2.43 Å versus Cu-P ) 2.28 Å.12
The helicates (MM,MM)-[M2{(R,R)-tetraphos}2](PF6)2 (M
) Ag, Au) and the closely related compounds (MM,MM)-
[M2{(R,R)-diphars}2](PF6)2 (M ) Cu, Ag, Au) crystallize
with idealized D2 double helix or parallel structures, in which
the ten-membered rings containing the two metal ions in each
case have the chiral twist-boat-chair-boat (TBCB) conforma-
tion,13 as represented by the diagrams shown in Figure 1b.
In the parallel helix conformer of the helicate, the TBCB
ring has a δ (clockwise) twist of 60° that is opposed to the
two anticlockwise twists of ∼60° of the two M(PP)2
stereocenters of M configuration, which leads to a net overall
helicity of ∼60° for the helicate in the M direction. Each
ligand strand in the parallel helicate completes ∼1.5 turns
of a left-handed helix down each side of the M‚‚‚M axis. In
the double-helix structure, the λ (counterclockwise) twist of
∼60° of the TBCB ring augments the two twists of similar
magnitude and direction for the two M(PP)2 stereocenters
of M configuration to give an overall twist of ∼180° of each
ligand strand in the M direction for the helicate. In the
dinuclear metal helicates so far investigated, the more
voluminous double-helix conformer of the molecules has
been observed only in silver(I) complexes of (S,S)-tetraphos
Results and Discussion
Stereochemical Considerations. The stereospecific co-
ordination of (S,S)-tetraphos to give a tetrahedral metal
stereocenter of M configuration can be rationalized by
(4) As a consequence of the CIP rules for specifying absolute configura-
tions,5 an apparent inversion of configuration occurs upon coordination
of a P-chiral tertiary phosphine to an atom of atomic number greater
than 12.
(5) Cahn, R. S.; Ingold, C. K.; Prelog, V. Angew. Chem., Int. Ed. Engl.
1966, 5, 385-415.
(6) Spirocyclic complexes of the type [M(AB)2] may be viewed as helices,
and their configuration may be denoted as P (right-handed or clockwise
turns of the chelate rings when viewed down the principal C2 axis,
from either end) or M (left-handed), or they can be considered to have
a chiral center, in which case they may be assigned an R (or aR) or S
(aS) descriptor, respectively. The correspondence of R with M and S
with P is general.7 The P,M nomenclature is more appropriate for di-
and oligonuclear metal helicates of the type considered here because
the bulk helicity of a particular helicate, also designated P or M, is
then a direct manifestation of the individual helicities of the chiral
components along the principal axis of the helicate.
(7) Eliel, E. L.; Wilen, S. H.; Mander, L. N. Stereochemistry of Organic
Compounds; John Wiley & Sons: New York, 1994.
(8) Blake, C. J.; Cook, V. C.; Keniry, M. A.; Kitto, H. J.; Rae, A. D.;
Swiegers, G. F.; Willis, A. C.; Zank, J.; Wild, S. B. Inorg. Chem.
2003, 42, 8709-8715.
(9) Kitto, H. J.; Rae, A. D.; Willis, A. C.; Zank, J.; Wild, S. B. Z.
Naturforsch. B 2004, 59, 1458-1461.
(10) The use of the relative configuration descriptors (R*,R*)-(() and
(R*,S*) for the racemic and meso diastereomers of the tetraphosphine,
respectively, conforms with current Chemical Abstracts indexing
practice and IUPAC recommendations. The enantiomers of the
tetraphosphine have been given simplified descriptors.
(12) Cook, V. C.; Willis, A. C.; Zank, J.; Wild, S. B. Inorg. Chem. 2002,
41, 1897-1906.
(13) Hendrickson, J. B. J. Am. Chem. Soc. 1967, 89, 7047-7061.
(11) King, R. B.; Kapoor, P. N. Inorg. Chem. 1972, 11, 1524-1527.
8060 Inorganic Chemistry, Vol. 46, No. 19, 2007

Di- and ZeroWalent Ni, Pd, and Pt Monomers and Dimers
Scheme 1
solution afforded the chloronickel(II)-tetraphos complex as
a deep red-brown powder. In the 31P{1H} NMR spectrum in
chloroform-d, the complex exhibits multiplets at 45.4 (PPh2)
18
546
and 103.5 ppm (PPh) and has [R]
) +16.1 (c 1,
acetone). The addition of trimethylsilyl triflate to a solution
of the chloro complex in dichloromethane at 0 °C resulted
in an immediate color change from deep red to yellow. The
removal of the solvent from the yellow solution furnished
[Ni{(R,R)-tetraphos}](OTf)2, which remained as a yellow
solid. Redissolution of the solid in acetonitrile afforded a
deep red solution of the acetonitrile adduct [Ni(NCMe)-
{(R,R)-tetraphos}](OTf)2 that was isolated by precipitation
with diethyl ether; the dark orange-red crystals of the
acetonitrile adduct lost acetonitrile upon evacuation and
reverted to the original yellow, square-planar complex (mp
231-234 °C).
The neutral complex [Ni{(R,R)-tetraphos}] was prepared
by the reaction of [Ni(COD)2] (COD ) cyclo-1,5-octadiene)
with (S,S)-tetraphos in benzene (Scheme 1b). After 1 h, the
orange solution was evaporated to dryness, and the orange
oil that remained was dissolved in diethyl ether; the solution
deposited an orange microcrystalline precipitate, which was
filtered off and washed with diethyl ether to give the pure
complex, having mp ) 98-101 °C and [R]1D8 ) -147.9 (c
1, benzene). Orange solutions of the mononuclear nickel
complex in diethyl ether, benzene, or toluene slowly changed
(over several months) into yellow solutions of [Ni2{(R,R)-
tetraphos}2]. Orange plates of monomeric complex and
yellow plates of the dimeric complex slowly crystallized
above a diethyl ether solution of the monomeric complex as
the solvent evaporated, both of which were shown by X-ray
crystallography to contain 0.5 diethyl ether molecule of
crystallization (see below).
Figure 1. Chiral crossed-bar representations of the stereospecific binding
of (S,S)-tetraphos to one tetrahedral metal stereocenter of M configuration
(left) or P configuration (right) (a), two tetrahedral metal stereocenters of
M configuration (b), and (R,S,S,R)-hexaphos to give three tetrahedral metal
stereocenters of P configuration (c).4
and (S,S)-diphars, where it is found alongside the more
compact parallel helix conformer in the unit cell of each
lattice. The parallel helix is the more usual structure for
univalent copper, silver, and gold complexes of the type
(MM,MM)-[M2{(S,S)-tetraphos}2](PF6)2 and (MM,MM)-[M2-
{(R,R)-diphars}2](PF6)2. The close relationship between the
direction of the twist of the ten-membered TBCB ring
containing the pairs of metal ions and the configurations of
the chiral M(PP)2 and M(PAs)2 stereocenters in the helicates
is preserved in the structure of (PCu,PCu,PCu)-[Cu3{S,R,R,S)-
hexaphos}3](PF6)3‚4C6H6, where the parallel helix conformer
of the helicate is found in the crystal lattice, as represented
by the diagram in Figure 1c.3 In this structure, the two
annulated ten-membered TBCB rings containing the three
copper ions each have the left-handed (λ) twist; these
anticlockwise twists of the TBCB rings are negated by the
clockwise (P) twists of the three Cu(PP)2 stereocenters, which
results in the parallel helix conformer of the helicate, where
each ligand strand completes ∼2.5 turns of a P helix in a
cation of idealized D2 symmetry.
Synthesis of Metal Complexes. (a) Nickel. The enan-
tiopure complex [NiCl{(R,R)-tetraphos}]Cl was prepared by
the addition of a solution of [Ni(H2O)6]Cl2 in water to a
solution of (S,S)-tetraphos in dichloromethane (Scheme 1a).
The addition of ethanol to the vigorously stirred mixture led
to a deep red solution. Removal of the solvents from the
(b) Palladium. The complex [Pd{(R,R)-tetraphos}](PF6)2
was prepared by the addition of [PdCl2(SEt2)2] in acetonitrile
to a suspension of (S,S)-tetraphos in the same solvent
(Scheme 2a). A bright yellow solution resulted, which was
Inorganic Chemistry, Vol. 46, No. 19, 2007 8061

Kitto et al.
