Free-energy relationships between the proton and hydride donor abilities of [HNi(diphosphine)2]+ complexes and the half-wave potentials of their conjugate bases

Article abstract of DOI:10.1021/om0100582

A linear free-energy relationship exists between the half-wave potentials of the (II/I) couples of Ni(diphosphine)₂ complexes and the hydride donor ability (ΔG°_H^-) of the corresponding [HNi(diphosphine)₂]⁺ complexes. A similar correlation is observed between the half-wave potentials of the (I/O) couples of Ni(diphosphine)₂ complexes and the pK_a values (or ΔG°_H⁺) of the corresponding [HNi(diphosphine)₂]⁺ complexes. As a result, it is possible to use the potentials of these two couples to predict the free energies of all three Ni-H bond cleavage reactions, ΔG°_H+, ΔG°_H·, and ΔG°_H-, for this class of nickel complexes. The molecular structures of Ni(Et₂PCH₂CH₂ PPh₂)₂ and Ni(Ph₂PCHCHPPh₂)₂ were determined by single-crystal X-ray diffraction studies.

Full text of DOI:10.1021/om0100582

1832
Organometallics 2001, 20, 1832-1839
F r ee-En er gy Rela tion sh ip s betw een th e P r oton a n d
Hyd r id e Don or Abilities of [HNi(d ip h osp h in e)2]+
Com p lexes a n d th e Ha lf-Wa ve P oten tia ls of Th eir
Con ju ga te Ba ses
Douglas E. Berning,† Alex Miedaner,† Calvin J . Curtis,† Bruce C. Noll,‡
Mary C. Rakowski DuBois,‡ and Daniel L. DuBois*,†
National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado,
and Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado
Received J anuary 24, 2001
A linear free-energy relationship exists between the half-wave potentials of the (II/I) couples
-
of Ni(diphosphine)2 complexes and the hydride donor ability (∆G°H ) of the corresponding
[HNi(diphosphine)2]+ complexes. A similar correlation is observed between the half-wave
+
potentials of the (I/0) couples of Ni(diphosphine)2 complexes and the pKa values (or ∆G°H
)
of the corresponding [HNi(diphosphine)2]+ complexes. As a result, it is possible to use the
potentials of these two couples to predict the free energies of all three Ni-H bond cleavage
+
•
-
reactions, ∆G°H , ∆G°H , and ∆G°H , for this class of nickel complexes. The molecular
structures of Ni(Et2PCH2CH2PPh2)2 and Ni(Ph2PCHCHPPh2)2 were determined by single-
crystal X-ray diffraction studies.
In tr od u ction
values of metal hydride complexes and the one-electron
oxidation potentials of the conjugate bases of the
hydrides.3-7 The constants (kcal/mol) shown in eqs 2 and
3 are appropriate when acetonitrile is used as a solvent
and the potentials are referenced to the ferrocene/
ferrocenium couple taken as 0 V. Recently, free-energy
measurements for reaction 3 have been reported. One
study was based on hydride transfer reactions from
organic reagents of known hydride donor ability.8 The
other was based on a thermodynamic cycle that uses
the pKa values of the hydrides and the two-electron
oxidation potentials of the conjugate bases of the hy-
drides to calculate the free energy of hydride transfer,
as shown in eq 3.9
For many metal hydrides, it is not possible to obtain
reversible electrochemical data for the conjugate base
because rapid dimerization reactions occur following
oxidation.3-7 The complexes formed on deprotonation of
the HM(diphosphine)2 and [HM(diphosphine)2]+ com-
plexes of the cobalt and nickel triads are unusual in that
they generally undergo two reversible one-electron
Transition metal hydride complexes are important
intermediates in a number of catalytic reactions, and a
better understanding of the factors controlling the
thermodynamic properties of the M-H bond would be
useful in designing catalytic reactions. The M-H bond
can be cleaved in three ways, as shown in reactions 1-3.