Scheme 2
to deep yellow and complete conversion into the mono-
nuclear palladium complex in 8 days. The addition of an
excess of a solution of either halide to an acetone-d6 solution
of the dipalladium complex resulted in immediate and
complete conversion into the monopalladium complex.
The complex [Pd2{(R,R)-tetraphos}2] resulted from the
reaction between [Pd(PPh3)4] and (S,S)-tetraphos in toluene
(Scheme 2d). After ∼0.5 h, the bright yellow solution of
the complex was filtered through Celite to remove a trace
of palladium. Removal of the solvent from the filtrate left a
yellow oil, which was dissolved in diethyl ether; yellow
needles of the product deposited overnight from the solution.
The needles were filtered off, washed with diethyl ether, and
dried under vacuum to afford the dipalladium(0) complex
in 85% yield, which had mp ) 125-128 °C (dec) and
[R]1D8 ) +40.2 (c 1, benzene).
(c) Platinum. The complex [Pt{(R,R)-tetraphos}](PF6)2
was prepared by the addition of [PtCl2(COD)] in acetonitrile
to a suspension of (S,S)-tetraphos in the same solvent
(Scheme 2a). The bright yellow solution that resulted was
treated with aqueous ammonium hexafluorophosphate. After
removal of the solvents, the residue was stirred in water to
remove excess ammonium hexafluorophosphate and the
ammonium chloride byproduct. The pale yellow powder that
remained was recrystallized from acetonitrile-diethyl ether
to give the almost colorless product (93%), which had mp
) 270-272 °C and [R]1D8 ) -35.0 (c 1, acetone). Crystals
of the complex suitable for X-ray crystallography could not
be obtained.
treated with aqueous ammonium hexafluorophosphate. The
solvent was removed from the solution under vacuum, and
the residue was stirred in water to remove excess ammonium
hexafluorophosphate and the ammonium chloride byproduct.
The pale orange powder that remained was filtered off and
recrystallized from acetonitrile-diethyl ether to give colorless
microcrystals of the pure complex (88%), which had mp )
174 °C (dec) and [R]D18 ) +61.2 (c 1.0, acetone).
The diplatinum complex was prepared as the triflate by
the addition of TMSOTf to a solution of [PtCl2(COD)] and
(S,S)-tetraphos in dichloromethane (Scheme 2b).9 Removal
of the solvent and trimethylsilyl chloride byproduct under
vacuum afforded a colorless residue that was redissolved in
acetonitrile; the product, [Pt2{(R,R)-tetraphos}2](OTf)2‚2H2O,
was obtained from the solution by precipitation with wet
diethyl ether and was isolated as a colorless solid in 91%
yield (mp ) 309-312 °C, [R]1D8 ) +160 (c 1.0, acetone)).
The addition of a trace of aqueous potassium chloride to a
solution of the diplatinum(II) complex in acetone-d6 led to
its gradual rearrangement into the monoplatinum(II) complex
(∼95% conversion after 90 days); the conversion of dimer
into monomer also occurred when aqueous sodium iodide
was added, but the rate was much slower (∼40% conversion
after 90 days) (Scheme 2c). The addition of an excess of
aqueous chloride or iodide solution to an acetone-d6 solution
of the platinum dimer led to the immediate, complete
formation of the platinum monomer.
The complex [Pd2{(R,R)-tetraphos}2](OTf)4 was prepared
by the addition of a dichloromethane solution of [PdCl2-
(SEt2)2] to a solution of (S,S)-tetraphos in the same solvent,
as indicated in Scheme 2b. The bright yellow solution that
resulted was treated with TMSOTf, whereupon there was
an immediate decolorization of the solution. The solvent and
trimethylsilyl chloride byproduct were removed from the
reaction mixture under vacuum and the residue was dissolved
in acetonitrile; dilution of the solution with diethyl ether
brought about the crystallization of the dipalladium(II)
complex as a colorless solid (87%), having mp ) 312-
315 °C and [R]1D8 ) -311.7 (c 1, acetone). The 31P{1H}
NMR spectrum of the complex was consistent with its
formulation as a dimer (see below). Crystals of the complex
suitable for X-ray crystallography could not be obtained. The
addition of a few drops of aqueous potassium chloride (0.13
M) to a solution of [Pd2{(R,R)-tetraphos}2](OTf)4 (∼50 mg)
in acetone-d6 resulted in the immediate rearrangement of the
dimer into ∼40% of the monopalladium complex (Scheme
2c). After 90 days, the proportion of dimer to monomer was
essentially unchanged. The addition of a trace of aqueous
sodium iodide to the dipalladium complex in acetone-d6 led
to an immediate color change in the solution from colorless
The complex [Pt2{(R,R)-tetraphos}2] was prepared by the
reaction of [Pt(PPh3)4] with (S,S)-tetraphos in benzene
(Scheme 2d). After 0.5 h, the bright yellow solution was
evaporated to dryness, and the yellow oil that remained was
dissolved in diethyl ether; a fine yellow powder separated
almost immediately. The powder was filtered off and washed
with diethyl ether; the pure complex formed feathery, yellow
needles (53%) having mp ) 162-166 °C and [R]D18
)
8062 Inorganic Chemistry, Vol. 46, No. 19, 2007

Di- and ZeroWalent Ni, Pd, and Pt Monomers and Dimers
Table 1. Crystallographic Data and Experimental Details for the X-ray Crystal Structure Analyses
(MNi,MNi)-
[Ni2{(R,R)-tetraphos}2]
‚0.5Et2O
(MPd,MPd)-
[Pd2{(R,R)-tetraphos}2]
‚2C6H6
[Ni(NCMe)-
(MNi)-[Ni{(R,R)-
{(R,R)-tetraphos}](OTf)2
tetraphos}]‚0.5Et2O
molecular formula
fw
C46H45F6NNiP4S2
1068.59
C44H47NiO0.5P4
766.46
C86H89Ni2O0.5P8
1495.86
C96H96P8Pd2
1710.33
cryst color, habit
crystal size (mm)
space group
cryst syst
a (Å)
orange, plate
0.22 × 0.16 × 0.06
P212121
orthorhombic
13.2784(1)
orange, plate
0.30 × 0.15 × 0.12
P21
monoclinic
14.6487(2)
14.4641(2)
19.0842(2)
yellow, plate
0.26 × 0.21 × 0.15
P21
monoclinic
14.7451(2)
9.9707(1)
yellow, plate
0.21 × 0.18 × 0.04
P1h
triclinic
14.8875(8)
16.5889(9)
19.0688(11)
105.936(2)
100.285(4)
90.298(3)
4448.3(4)
2
b (Å)
c (Å)
19.3240(2)
37.3338(4)
25.8889(2)
R (deg)
â (deg)
90.1252(9)
90.3454(5)
γ (deg)
V (Å3)
9579.55(16)
8
1.482
4043.56(9)
4
1.259
3806.09(7)
2
1.305
Z
d
calcd (g cm-3
)
1.277
5.92
µ (cm-1
)
6.98
6.69
7.08
instrument
radiation
Nonius KappaCCD
Mo KR
Nonius KappaCCD
Mo KR
Nonius KappaCCD
Mo KR
Nonius KappaCCD
Mo KR
no. unique reflns
no. reflns obsd
2θ range (deg)
scan technique
temp (K)
16 875
18 351
17 368
28 412
11 403 (I > 3σ(I))
13 018 (I > 3σ(I))
13 745 (I > 3σ(I))
15 951 (I > 3σ(I))
6-55
æ and ω
200
6-55
æ and ω
200
6-55
æ and ω
200
6-50
æ and ω
200
structural refinement
CRYSTALS32
maXus33
0.0404
CRYSTALS32
maXus33
0.0261
CRYSTALS32
maXus33
0.0255
RAELS200034
R1a
0.080
0.104
wR2a
0.0463
0.0250
0.0259
a R1 ) [∑||F0| - |FC||]/∑|F0|; wR2 ) {[∑w(F0 - FC2)2]/[∑w(F0 )2]}1/2
.
2
2
-117.6 (c 1, benzene). The needles were not suitable for
X-ray crystallography.
and 0.274 Å. As is the case with all crystallographically
characterized, monomeric nickel(II)-tetraphos complexes,
the distances between the nickel ion and the inner-phosphorus
atoms (Ni-PPh) are shorter than those to the outer-
phosphorus atoms (Ni-PPh2). The central five-membered
ring in the cation shown (containing P2 and P3) has a
symmetric skew conformation; a counterclockwise direction
of rotation is required to bring the axis passing through the
two phosphorus atoms into coincidence with the non-
orthogonal skew-line passing through the two carbon atoms,
which leads to the assignment of λ to the conformation of
the ring. The two outer five-membered chelate rings in the
cation adopt enantiomorphic asymmetric envelope conforma-
tions.16
The crystallographic asymmetric unit of [Ni{(R,R)-
tetraphos}]‚0.5Et2O contains two crystallographically distinct
molecules in the unit cell. The structure of one of the
molecules is shown in Figure 3. The coordination geometry
around the nickel is tetrahedral and is similar to that of the
copper in (MCu)-[Cu{(R,R)-tetraphos}]PF6‚EtOH.2 The R
configurations of the inner-phosphorus stereocenters in both
complexes generate metal stereocenters of M configuration.
The three five-membered chelate rings in the complex have
the symmetric skew conformation, with the twist in each
case being in the counterclockwise, or λ, direction.
Crystal Structures. (a) Nickel. Dark orange plates of [Ni-
(NCMe){(R,R)-tetraphos}](OTf)2 crystallized from an ac-
etonitrile solution of [Ni{(R,R)-tetraphos}](OTf)2 upon the
addition of diethyl ether. Crystals of the nickel(0) complexes
were obtained from a diethyl ether solution: orange and
yellow crystals of the complexes slowly crystallized on the
walls of the flask, above the solution. X-ray crystal structure
analyses identified the orange crystals as (MNi)-[Ni{(R,R)-
tetraphos}]‚0.5Et2O and the yellow crystals as (MNi,MNi)-
[Ni2{(R,R)-tetraphos}]‚0.5Et2O. Crystallographic data and
associated experimental details for the complexes are given
in Table 1. The crystal structures of the racemates (()-[NiCl-
{(R*,R*)-tetraphos}]PF614 and (()-[Ni{(R*,R*)-tetraphos}]-
15
(ClO4)2 have been determined previously.
The crystallographic asymmetric unit of [Ni(NCMe)-
{(R,R)-tetraphos}2](OTf)2 consists of two distinct cations of
the complex and four triflate ions (one of which is disor-
dered). The structure of one of the two cations of the complex
is shown in Figure 2; the nickel(II) ion has square-pyramidal
coordination geometry, with the acetonitrile molecule oc-
cupying an apical site. The Ni-N distance in the cation is
2.018(5) Å in one independent molecule and 2.089(4) Å in
the other molecule; the nickel atom in the former molecule
sits 0.354 Å above the least-squares P4 plane, in the direction
of the acetonitrile ligand. The phosphorus atoms P1 and P3
in the cation deviate above the least-squares plane by 0.352
The crystallographic asymmetric unit of (MNi,MNi)-[Ni2-
{(R,R)-tetraphos}2]‚0.5Et2O contains one molecule of the
complex and the disordered solvent of crystallization. The
coordination geometry around each nickel atom in the
(14) Airey, A. L.; Hockless, D. C. R.; Swiegers, G. F.; Wild, S. B. Z.
Kristallogr. 1996, 211, 939-941.
(15) Airey, A. L.; Hockless, D. C. R.; Swiegers, G. F.; Wild, S. B. Z.
Krystallogr. 1996, 211, 937-938.
(16) Hawkins, C. J. Absolute Configurations of Metal Complexes; Wiley-
Interscience: New York, 1971; Chapter 2.
Inorganic Chemistry, Vol. 46, No. 19, 2007 8063

Kitto et al.
Figure 4. Molecular structure of (MNi,MNi)-[Ni2{(R,R)-tetraphos}2] show-
ing 50% probability ellipsoids (a) and view showing the parallel arrangement
of the tetraphosphine ligand strands along each side of the Ni‚‚‚Ni axis
(b); hydrogen atoms and solvent molecules have been omitted for clarity.
Selected bond lengths (Å) and angles (deg): Ni1-P1 ) 2.1659(6), Ni1-
P2 ) 2.1623(6), Ni1-P5 ) 2.1529(6), Ni1-P6 ) 2.1667(6), Ni2-P3 )
2.1689(6), Ni2-P4 ) 2.1665(6), Ni2-P7 ) 2.1586(6), Ni2-P8 ) 2.1511-
(6), Ni1‚‚‚Ni2 ) 6.205(1), P1-Ni1-P2 ) 90.30(2), P1-Ni1-P5 ) 115.47-
(2), P1-Ni1-P6 ) 125.78(2), P2-Ni1-P5 ) 116.87(2), P2-Ni1-P6 )
119.47(2), P5-Ni1-P6 ) 91.44(2), P3-Ni1-P4 ) 91.52(2), P3-Ni2-
P7 ) 121.83(2), P3-Ni2-P8 ) 117.43(2), P4-Ni2-P7 ) 118.39(2), P4-
Ni2-P8 ) 119.27(2), P7-Ni2-P8 ) 91.16(2).
Figure 2. Molecular structure of one of the two independent molecules
of [Ni(NCMe){(R,R)-tetraphos}]2+ showing 50% probability ellipsoids (a)
and view showing the distorted square-pyramidal geometry of the nickel-
(II) atom (b); hydrogen atoms have been omitted for clarity. Selected bond
lengths (Å) and angles (deg): Ni1-P1 ) 2.2236(14), Ni1-P2 ) 2.1878-
(14), Ni1-P3 ) 2.1945(14), Ni1-P4 ) 2.2515(14), Ni1-N1 ) 2.018(5),
P1-Ni1-P2 ) 82.51(5), P1-Ni1-P3 ) 167.67(6), P1-Ni1-P4 ) 106.11-
(5), P2-Ni1-P3 ) 85.31(5), P2-Nil-P4 ) 143.60(6), P3-Ni1-P4 )
84.91(5), P1-Ni1-N1 ) 89.42(13), P2-Ni1-N1 ) 111.73(13), P3-Ni1-
N1 ) 93.35(13), P4-Ni1-N1 ) 103.79(13).
have M configurations, which are generated by the four inner-
phosphorus stereocenters of R configuration. The inner Ni-
P(Ph) and the outer Ni-P(Ph2) distances in the complex are
similar. The Ni‚‚‚Ni distance in the complex is 6.205(1) Å;
the corresponding distances in the analogous silver and gold
complexes are 6.072(4) and 6.244(2) Å, respectively.2 The
three five-membered rings in the dinuclear nickel complex
associated with the atoms P1/P2, P3/P4, and P7/P8 have the
δ conformation, but the five-membered ring containing P5/
P6 has the λ conformation, which reduces the symmetry of
the cation in the solid state to C1. The ten-membered ring
containing the two nickel atoms adopts the δ-TBCB con-
formation, which, in conjunction with the two negative twists
arising from the two M(PP)2 stereocenters of M configura-
tion, generates an overall M twist for the parallel helix
conformer of the helicate.
The differences in the P-Ni-P angles in (MNi)-[Ni{(R,R)-
tetraphos}], (MNi,MNi)-[Ni2{(R,R)-tetraphos}2], and [Ni(d-
ppe)2]17 reflect the strain imposed on the tetrahedral structure
by the central 1,2-ethano spacer group of the tetraphosphine
ligand. The P-Ni-P angles in the three complexes (fol-
lowing the labeling scheme shown in Figure 5) are listed in
Table 2. All of the angles in the three complexes that involve
five-membered chelate rings are close to 90° because of the
constraints within these rings. It is also evident in the
mononuclear metal complex that the five-membered chelate
ring containing the central ethylene spacer adds considerable
Figure 3. Molecular structure of one of the two independent molecules
of (MNi,MNi)-[Ni{(R,R)-tetraphos}] showing 50% probability ellipsoids (a)
and view showing the distorted tetrahedral geometry at nickel (b); hydrogen
atoms and solvent molecules have been omitted for clarity. Selected bond
lengths (Å) and angles (deg): Ni1-P1 ) 2.1440(7), Ni1-P2 ) 2.1254(7),
Ni1-P3 ) 2.1257(7), Ni1-P4 ) 2.1419(6), P1-Ni1-P2 ) 89.21(3), P1-
Ni1-P3 ) 127.73(3), P1-Ni1-P4 ) 128.22(3), P2-Ni1-P3 ) 92.68-
(3), P2-Ni1-P4 ) 127.84(3), P3-Ni1-P4 ) 89.66(3).
complex is tetrahedral (Figure 4). The overall structure of
the complex is similar to that of the analogous silver(I) and
gold(I) complexes, where the two ligand strands wrap around
each other on either side of the metal-metal axis, as shown
in Figure 4b. The two nickel stereocenters in the molecule
(17) Hartung, H.; Baumeister, U.; Walther, B.; Maschmeier, M. Z. Anorg.