LnMH+ f LnM + H+
LnMH+ f LnM+ + H•
∆G°H+ ) 1.37pKa
(1)
∆G°H• ) 1.37pKa +
23.06E°(I/0) + 53.6 (2)
LnMH+ f LnM2+ + H-
∆G°H- ) 1.37pKa +
46.1E°(II/0) + 79.6 (3)
The free energy of deprotonation, reaction 1, is generally
obtained by measuring the equilibrium constant for a
proton transfer reaction between the metal hydride and
a base whose pKa value is known.1 The energetics for
the homolytic cleavage of the M-H bond, reaction 2,
have been measured by several techniques.2 One of the
easiest is based on a thermodynamic cycle that uses pKa
(2) (a) Simo˜es, J . A. M.; Beauchamp, J . L. Chem. Rev. 1990, 90, 629-
688. (b) Kiss, G.; Zhang, K.; Mukerjee, S. L.; Hoff, C. D. J . Am. Chem.
Soc. 1990, 112, 5657-5658. (c) Schock, L. E.; Marks, T. J . J . Am. Chem.
Soc. 1988, 119, 7701-7715. (d) Wei, M.; Wayland, B. B. Organome-
tallics 1996, 15, 4681-4683.
† National Renewable Energy Laboratory.
‡ University of Colorado.
(3) Tilset, M.; Parker, V. D. J . Am. Chem. Soc. 1989, 111, 6711-
6717; (b) 1990, 112, 2843.
(1) (a) Kristja´nsdo´ttir, S. S.; Norton, J . R. In Transition Metal
Hydrides: Recent Advances in Theory and Experiment; Dedieu, A., Ed.;
VCH: New York, 1991; pp 309-359. (b) J ordan, R. F.; Norton, J . R. J .
Am. Chem. Soc. 1982, 104, 1255-1263. (c) Moore, E. J .; Sullivan, J .
M.; Norton, J . R. J . Am. Chem. Soc. 1986, 108, 2257-2263. (d)
Kristja´nsdo´ttir, S. S.; Moody, A. E.; Weberg, R. T.; Norton, J . R.
Organometallics 1988, 7, 1983-1987. (e) J essop, P. G.; Morris, R. H.
Coord. Chem. Rev. 1992, 121, 155. (f) Bullock, R. M.; Song, J .-S.; Szalda,
D. J . Organometallics 1996, 15, 2504-2516. (g) Davies, S. C.; Hend-
erson, R. A.; Hughes, D. L.; Oglieve, K. E. J . Chem. Soc., Dalton Trans.
1998, 425-431. (h) Abdur-Rashid, K.; Fong, T. P.; Greaves, B.; Gusev,
D. G.; Hinman, J . G.; Landau, S. E.; Lough, A. J .; Morris, R. H. J .
Am. Chem. Soc. 2000, 122, 9155-9171.
(4) Parker, V. D.; Handoo, K. L.; Roness, F.; Tilset, M. J . Am. Chem.
Soc. 1991, 113, 7493-7498.
(5) Ryan, B. O.; Tilset, M.; Parker, V. D. J . Am. Chem. Soc. 1990,
112, 2618-2626.
(6) Wayner, D. D. M.; Parker, V. D. Acc. Chem. Res. 1993, 26, 287-
294.
(7) Wang, D.; Angelici, R. J . J . Am. Chem. Soc. 1996, 118, 935-
942.
(8) (a) Sarker, N.; Bruno, J . W. J . Am. Chem. Soc. 1999, 121, 2174-
2180. (b) Sarker, N.; Bruno, J . W. Organometallics 2001, 20, 55-61.
(9) Berning, D. E.; Noll, B. C.; DuBois, D. L. J . Am. Chem. Soc. 1999,
121, 11432-11447.
10.1021/om0100582 CCC: $20.00 © 2001 American Chemical Society
Publication on Web 03/22/2001

Proton and Hydride Donor Abilities
Organometallics, Vol. 20, No. 9, 2001 1833
oxidations or a single reversible two-electron oxida-
tion.9-11 If two reversible one-electron oxidations of the
deprotonated metal hydride are observed and one of the
three bond-cleavage reactions, 1-3, can be measured,
then the free energy for all three reactions can be
determined. For those complexes undergoing reversible
two-electron processes, measurement of the pKa values
allows the hydride transfer potentials to be calculated,
but free energies for the homolytic bond dissociation
reaction cannot be obtained. Because the Ni(diphos-
phine)2 complexes formed on deprotonation of the [HNi-
(diphosphine)2]+ complexes undergo two reversible one-
electron oxidation reactions, they represent an ideal
opportunity for studying how various electronic and
steric parameters influence all three bond-cleavage
reactions, 1-3.