Allg. Chem. 1989, 578, 177-184.
8064 Inorganic Chemistry, Vol. 46, No. 19, 2007

Di- and ZeroWalent Ni, Pd, and Pt Monomers and Dimers
Figure 5. Atom labeling for [Ni(dppe)2] (a), [Ni{(R,R)-tetraphos}] (b),
and [Ni2{(R,R)-tetraphos}2] (c).
Table 2. Core Angles (deg) in [Ni(dppe)2] (a),
(MNi)-[Ni{(R,R)-tetraphos}] (b), and (MNi,MNi)-Ni2{(R,R)-tetraphos}2] (c)
a
b
c
PA-Ni-PB
PA-Ni-PC
PA-Ni-PD
PB-Ni-PC
PB-Ni-PD
PC-Ni-PD
90.8(1)
120.2(1)
113.8(1)
129.3(1)
114.4(1)
90.1(1)
89.12(3)
127.73(3)
128.22(3)
92.68(3)
127.84(3)
89.66(3)
90.30(2)
125.78(2)
115.47(2)
119.47(2)
116.87(2)
91.44(2)
strain to the structure; the PA-Ni-PD angle in the monomer,
128.22(3)°, is much greater than the corresponding angles
in the dinickel complex, 115.47(2)°, and the dppe complex,
113.8(1)°. With the exception of the large PB-Ni-PC angle,
the angles in [Ni(dppe)2] and the dimer are similar, but they
differ greatly from those in the mononuclear nickel-
tetraphos complex because of the constraint placed on this
molecule by the small angle PB-Ni-PC.
Figure 6. Molecular structure of one of two independent molecules of
(MPd,MPd)-[Pd2{(R,R)-tetraphos}2] showing 50% probability ellipsoids (a)
and view showing the parallel arrangement of the tetraphosphine ligand
strands down either side of the metal-metal axis (b); hydrogen atoms and
solvent molecules have been omitted for clarity. Selected bond lengths (Å)
and angles (deg): Pd1-P1 ) 2.336(6), Pd1-P2 ) 2.330(5), Pd1-P5 )
2.335(5), Pd1-P6 ) 2.308(6), Pd2-P3 ) 2.331(6), Pd2-P4 ) 2.362(6),
Pd2-P7 ) 2.345(5), Pd2-P8 ) 2.357(6), Pd1‚‚‚Pd2 ) 6.300(8), P1-Pd1-
P2 ) 86.6(2), P1-Pd1-P5 ) 116.6(2), P1-Pd1-P6 ) 128.2(2), P2-
Pd1-P5 ) 127.7(2), P2-Pd1-P6 ) 116.8(2), P5-Pd1-P6 ) 86.1(2),
P3-Pd2-P4 ) 86.9(2), P3-Pd2-P7 ) 116.5(2), P3-Pd2-P8 ) 128.4-
(2), P4-Pd2-P8 ) 117.4(2), P7-Pd2-P8 ) 86.5(2).
(b) Palladium. The complex (MPd,MPd)-[Pd2{(R,R)-
tetraphos}2]‚2C6H6 crystallizes as yellow plates from benzene-
diethyl ether. The crystallographic asymmetric unit of the
complex contains two similar molecules and four benzene
molecules of crystallization. The Z ) 2, space group P1
crystal structure has four twin components in the ratio 0.540:
0.158(2):0.286(2):0.016(2), resulting from a pseudo-P21212
interface between layers perpendicular to the c* direction.
By refining the contributions to intensities of the minor twin
components as a function of spot separation in reciprocal
space, we obtained a satisfactory refinement. One of the
molecules is shown in Figure 6; the two palladium(0) atoms
have distorted tetrahedral coordination geometries, and the
overall structure of the molecule is similar to that of the
corresponding dinickel(0) complex. The metal stereocenters
in the palladium(0) complex have M configurations imposed
by the R configurations of the inner-phosphorus stereocenters.
The average M-P distances in (MPd,MPd)-[Pd2{(R,R)-tetra-
phos}2] (2.338 Å) are longer than those in the nickel(0)
complex (2.162 Å). The P-M-P angles in the two com-
plexes are comparable, with the exceptions of P2-M-P5,
P3-M-P7, and P3-M-P8, which are 7-11° larger in the
palladium(0) complex. The Pd‚‚‚Pd interatomic distance is
6.300(8) Å. Four five-membered chelate rings are present
in the molecule; the rings associated with P1/P2 and P3/P4
have δ conformations, and those associated with P5/P6 and
P7/P8 have λ conformations. The central ten-membered ring
containing the two palladium atoms has the δ-TBCB
conformation, and the two ligand strands in the molecule
complete ∼1.5 turns down each side of the metal-metal axis.
The negative twists associated with the two Pd(PP)2 stereo-
centers of M configuration overcome the positive twist of
the TBCB ring to generate an overall parallel helicate of M
configuration. The crystal structure of (()-[Pd{(R*,R*)-
tetraphos}]Cl2 has been reported previously.18
(c) Platinum. The crystal structure of [Pt2{(R,R)-tetraphos}2]-
(CF3SO3)4‚4.5H2O has been reported elsewhere.9 The cation
of the salt consists of a double-stranded diplatinum(II)
helicate that completes an approximately one-eighth turn of
a double helix in the M direction, as measured by the angle
between the two Pt(PP)2 square planes. The ten-membered
ring containing the two platinum atoms has a distorted twist-
boat-chair-boat conformation of λ helicity, which is respon-
sible for the M twist of the helicate. The crystal structures
of (()-[Pt{R*,R*)-tetraphos}](BPh4)2,19 [Pt{(R*,S*)-tetraphos}]-
21
(BPh4)2,20 and (()-[PtH{(R*,R*)-tetraphos}]BPh4 have
been previously reported.
NMR Spectra. (a) Mononuclear Metal Complexes. The
31P{1H} NMR spectrum of [Ni{(R,R)-tetraphos}](OTf)2 in
acetone-d6 corresponds to an AA′BB′ spin system because
of the magnetic inequivalence of the four phosphorus nuclei.
The spectrum is shown in Figure 7a and contains second-
order multiplets in the vicinity of 45.5 ppm for the outer-
phosphorus nuclei (PPh2) and 110.3 ppm for the inner-
phosphorus nuclei (PPh). These signals arise from the two
overlapping and mutually coupled resonances for PA and PA′
(45 ppm) and PB and PB′ (110 ppm). The resonances in the
(18) Oberhauser, W.; Bachmann, C.; Bru¨geller, P. Polyhedron 1995, 14,
787-792.
(19) Bru¨geller, P.; Hu¨bner, T. Acta Crystallogr. C 1990, 46, 388-391.
(20) Bru¨geller, P.; Nah, H.; Messerschmidt, A. Acta Crystallogr. C 1992,
48, 817-821.
(21) Bru¨geller, P. Acta Crystallogr. C 1992, 48, 445-449.
Inorganic Chemistry, Vol. 46, No. 19, 2007 8065

Kitto et al.
gives rise to the typical AA′BB′ 31P{1H} NMR spectrum in
acetone-d6 with resonances for the cation being centered at
42.0 (PPh2) and 95.7 ppm (PPh), along with 31P-195Pt
1
coupling, namely, JPtP ) 2133 (PPh) and 2413 Hz (PPh2).
(b) Dinuclear Metal(II) Complexes. The 31P{1H} NMR
spectrum in acetone-d6 of [Pd2{(R,R)-tetraphos}2](OTf)4
gives rise to two broad peaks at 51.6 and 56.0 ppm for the
rapidly equilibrating mixture of double-helix and parallel-
helix conformers. The assignment of the peaks in the
spectrum to the inner- and outer-phosphorus nuclei was made
by comparison with spectrum of [Pd(dppe)2]Cl2. The singlet
peak in the 31P{1H} NMR spectrum of [Pd(dppe)2]Cl2 in
chloroform-d occurs at 56.7 ppm;25 on this basis, the peak
at 56.0 ppm in the dipalladium(II) complex was assigned to
the outer-phosphorus nuclei (PPh2).