In this publication, we demonstrate that a linear free-
energy correlation exists between the Ni(II/I) half-wave
potentials for Ni(diphosphine)2 complexes and the free
energy associated with hydride transfer reactions of
[HNi(diphosphine)2]+ complexes. A similar free-energy
relationship exists between the Ni(I/0) half-wave po-
tentials and the pKa values of the Ni-H complexes.
has been reported previously.14 Spectral data are pro-
vided in the Experimental Section. Both complexes were
characterized by X-ray diffraction studies, as discussed
below.
Protonation of Ni(dppv)2 and Ni(dedpe)2 with p-bromo-
aniline‚HBF4 and anisidine‚HBF4, respectively, results
in formation of the corresponding Ni-H complexes (step
2 of reaction 5). The formation of the hydride species is
confirmed by their distinctive hydride resonances in the
1H NMR spectrum. For [HNi(dedpe)2]+, a triplet of
triplets is observed at -13.1 ppm in the 1H NMR
spectrum. This pattern arises from coupling of the
hydride ligand to two sets of two equivalent phosphorus
atoms. The structure of this complex is likely a trigonal
bipyramid, with an apical hydride ligand similar to
those observed for HCo(dppp)2 and HCo(dppe)2 (where
dppp is bis(diphenylphosphino)propane and dppe is bis-
(diphenylphosphino)ethane).15,16 The equivalence of the
two phosphorus nuclei of the diethylphosphino groups
and the two phosphorus nuclei of the diphenylphosphino
groups is attributed to an exchange process. In an
attempt to slow the exchange, low-temperature 31P
NMR and 1H NMR spectra were collected in acetone
down to -90 °C. The high-temperature spectra broad-
ened at the lowest temperature, but no low-temperature
limiting spectra were observed. An IR band assigned
Resu lts
Syn t h eses a n d Sp ect r a l Ch a r a ct er iza t ion of
Liga n d s a n d Meta l Com p lexes. A new ligand, Et2-
PCH2CH2PPh2 (dedpe), was prepared using the free-
radical addition of diethylphosphine to vinyldiphe-
nylphosphine, reaction 4.
to the Ni-H stretching mode is observed at 1934 cm-1
.
The hydride resonance of [HNi(dppv)2]+ is a quintet at
-11.9 ppm. This complex is also thought to be a
distorted trigonal bipyramid, and the apparent equiva-
lence of the phosphorus atoms indicated by the 31P and
1H NMR spectra is also attributed to a fluxional process.
p Ka Mea su r em en ts. The pKa values of [HNi(diphos-
phine)2]+ complexes can be obtained by measuring
equilibrium constants for the reaction of the hydrides
with bases whose pKa values are known in acetonitrile
or its reverse, reaction 6.1,9 The pKa value of the hydride
This free-radical reaction is a common method used for
the synthesis of bidentate phosphine ligands.12 The 31P
NMR spectrum of dedpe consists of two doublets arising
from the coupling of the two different phosphorus atoms.
The coupling constants and chemical shifts are in the
range expected (see Experimental Section).13 Reaction
of Ni(COD)2 with dedpe produces Ni(dedpe)2 (step 1 of
reaction 5).
is the sum of the pKa value of the protonated base and
+
pKeq. In the current study, triethylamine (pKBH
)
18.5)17 was reacted with [HNi(dedpe)2]+ to form equi-
librium mixtures of the Ni(0) and Ni-H complexes.
Similarly, Ni(dppv)2 was dissolved in acetonitrile and
benzonitrile solutions containing different ratios of
anisidine and protonated anisidine (pKBH ) 11.3).1c
+
Constant ratios of the metal complexes were observed
after approximately 30 min at room temperature (21
1
°C). Integrations of the H and 31P NMR spectra were
used to determine the equilibrium constants.
Ni(diphosphine)2 + BH+
[HNi(diphosphine)2]+ + B (6)
/
This complex exhibits an AA′XX′ 31P NMR spectrum,
as expected. Ni(dppv)2 [where dppv is bis(diphenylphos-
phino)ethylene] was prepared in a similar manner and
diphosphine
dppv
B
Keq
87
75
0.016
solvent
pKa
anisidine
acetonitrile
benzonitrile
benzonitrile
13.2 ( 0.2
13.1 ( 0.2
20.3 ( 0.2
(10) Miedaner, A.; Haltiwanger, R. C.; DuBois, D. L. Inorg. Chem.
1991, 30, 417-427.
dedpe
NEt3
(11) (a) Pilloni, G.; Zotti, K. G.; Martelli, M. J . Electroanal. Chem.
1974, 50, 295-297. (b) Pilloni, G.; Vecchi, E.; Martelli, M. J . Electroa-
nal. Chem. 1973, 45, 483-487. (c) Longato, B.; Riello, L.; Bandoli, G.;
Pilloni, G. Inorg. Chem. 1999, 38, 2818-2823.
(12) (a) Uriarte, R.; Mazanec, T. J .; Tau, K. D.; Meek, D. W. Inorg.
Chem. 1980, 19, 79. (b) DuBois, D. L.; Meyers, W. H.; Meek, D. W. J .
Chem. Soc., Dalton Trans. 1975, 1011-1015. (c) Heitkamp, S.; Lebbe,
T.; Spiegel, G. U.; Stelzer, O. Z. Naturforsch. 1987, 42b, 177.
(13) Garrou, P. E. Chem. Rev. 1981, 81, 229.
(14) Proft, B.; Porschke, K.-R. Z. Naturforsch. 1993, 48b, 919-927.
(15) Holah, D. G.; Hughes, A. N.; Maciaszek, S.; Magnuson, V. R.;
Parker, K. O. Inorg. Chem. 1985, 24, 3956-3962.
(16) Ciancanelli, R.; Rakowski DuBois, M. C.; DuBois, D. L. Un-
published results.
(17) Kolthoff, I. M.; Chantooni, M. K., J r.; Bhowmik, S. J . Am. Chem.
Soc. 1968, 90, 23-28.

1834 Organometallics, Vol. 20, No. 9, 2001
Berning et al.
Ta ble 1. Cr ysta llogr a p h ic Da ta for Ni(d ed p e)2
(d ed p e ) Et2P CH2CH2P P h 2) a n d Ni(d p p v)2 (d p p v )
P h 2P CHCHP P h 2)
Ni(dedpe)2
Ni(dppv)2
empirical formula
formula mass
crystal system
space group
a, Å
b, Å
c, Å
R, deg
â, deg
C36H48NiP4
663.33
C52H44NiP4
851.46
triclinic
orthorhombic
Pbca
18.4789(9)
19.0693(9)
19.6335(9)
90
P1h
11.5923(6)
17.0717(8)
22.9546(11)
72.0620(10)
81.5110(10)
83.4050(10)
4262.9(4)
4
90
90
γ, deg
volume, Å3
Z, formula units/cell
density(calcd), Mg‚m-3
abs coeff, mm-1
temp, K
6918.4(6)
8
1.274
0.769
143(2)
R1 ) 0.0382,
wR2 ) 0.0993
8281
R1 ) 0.0544
wR2 ) 0.1096
1.038
1.327
0.641
143(2)
R1 ) 0.0523,
wR2 ) 0.1273
19 110
R1 ) 0.0742
wR2 ) 0.1380
1.051
F igu r e 1. Cyclic voltammogram of [Ni(dppv)2](BF4)2 in a
0.3 N NEt4BF4 acetonitrile solution. The scan rate was
0.05 V/s, the working electrode was glassy carbon, and
the potentials are given in volts vs the ferrocene/ferroce-
nium couple. Cathodic current is up, and anodic current is
down.