The 31P{1H} NMR spectrum of [Pt2{(R,R)-tetraphos}2]-
(OTf)4 in acetone-d6 contains two broad resonances at 42.2
(PPh) and 47.8 ppm (PPh2), together with associated platinum
satellites. The resonance at 47.8 ppm has been assigned to
the outer-phosphorus nuclei in the complex on the basis of
the similarity of the shift to that for the PPh2 group in [Pt-
(dppe)2]Cl2, namely, 47.0 ppm in benzene-d6.26 In the
Figure 7. 31P{1H} NMR spectrum of [Ni{(R,R)-tetraphos}](OTf)2 in
acetone-d6 (a) and simulated spectrum (b).
spectrum of the complex cannot be resolved by implying
that the differences in chemical shifts for PA and PA′ and PB
and PB′ are small, even within the context of second-order
behavior (∆A < 5JAA′). Given this constraint and the capacity
for resonances within a second-order regime to exhibit
different line-widths [the signs and magnitudes typically
observed for P-P coupling constants within square-planar
complexes are the following: Jcis ) -10 to -50 Hz, Jtrans
1
diplatinum complex, JPtP ) 2236 Hz (PPh) and 2405 Hz
(PPh2). The difference in the values of the coupling constants
for the inner and outer phosphorus nuclei in the complex is
similar to the difference in the couplings observed for the
corresponding phosphorus nuclei in [Pt{(R,R)-tetraphos}]-
(PF6)2.
(c) Mono- and Dinuclear Metal(0) Complexes. The 31P-
{1H} NMR spectrum of [Ni{(R,R)-tetraphos] in benzene-d6
contains apparent triplets centered at 49.5 (PPh2) and 69.1
ppm (PPh). In an ideal tetrahedral system, the inner- and
outer-phosphorus nuclei will each give a doublet in the
spectrum. The apparent triplets in the spectrum are overlap-
ping doublets that arise because of a lowering of symmetry
of the molecule brought about by the restrictions of the
ligand. The 31P{1H} NMR spectrum in benzene-d6 of a
solution of the complex after 2 months contained broad peaks
at 45.4 and 48.6 ppm for [Ni2{(R,R)-tetraphos}2]. After a
further month, the NMR spectrum of the solution showed
only the resonances for the dimer.
The 500 MHz 31P{1H} NMR spectrum of [Pd2{(R,R)-
tetraphos}2] at 25 °C in benzene-d6 contains a single peak
at 33.7 ppm. The 300 MHz 31P{1H} NMR spectrum in
toluene-d8 also contained a singlet, but at -40 °C, the
resonance broadens and emerges as a broad doublet for the
inner- and outer-phosphorus nuclei of the D2 molecule.
3
) 100-500 Hz, and JPP ) 20-40 Hz (bond-mediated
interactions)]22 and that Jcis and JPP are rarely resolved
individually, a simulation of the observed spectrum was
carried out with use of the program gNMR, version 4.1.0.23
One viable solution for the observed spectrum is shown in
Figure 7b and corresponds to an AA′BB′ spin system with
2
3
2
2
the following couplings: JAA′ ) -33 Hz, JAB′ ) 177 Hz,
2+3
2+3
2+3
2
J
) -27 Hz,
J
) -28 Hz, JA′B ) 180 Hz, and
AB
A′B′
J
BB′
) 20 Hz. The 31P{1H} NMR spectrum of the racemate
(()-[Ni{(R*,R*)-tetraphos}](BPh4)2 is reported to be solvent
dependent.24 For the complex [Ni{(R,R)-tetraphos}](OTf)2,
an upfield shift was observed for the resonances for the
phosphorus nuclei in acetonitrile-d3 compared to the values
in acetone-d6 and nitromethane-d3, together with a slight
broadening of the peaks; the latter phenomenon can be
attributed to exchange of the coordinated acetonitrile with
the solvent. The complex [Pd{(R,R)-tetraphos}](PF6)2 has a
31P{1H} NMR spectrum in acetone-d6 similar to that of the
corresponding nickel(II) complex, with the multiplets for the
AA′BB′ spin system being centered at 42.2 (PPh2) and 108.9
ppm (PPh). The complex [Pt{(R,R)-tetraphos}](PF6)2 also
The complex [Pt2{(R,R)-tetraphos}2] exhibits multiplets
at 29.2 and 38.4 ppm for the equilibrating mixture of
conformers in the 31P{1H} NMR spectrum in benzene-d6,
which are superimposed on broad resonances. Both multiplets
in the spectrum consist of two doublets, each being flanked
by platinum satellites. The appearance of two doublets for
the two types of phosphorus nuclei in the complex indicates
(22) (a) Quin, L. D., Verkade, J. G., Eds. Phosphorus-31 NMR Spectral
Properties in Compound Characterization and Structural Analysis;
VCH: New York, 1994. (b) Pregosin, P. S.; Kunz, R. W. Phosphorus-
31 and Carbon-13 NMR of Transition Metal Phosphine Complexes;
Springer-Verlag: Berlin, 1979.
(23) Budzelaar, P. H. M. gNMR, version 4.1.0; Cherwell Scientific
Publishing: Oxford, U.K., 1995-1997.
(24) Bachmann, C.; Oberhauser, W.; Bru¨geller, P. Polyhedron 1996, 15,
2223-2230.
(25) Jarrett, P. S.; Ni Dhubhghaill, O. M.; Sadler, P. J. J. Chem. Soc., Dalton
Trans. 1993, 1863-1870.
(26) Davies, J. A.; Staples, R. J. Polyhedron 1991, 10, 899-908.
8066 Inorganic Chemistry, Vol. 46, No. 19, 2007

Di- and ZeroWalent Ni, Pd, and Pt Monomers and Dimers
Figure 8. Cyclic voltammograms at 20 °C of a 0.5 mM solution of [Ni-
{(R,R)-tetraphos}](OTf)2 in dichloromethane containing 0.25 M [(n-Bu)4N]-
PF6 at a scan rate of 100 mV s-1 using a GC electrode (left) and Pt electrode
(right).
Figure 9. Cyclic voltammograms at 20 °C of a 0.5 mM solution of [Ni-
{(R,R)-tetraphos}](OTf)2 in acetonitrile containing 0.25 M [(n-Bu)4N]PF6
at a scan rate of 100 mV s-1 using a GC electrode (left) or Pt electrode
(right).
Table 3. Cyclic Voltammetric Dataa for 0.5 mM Solutions of
[Ni{(R,R)-tetraphos}](OTf)2 in Dichloromethane and Acetonitrile
Recorded at a Scan Rate of 0.1 V s-1 at a 1 mm Diameter Planar GC
Electrode at 20 °C with 0.25 M [(n-Bu)4N]PF6 as the Supporting
Electrolyte
conditions, which suggests two one-electron reduction
processes for the divalent metal complexes. It was therefore
assumed that the first process was associated with the
reduction of nickel(II) to nickel(I), and the second process
was the result of the reduction of nickel(I) to nickel(0) (eqs
1 and 2).
solvent
CH2Cl2
Epred (V)b
Epox (V)c
E1/2r (V)d
∆E (mV)e
-0.769
-1.065
-0.890
-1.040
-0.699
-0.990
-0.814
-0.960
-0.73
-1.03
-0.85
-1.00
70
75
76
80
[Ni{(R,R)-tetraphos}]2+ + 1e- h [Ni{(R,R)-tetraphos}]+ (1)
[Ni{(R,R)-tetraphos}]+ + 1e- h [Ni{(R,R)-tetraphos}] (2)
CH3CN
+
red
a Potentials are relative to the FC/FC redox couple. b Ep ) reductive
peak potential. c Epox ) oxidative peak potential. E1/2r ) (Epred + Epox)/2.
d
ox
red
e ∆E ) |Ep - Ep |.
As the temperature of the solution containing [Ni{(R,R)-
red
tetraphos}](OTf)2 was lowered from 20 to -40 °C, the ip
a distorted tetrahedral arrangement of each ligand around
the metal atoms, which gives rise to four different phosphorus
environments. Assignments of peaks in the 31P{1H} NMR
spectrum of dimer in benzene-d6 were made by comparison
with those for [Pt(dppe)2], which has a peak at 30.2 ppm in
benzene-d6;27 the signal at 29.2 ppm in the dimer was
accordingly assigned to the outer-phosphorus (PPh2) nuclei.
Electrochemistry. The in situ UV-vis spectra of [Ni-
{(R,R)-tetraphos}](OTf)2 were recorded during the sequential
reduction of nickel(II) to nickel(I) and nickel(0) in dichlo-
romethane (see Supporting Information). The UV-vis
spectra for the complex indicated that the electrochemical
reactions were completely reversible; the starting material
was regenerated by application of an oxidizing potential to
the reduced solutions, and the reduced species were stable
at -20 °C in this solvent.
(and ipox) values for the complexes obtained on a glassy
carbon electrode (GC) decreased because of diminishing
red
diffusion coefficients. Below -20 °C, the ip values
observed on a platinum electrode (Pt) were lower than could
be accounted for by decreasing diffusion coefficient values
alone; however, the ∆Epp values were larger than could
reasonably be explained by increased solution resistance.
Instead, it appears that the voltammetric data obtained on Pt
in dichloromethane were affected by a slow rate of hetero-
geneous electron transfer that was most pronounced at low
temperatures.
The cyclic voltammograms of [Ni{(R,R)-tetraphos}](OTf)2
in acetonitrile (Figure 9) were similar to those in dichlo-
romethane at 20 °C, but the first process was shifted by
approximately -0.1 V (Table 3). As the temperature was
red
lowered, the ip value for the first process decreased,
The electrochemical properties of the nickel(II), palladium-
(II), and platinum(II) complexes of (S,S)-tetraphos were
investigated by cyclic voltammetry (CV). The cyclic volta-
mmograms obtained for a dichloromethane solution contain-
ing [Ni{(R,R)-tetraphos}](OTf)2 showed reduction processes
at -0.73 and -1.03 V vs Fc/Fc+ (Fc ) ferrocene) at 20 °C.
red
whereas the ip value for the second process remained
almost constant; at -40 °C, only the second process could
be detected on the forward scan. Since it is expected that
the peak current will decrease as the temperature is lowered
because of the decreasing diffusion coefficients (as seen in
dichloromethane), the apparently constant ipred value observed
for the second process in acetonitrile as the temperature was
lowered is most likely caused by the transfer of an increasing
number of electrons (that is, the process at -1 V vs Fc/Fc+
at -40 °C is associated with a two-electron transfer).
The CV data obtained in acetonitrile can most easily be
accounted for by a mechanism that involves the reversible
coordination of a solvent molecule to the nickel. At higher
temperatures, there is a rapid equilibrium between the
acetonitrile adduct [Ni(NCMe){(R,R)-tetraphos}]2+ and its
parent [Ni{(R,R)-tetraphos}]2+, so that cyclic voltammo-
gramss detect the parent nickel(II) complex being reduced
r
The reversible half-wave reduction potentials (E1/2 ), which
approximate the formal potentials (E0), were calculated from
the CV data under conditions where the ratio of the reductive
(ipred) to oxidative (ipox) peak currents were equal to unity,
r
red
with use of the relationship E1/2 ) (Ep + Epox)/2, where
ox
red
Ep and Ep are the anodic and cathodic peak potentials,
respectively (Figure 8 and Table 3). The anodic to cathodic
peak-to-peak separations (∆Epp) for the reduction processes
were similar to those observed for ferrocene under identical
(27) Clark, H. C.; Kapoor, P. N.; McMahon, I. J. J. Organomet. Chem.
1984, 265, 107-115.
Inorganic Chemistry, Vol. 46, No. 19, 2007 8067

Kitto et al.
plexes were restricted to measurements in acetonitrile because
of the poor solubilities of [M{(R,R)-tetraphos}(PF6)2 (M )
Pd, Pt) in dichloromethane. The principal difference between
the CV data for the three complexes is that the nickel
complex displays two reduction processes in acetonitrile (at
higher temperatures), whereas the palladium and platinum
complexes show only one response, albeit with a complex
shape. The peak currents for the reduction processes for [M{-
(R,R)-tetraphos](PF6)2 (M ) Pd, Pt) were similar to the
overall current for the two one-electron reduction processes
of [Ni{(R,R)-tetraphos}](PF6)2 (or the one two-electron
processes at low temperatures). On the basis of the peak
current similarities, it was concluded that the reductions of
[M{(R,R)-tetraphos}](PF6)2 (M ) Pd, Pt) occur in one two-
electron step over all temperatures measured, assuming that
all three complexes have similar diffusion coefficients.
Although the cyclic voltammograms of [M{(R,R)-tetraphos}]-
(PF6)2 (M ) Pd, Pt) show reverse peaks when the scan
directions are switched, the wide ∆Epp values observed on
Pt and the very sharp peaks observed on GC indicate that
the processes are more complicated than simple reversible
electron transfer. It is possible that the reduction processes
are associated with equilibria involving acetonitrile coordina-
tion to the palladium or platinum ions, as observed for the
corresponding nickel complex.
Figure 10. Cyclic voltammograms recorded in acetonitrile containing 0.25
M [(n-Bu)4N]PF6 at a scan rate of 100 mV s-1 at (---) 20 °C and
(‚‚‚‚‚‚‚) -30 °C for 0.5 mM solutions of [Ni{(R,R)-tetraphos}](OTf)2 (a),
[Pd{(R,R)-tetraphos}](PF6)2 (b), and [Pt{(R,R)-tetraphos}](PF6)2 (c) using
a GC electrode (left) or Pt electrode (right).
Attempts to obtain cyclic voltammograms for [M2{(R,R)-
tetraphos}2](OTf)4 (M ) Pd, Pt) were unsuccessful because
the compounds could not be reduced within the potential
window of the solvent/electrolyte (-3 V vs ferrocene).
sequentially to the corresponding nickel(I) and nickel(0)
complexes (eqs 1 and 2). As the temperature of the solution
is lowered to -40 °C, the position of the equilibrium changes
so that only the five-coordinate complex is detected, and the
CV represents the two-electron reduction of nickel(II)-
MeCN to nickel(0)-MeCN (eq 3). At -40 °C, the aceto-
nitrile-nickel(0) complex quickly dissociates into the parent
nickel(0) complex and acetonitrile (eq 4, Figure 9). Thus,
when the CV scan direction was reversed at -40 °C,
oxidative current was observed because of the reoxidation
of nickel(0) to nickel(I) and nickel(II) (the reverse of eqs 1
and 2). It was concluded from the results that the 100 mV
shift in potential of the first oxidation process in acetonitrile
was the result of the equilibrium that exists between the
acetonitrile-free and acetonitrile-nickel(II) complexes.
Conclusion
The enantiomerically pure, linear, tetra(tertiary phosphine)
(S,S)-tetraphos combines with nickel(II) chloride to give
[NiCl{(R,R)-tetraphos}]Cl, which can be converted into [Ni-
{R,R)-tetraphos}](OTf)2 by treatment with trimethylsilyl
triflate; the latter complex shows reversible, sequential
reductions to nickel(I) and nickel(0) in acetonitrile and
dichloromethane at 20 °C. The complex [PdCl2(SEt2)2]
combines with (S,S)-tetraphos in acetonitrile to give a yellow
solution from which colorless [Pd{(R,R)-tetraphos}](PF6)2
can be isolated by the addition of aqueous ammonium
hexafluorophosphate. The mononuclear platinum(II) complex
can be prepared similarly from [PtCl2(COD)]. The CV of
the mononuclear nickel(II) complex displays two one-
electron reduction processes in acetonitrile at room temper-
ature, whereas the corresponding palladium and platinum
complexes each show one two-electron response of complex
shape. When the reactions of (S,S)-tetraphos with [PdCl2-
(SEt2)2] or [PtCl2(COD)] are carried out in dichloromethane
and the intermediates are treated with trimethylsilyl triflate,
the yellow solutions are decolorized immediately, and the
complexes [M2{R,R)-tetraphos}2](OTf)4 (M ) Pd, Pt) can
be isolated. The addition of a few drops of aqueous potassium
chloride or sodium iodide to acetone-d6 solutions of the
dinuclear metal complexes brings about the immediate
establishment of equilibrium mixtures containing the corre-
sponding mononuclear metal complexes. The tetrahedral
complex [Ni{(R,R)-tetraphos}] can be prepared from [Ni-
[Ni(NCMe){(R,R)-tetraphos}]2+ + 2e- h
[Ni(NCMe){(R,R)-tetraphos}] (3)
[Ni(NCMe){(R,R)-tetraphos}] h
[Ni{(R,R)-tetraphos}] + MeCN (4)
Cyclic voltammograms of 0.5 mM solutions of [Pd{(R,R)-
tetraphos}](PF6)2 and [Pt{(R,R)-tetraphos}](PF6)2 in aceto-
nitrile at -30 and 20 °C are shown in Figure 10, along with
cyclic voltammograms of [Ni{(R,R)-tetraphos}](OTf)2 that
were obtained under identical conditions. In general, it
appears that the potential of reduction varies in the order of
[Ni{(R,R)-tetraphos}]2+ < [Pd{(R,R)-tetraphos}]2+ < [Pt-
{(R,R)-tetraphos]2+, although there are difficulties in assign-
ing the true reduction potentials for the palladium and
platinum complexes because of the complex shapes of the
cyclic voltammograms. Electrochemical data for these com-
8068 Inorganic Chemistry, Vol. 46, No. 19, 2007

Di- and ZeroWalent Ni, Pd, and Pt Monomers and Dimers
under vacuum to leave an orange oil. The oil was dissolved in
diethyl ether (7 mL); the pure product separated from the solution
as a bright orange solid. Yield: 0.18 g (82%). mp: 98-101 °C.