final R indicesa
[I > 2σ(I)]
no. of reflns obsd
R indices (all data)
goodness-of-fitb on F2
pKa values of 13.1 ( 0.2 and 13.2 ( 0.2 were determined
for Ni(dppv)2 in benzonitrile and deuteroacetonitrile,
respectively. Each value represents five different mea-
surements. Similarly, a pKa value of 20.3 ( 0.2 was
determined for Ni(dedpe)2 in benzonitrile. As discussed
in a preceding publication, benzonitrile has an advan-
tage over acetonitrile for these complexes because of the
higher solubility of the Ni(0) complexes in benzonitrile
compared to acetonitrile.9
largest diff peak and hole 0.614 and -0.802 2.051 and -1.498
2
R1 ) (||Fo| - |Fc||)/|Fo|; wR2 ) w(F - Fc2)2/w(Fo2)2. GooF
a
b
x
o
) S ) w(F - Fc2)2/(M - N), where M is the number of reflec-
tions and N is the number of parameters refined.
2
x
o
Ta ble 2. Selected Bon d Dista n ces a n d An gles for
Ni(d ed p e)2 a n d Ni(d p p v)2
Bond Distances (Å) for Ni(dedpe)2
Ni(1)-P(1)
Ni(1)-P(2)
P(1)-C(1)
P(1)-C(13)
P(2)-C(15)
P(3)-C(19)
P(3)-C(31)
P(4)-C(33)
C(13)-C(14)
2.1407(5)
2.1527(5)
1.8388(17)
1.8619(17)
1.8581(18)
1.8361(18)
1.8574(17)
1.8531(17)
1.534(2)
Ni(1)-P(3)
Ni(1)-P(4)
P(1)-C(7)
P(2)-C(17)
P(2)-C(14)
P(3)-C(25)
P(4)-C(35)
P(4)-C(32)
C(31)-C(32)
2.1437(5)
2.1654(5)
1.8487(17)
1.8462(18)
1.8610(17)
1.8495(17)
1.8479(18)
1.8533(19)
1.533(3)
Electr och em ica l Mea su r em en ts. A cyclic voltam-
mogram of [Ni(dppv)2](BF4)2 in acetonitrile is shown in
Figure 1. It consists of two reversible one-electron
reductions at -0.52 and -0.83 V vs the ferrocene/
ferrocenium couple. The peak-to-peak separations of the
Ni(II/I) and Ni(I/0) couples are 65 and 67 mV, respec-
tively, compared to an expected value of 60 mV for a
fully reversible one-electron process with no resistance
effects, and the ratios of the cathodic and anodic peak
heights are unity within experimental error (1.0 ( 0.1).
The waves are diffusion-controlled, as indicated by a
linear plot of the peak height vs the square root of the
scan rate. The electrochemical behavior of Ni(dedpe)2
is comparable with half-wave potentials at -0.99 V (II/
I) and -1.08 V (I/0) vs the ferrocene/ferrocenium couple
in acetonitrile.