(COD)2] and (S,S)-tetraphos in benzene, but it slowly
rearranges into the more stable double-stranded helicate [Ni2-
{R,R)-tetraphos}2] in solution, with which it is in equilibrium.
The complexes [M2{(R,R)-tetraphos}2] (M ) Pd, Pt) can be
prepared in high yields by the addition of (S,S)-tetraphos to
solutions of [M(PPh3)4] in toluene (M ) Pd) or benzene (M
) Pt).
[R]18: -147.9 (c 1, benzene). Anal. Calcd for C42H42NiP4: C,
D
69.16; H, 5.80. Found: C, 69.55; H, 5.56. 31P{1H} NMR (C6D6):
δ 49.5 (overlapping doublets, 2 P, 2JPP ) 51.8 and 54.9 Hz, PPh2),
2
69.1 (overlapping doublets, 2 P, JPP ) 54.9 and 51.8 Hz, PPh).
Orange plates of the monomeric complex and yellow plates of
(MNi,MNi)-[Ni2{(R,R)-tetraphos}2] crystallized above a diethyl ether
solution of the monomeric complex, both of which were shown by
X-ray crystallography to contain 0.5 diethyl ether molecule of
crystallization.
Experimental Section
Reactions involving the air-sensitive zerovalent metal complexes
were performed under a positive pressure of nitrogen with the use
of standard Schlenk techniques. Solvents were dried following the
usual procedures and distilled under nitrogen before use. 31P{1H}
NMR spectra were recorded in the solvents specified at 25 °C on
Varian Inova 300 and 500 spectrometers operating at 121.42 and
202.42 MHz, respectively. The chemical shifts (δ) are reported in
parts per million relative to external 85% aqueous H3PO4. Optical
rotations were measured on the specified solutions in a 1 dm cell
at 18 °C with a Perkin-Elmer Model 241 polarimeter. Specific
rotations were estimated to be within (0.5 deg cm2 g-1. ES MS
were recorded on a Bruker Apex II 4.7T FTIR spectrometer with
an Apollo ESI source. Elemental analyses were performed by staff
within the Research School of Chemistry.
Enantiopure (S,S)-tetraphos2 and the complexes [PdCl2(SEt2)2],28
[Pd(PPh3)4],29 [PtCl2COD],30 and [Pt(PPh3)4]31 were prepared by
literature methods. [Ni(COD)2] was purchased from the Aldrich
Chemical Co., Inc., and stored under argon in a Schlenk tube.
[SP-4-(4RP,7RP)]-(+)546-(1,1,4,7,10,10-Hexaphenyl-1,4,7,10-tet-
raphosphadecane)nickel(II) Trifluoromethanesulfonate, [Ni-
{(R,R)-tetraphos}](OTf)2. A solution of [Ni(H2O)6]Cl2 (0.048 g,
0.20 mmol) in water (5 mL) was added to a solution of (S,S)-
tetraphos (0.13 g, 0.30 mmol) in dichloromethane (5 mL). The
dropwise addition of ethanol (20 mL) to the vigorously stirred
mixture afforded a deep red-brown precipitate of [NiCl((R,R)-
tetraphos]Cl (0.15 g, 0.19 mmol). The solid was dissolved in
dichloromethane (10 mL), and the deep red solution was cooled to
0 °C; TMSOTf (0.25 g, 1.11 mmol) was added to the solution,
which changed color from deep red to yellow. After the mixture
was stirred for 0.5 h, the solvent was removed from the reaction
mixture under vacuum. The resulting solid was washed with diethyl
ether to remove the TMSCl and excess TMSOTf. The solid was
then dissolved in acetonitrile; dilution of the solution with diethyl
ether afforded deep red crystals of the acetonitrile adduct, which,
when filtered off, lost acetonitrile in vacuo to give the parent product
[SP-4-(4RP,7RP)]-(+)-(1,1,4,7,10,10-Hexaphenyl-1,4,7,10-tet-
raphosphadecane)palladium(II) Hexafluorophosphate, [Pd-
{R,R)-tetraphos}](PF6)2. A mixture of (S,S)-tetraphos (0.10 g, 0.15
mmol) and [PdCl2(SEt2)2] (0.053 g, 0.15 mmol) was dissolved in
acetonitrile (5 mL). The yellow solution was stirred for 10 min
before the addition of ammonium hexafluorophosphate (0.50 g, 3.07
mmol) in water (3 mL), which caused a color change in the solution
to pale orange. The solvents were removed under vacuum, and water
(10 mL) was added to dissolve the ammonium salts. The solid was
filtered off and washed with water and diethyl ether, and the pale
orange solid that remained was recrystallized from acetonitrile (4
mL) by the addition of diethyl ether (8 mL). The pure product
crystallized from this solution as a colorless solid. Yield: 0.14 g
(88%). mp: 174 °C (dec). [R]1D8: +61.2 (c 1.0, acetone). Anal.
Calcd for C42H42F12P6Pd: C, 47.28; H, 3.97. Found: C, 47.12; H,
4.02. 31P{1H} NMR (acetone-d6): δ 42.2 (m, 2 P, 2JPP(trans) ) 266
Hz, PPh2), 108.9 (m, 2 P, 2JPP(trans) ) 268 Hz, PPh), -143.1 (sept,
1
2P, JPF ) 711.1 Hz, PF6). ES MS: m/z 923.1 [M + PF6 + H]+,
777.2 [M]+, 777.2 [M]+, 389.2 [M]2+
.
[SP-4-(4RP,7RP)]-MH-(-)-Bis(1,1,4,7,10,10-hexaphenyl-1,4,7,-
10-tetraphosphadecane)dipalladium(II) Trifluoromethane-
sulfonate-2-Hydrate, [Pd2{(R,R)-tetraphos}2](OTf)4‚2H2O. A
solution of (S,S)-tetraphos (0.10 g, 0.15 mmol) in dichloromethane
(6 mL) was treated with a solution of [PdCl2(SEt2)2] (0.053 g, 0.15
mmol) in dichloromethane (5 mL). After the mixture was stirred
for 5 min, TMSOTf (0.067 g, 0.30 mmol) was added to the yellow
solution, which was immediately decolorized. The reaction mixture
was evaporated to dryness, and the colorless residue was recrystal-
lized by dissolution in wet acetonitrile (8 mL) and slow addition
of diethyl ether (12 mL). The pure complex was thus isolated as a
colorless solid. Yield: 0.14 g (87%). mp: 312-315 °C. [R]1D8:
-311.7 (c 1, acetone). Anal. Calcd for C88H84F12O12P8Pd2S4‚
2H2O: C, 48.34; H, 4.06. Found: C, 48.26; H, 4.14. 31P{1H} NMR
(acetone-d6): δ 51.6 (br s, 4 P, PPh), 56.0 (br s, 4 P, PPh2).
as a bright yellow solid. Yield: 0.17 g (90%). mp: 231-234 °C.
18
546
[R] : +16.1 (c 1, acetone). Anal. Calcd for C44H42F6NiO6P4S2:
C, 51.43; H, 4.12. Found: C, 51.19; H, 4.35. 31P{1H} NMR
(acetone-d6): δ 45.4 (m, 2 P, 2JPP(trans) ) 151 Hz, PPh2), 110.3 (m,
2 P, 2JPP(trans) ) 151 Hz, PPh). ES MS: m/z 1069.3 [M + 2OTf +
CH3CN + H]+, 729.3 [M]+, 366.3 [M + H]2+. The crystallographic
determination was carried out on a deep red crystal of the complex
that had not been exposed to evacuation.