Bond Distances (Å) for Ni(dppv)2
Ni(1)-P(1)
Ni(1)-P(2)
P(1)-C(1)
2.1414(6)
2.1586(6)
1.836(2)
1.850(2)
1.834(2)
1.833(2)
1.851(2)
1.836(2)
1.334(3)
Ni(1)-P(3)
Ni(1)-P(4)
P(1)-C(13)
P(2)-C(21)
P(2)-C(15)
P(3)-C(39)
P(4)-C(40)
P(4)-C(47)
C(39)-C(40)
2.1484(6)
2.1594(6)
1.842(2)
1.832(2)
1.852(2)
1.838(2)
1.835(2)
1.850(2)
1.331(3)
P(1)-C(7)
P(2)-C(14)
P(3)-C(27)
P(3)-C(33)
P(4)-C(41)
C(13)-C(14)
Bond Angles (deg) for Ni(dedpe)2
117.645(19) P(1)-Ni(1)-P(2)
116.976(18) P(1)-Ni(1)-P(4)
90.583(18) P(2)-Ni(1)-P(4)
Str u ctu r a l Stu d ies. Single crystals of Ni(dedpe)2
and Ni(dppv)2 were grown by deprotonation of the
corresponding hydrides with an appropriate base in
acetonitrile. Crystallographic data for these two com-
plexes are given in Table 1. Two independent molecules
are observed in the unit cell of Ni(dppv)2. One of these
independent molecules exhibits a 10% disorder with a
third molecule. Selected bond distances and angles for
Ni(dedpe)2 and Ni(dppv)2 are given in Table 2. Drawings
of Ni(dedpe)2 and Ni(dppv)2 are shown in Figure 2. Both
complexes exhibit a distorted tetrahedral structure. The
dihedral angle between the two planes formed by the
Ni atom and the two phosphorus atoms of each dedpe
ligand is 84.9°. The two independent molecules of Ni-
(dppv)2 have dihedral angles of 83.6° and 81.2°. The
mean of the P-Ni-P bond angles of the diphosphine
ligands is 90.9° for Ni(dedpe)2 and 90.7° for Ni(dppv)2,
respectively. These values are significantly larger than
P(1)-Ni(1)-P(3)
P(3)-Ni(1)-P(2)
P(3)-Ni(1)-P(4)
91.297(17)
115.375(18)
127.538(19)
C(13)-P(1)-Ni(1) 106.49(5)
C(14)-P(2)-Ni(1) 106.81(6)
C(31)-P(3)-Ni(1) 106.51(6)
C(32)-P(4)-Ni(1) 105.79(6)
C(13)-C(14)-P(2) 110.11(11) C(31)-C(32)-P(4) 108.33(12)
C(14)-C(13)-P(1) 108.96(11) C(32)-C(31)-P(3) 107.80(12)
Bond Angles (deg) for Ni(dppv)2
P(1)-Ni(1)-P(3)
P(3)-Ni(1)-P(2)
P(3)-Ni(1)-P(4)
131.83(2)
115.65(2)
91.05(2)
P(1)-Ni(1)-P(2)
P(1)-Ni(1)-P(4)
P(2)-Ni(1)-P(4)
90.09(2)
114.43(2)
115.85(2)
C(13)-P(1)-Ni(1) 105.13(8)
C(14)-P(2)-Ni(1) 105.22(7)
C(39)-P(3)-Ni(1) 106.11(7)
C(40)-P(4)-Ni(1) 106.17(7)
C(13)-C(14)-P(2) 117.22(17) C(39)-C(40)-P(4) 118.08(17)
C(14)-C(13)-P(1) 118.18(17) C(40)-C(39)-P(3) 118.59(17)
P-Ni-P bond angles (85-86°) observed for [Ni(diphos-
phine)2]2+ complexes with similar ligands, indicating
that these ligands accommodate changes in bite sizes

Proton and Hydride Donor Abilities
Organometallics, Vol. 20, No. 9, 2001 1835
F igu r e 2. Drawings of Ni(dedpe)2 and Ni(dppv)2 showing the atom-numbering schemes.
Ta ble 3. Th er m od yn a m ic P r op er ties of [HNi(d ip h osp h in e)2]+ Com p lexes (k ca l/m ol) a n d Ha lf-Wa ve
P oten tia ls (V vs fer r ocen e) of Ni(d ip h osp h in e)2/[Ni(d ip h osp h in e)2]2+ Com p lexes in Aceton itr ile
b
b
+c
La
E1/2(II/I)
E1/2(I/0)
pKa (eq 7)b
∆G°H (eqs 2&7, 8&9)
∆G°H (eq 8)
pKBH
•
-
dppv
-0.52
-0.70
-0.89
-0.99
-1.16
-1.35
-0.83
-0.88
-1.33
-1.08
-1.29
-1.35
13.2 (13.6)
14.2 (14.6)
23.9 (24.0)
20.2 (18.8)
23.8 (23.2)
24.3 (25.3)
52.6 (53.2, 52.9)
52.8 (53.4, 53.8)
55.6 (55.8, 54.9)
56.4 (54.5, 55.5)
56.5 (55.6, 56.4)
55.8 (57.1, 57.6)
66.4 (66.9)
62.7 (63.7)
61.2 (60.4)
59.8 (58.7)
56.0 (55.7)
50.7 (51.6)
7.0
9.7
10.8
11.8
14.6
18.5
dpped
dmppd
dedpe
deped
dmped
a
Ligand abbreviations: dppv ) Ph2PCHCHPPh2, dppe ) Ph2PCH2CH2PPh2, dmpp ) Me2PCH2CH2CH2PMe2, dedpe ) Et2PCH2CH2PPh2,
b
depe ) Et2PCH2CH2PEt2, and dmpe ) Me2PCH2CH2PMe2. Data not in parentheses represent experimentally measured values, and
data in parentheses represent values calculated from the observed half-wave potentials and the indicated equations. c Calculated using
d
-
eq 14. Electrochemical data and ∆G°H taken from ref 9.