[T-4-[(MPd,MPd)-(4RP,7RP)]]-(+)-Bis(1,1,4,7,10,10-hexaphenyl-
1,4,7,10-tetraphosphadecane)dipalladium(0), (MPd,MPd)-[Pd2-
{(R,R)-tetraphos}2]. A solution of (S,S)-tetraphos (0.23 g, 0.35
mmol) and [Pd(PPh3)4] (0.4 g, 0.35 mmol) in toluene (5 mL) was
stirred for 0.5 h. The yellow solution was then filtered through Celite
to remove a trace of palladium. The solvent was removed from the
solution under vacuum, and the resulting oil was dissolved in diethyl
ether (8 mL). Bright yellow needles of the product separated
overnight, which were collected, washed with diethyl ether, and
dried under vacuum. Yield: 0.23 g (85%). mp: 125-128 °C (dec).
[T-4-[MNi,(4RP,7RP)]]-(-)-(1,1,4,7,10,10-Hexaphenyl-1,4,7,10-
tetraphosphadecane)nickel(0), (MNi)-[Ni{(R,R)-tetraphos}]. A
mixture of [Ni(COD)2] (0.086 g, 0.30 mmol) and (S,S)-tetraphos
(0.20 g, 0.30 mmol) was dissolved in benzene (5 mL) under argon.
After 1 h, the solvent was removed from the bright yellow solution
[R]18: +40.2 (c 1, benzene). Anal. Calcd for C84H84P8Pd2: C,
D
(28) Clark, R. J. H.; Natile, G.; Belluco, U.; Cattalini, L.; Filippin, C. J.
Chem. Soc. A 1970, 659-663.
(29) Coulson, D. R. Inorg. Synth. 1990, 28, 107-109.
(30) Drew, D.; Doyle, J. R. Inorg. Synth. 1990, 28, 346-347.
(31) Ugo, R.; Cariati, F.; La Monica, G. Inorg. Synth. 1990, 28, 123-126.
64.92; H, 5.45. Found: C, 64.56; H, 5.41. 31P{1H} NMR (C6D6):
δ 33.7 (s). A small quantity of the complex was recrystallized from
hot benzene, whereupon a yellow plate of the 2-benzene solVate
was isolated for the X-ray crystal structure determination.
Inorganic Chemistry, Vol. 46, No. 19, 2007 8069

Kitto et al.
[SP-4-(4RP,7RP)]-(-)-(1,1,4,7,10,10-Hexaphenyl-1,4,7,10-tet-
raphosphadecane)platinum(II) Hexafluorophosphate, [Pt{(R,R)-
tetraphos}](PF6)2. A solution of (S,S)-tetraphos (0.10 g, 0.15 mmol)
and [PtCl2(COD)] (0.055 g, 0.15 mmol) in acetonitrile (7 mL) was
stirred for 10 min, and then a solution of ammonium hexafluoro-
phosphate (0.50 g, 3.07 mmol) in water (3 mL) was added. After
the mixture was stirred for a further 10 min, the solvents were
removed from the solution by evacuation, and the pale yellow
residue was stirred with water (10 mL) to dissolve the ammonium
salts. The solid that remained was filtered off, washed with water
and diethyl ether, and recrystallized from acetonitrile (5 mL) by
the addition of diethyl ether (10 mL); the pure product crystallized
from the solution as colorless microcrystals. Yield: 0.16 g (93%).
mp: 270-272 °C. [R]1D8: -35.0 (c 1, acetone). Anal. Calcd for
C42H42F12P6Pt: C, 43.65; H, 3.66. Found: C, 43.67; H, 3.92.
31P{1H} NMR (acetone-d6): δ 42.0 (m + dm, 2 P, 2JPP(trans) ) 272
-117.6 (c 1, benzene). Anal. Calcd for C84H84P8Pt2: C, 58.27; H,
4.89. Found: C, 58.14; H, 4.76. 31P{1H} NMR (C6D6): δ 29.2
(m, 4 P, PPh2), 38.4 (m, 4 P, PPh).
Crystal Structures. Crystal data and experimental parameters
for the complexes are given in Table 1. The software used for the
structural solutions of the four complexes was SIR92.35
Spectroscopy and Electrochemistry. In situ UV-vis spectra
were obtained with use of a Varian Cary 5E spectrophotometer in
an optically transparent thin layer electrochemical (OTTLE) cell
(path length ) 0.05 cm) at -20 °C using a platinum mesh working
electrode. The typical exhaustive electrolysis period for the one-
electron reduction of 1 mM analyte in dichloromethane containing
0.5 M [(n-Bu)4N]PF6 was 1 h. Voltammetric experiments were
conducted with a computer-controlled Eco Chemie mAutolab III
potentiostat. The electrochemical cell was jacketed in a glass sleeve
and cooled from 20 to -40 °C using a Lamda RL6 variable
temperature methanol-circulating bath. Cyclic voltammetry experi-
ments were conducted with 1 mm diameter planar glassy carbon
(GC) or platinum (Pt) electrodes.
1
2
Hz, JPtP ) 2413 Hz, PPh2), 95.7 (m + dm, 2 P, JPP(trans) ) 283
Hz, 1JPtP ) 2133 Hz, PPh), -142.8 (sept, 2 P, 1JPF ) 708 Hz, PF6).
ES MS: m/z 1178.1 [M + 2PF6 + Na]+, 1013.3 [M + PF6]+, 866.2
[M + H]+, 432.8 [M]2+
.
Acknowledgment. We thank Dr Ian Crossley for per-
forming the simulation of the 31P{1H} NMR spectrum of
[Ni{(R,R)-tetraphos}](OTf)2.
[SP-4-(4RP,7RP)]-(-)-Bis(1,1,4,7,10,10-hexaphenyl-1,4,7,10-
tetraphosphadecane)diplatinum(II) trifluoromethanesulfonate-
4.5-Hydrate, [Pt2{(R,R)-tetraphos}2](OTf)4‚4.5H2O). This com-
pound was prepared and isolated as previously described.9 31P{1H}
NMR (300 MHz; acetone-d6): δ 42.2 (br m, 4 P, 1JPtP ) 2236 Hz,
PPh), 47.8 (br m, 4 P, 1JPtP ) 2405 Hz, PPh2). ES MS: m/z 1165.2
Supporting Information Available: Additional crystallographic
data in CIF format and electrochemical data. This material is
available free of charge via the Internet at http://pubs.acs.org.
[M + 2OTf + 2H]2+, 866.5 [M + H]2+
.
IC700912Q
[T-4-(4RP,7RP)]]-(-)-Bis(1,1,4,7,10,10-hexaphenyl-1,4,7,10-tet-
raphosphadecane)diplatinum(0), [Pt2{(R,R)-tetraphos}2]. A mix-
ture of (S,S)-tetraphos (0.20 g, 0.30 mmol) and [Pt(PPh3)4] (0.30
g, 0.24 mmol) was dissolved in benzene (5 mL). After the mixture
was stirred for 0.5 h, the solvent was removed under vacuum from
the yellow solution. The yellow oil that remained was dissolved in
diethyl ether (8 mL), whereupon a yellow solid separated im-
mediately. The solid was collected, washed thoroughly with diethyl
ether, and dried under vacuum to give the pure complex as a deep
yellow powder. Yield: 0.11 g (53%). mp: 162-166 °C. [R]1D8:
(32) Watkin, D. J.; Prout, C. K.; Carruthers, J. R.; Betteridge, P. W.
CRYSTALS, issue 10; Chemical Crystallography Laboratory: Oxford,
U.K., 1996.
(33) Mackay, S.; Gilmore, C. J.; Edwards, C.; Stewart, N.; Shankland, K.
maXus Computer Program for the Solution and Refinement of Crystal
Structures; Nonius: Delft, The Netherlands; MacScience: Japan;
University of Glasgow: Glasgow, U.K., 1999.
(34) Rae, A. D. RAELS2000; Australian National University: Canberra,
Australia, 2000.
(35) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Burla,
M. C.; Polidari, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 435.
8070 Inorganic Chemistry, Vol. 46, No. 19, 2007