equally well.9,10,18 The nearly identical P-Ni-P bite
angles for the ethylene and vinyl bridges suggest that
differences in half-wave potentials observed for the Ni-
(dppe)2 and Ni(dppv)2 complexes (Table 3) are caused
by differences in the electron-donating ability of the
ligand bridge and not by differences in the chelate bites
of these two ligands.9,10
Discu ssion
In previous studies of [HNi(diphosphine)2](BF4) and
[HPt(diphosphine)2](PF6) complexes, the diphosphine
ligands dmpe, depe, dppe, and dmpp were chosen to
provide information on the effects of substituents and
chelate bite sizes on the thermodynamic properties of
these hydrides.9 It was found that the hydride donor
ability of these complexes is greatest for ligands with
electron-donating substituents and a small chelate bite.
Platinum complexes were also found to be better hydride
donors than the corresponding nickel complexes. The
pKa values of these nickel and platinum hydrides
depend on the electron-donating ability of the substit-
uents and on the metal, but not on the chelate bite size
of the ligand. On the basis of the limited data available
for the nickel hydride complexes, it appeared that pKa
values correlated best with the Ni(I/0) couple, but three
of the Ni(I/0) potentials were very similar, which
prevented drawing firm conclusions. The hydricity of the
nickel hydride complexes paralleled the Ni(II/I) poten-
tials, but again, these preliminary observations were
based on only four data points. In this study, we have
attempted to extend this data set by carefully selecting
two metal complexes for further study. The previously
synthesized complex, Ni(dppv)2, was chosen because its
(II/I) (-0.52 V) and (I/0) (-0.83 V) couples are more
positive than those of Ni(dppe)2 (-0.70 and -0.88 V,
respectively).10,14 Similarly, the (I/0) couple of Ni(dedpe)2
should lie between those of Ni(dppe)2 (-0.89 V) and the
three basic Ni(0) complexes, Ni(dmpe)2 (-1.39 V), Ni-
The average Ni-P bond distance in both complexes
is 2.15 Å. This value is 0.06 Å shorter on average than
the values found for analogous Ni(II) complexes with
square-planar, tetrahedrally distorted square-planar,
trigonal-bipyramidal, and square-pyramidal geometries.
Although the orbital overlaps, steric interactions, and
geometries are all different for these complexes, the
Ni-P bond distances all fall within the range 2.21-2.23
Å.9,10,18 The shortening of the Ni-P bond lengths on
reduction from Ni(II) to Ni(0) is also accompanied by a
small increase (0.02 Å or more) in the average P-C bond
lengths of the diphosphine ligands. The contraction of
the Ni-P bond distances and the lengthening of the
P-C bond distances are consistent with increased back-
bonding to the diphosphine ligands in the Ni(0) com-
plexes compared to the Ni(II) complexes.19
(18) (a) Alyea, C. E.; Meek, D. W. Inorg. Chem. 1972, 11, 1029. (b)
Mari, A.; Gleizes, A.; Dartiguenave, M.; Dartiguenave, Y. Inorg. Chim.
Acta 1981, 52, 83-85. (c) Po¨rschke, K.; Mynott, R.; Kru¨ger, C.; Roma˜o,
M. J . Z. Naturforsch. 1984, 39b, 1076-1081. (d) Po¨rschke, K.; Mynott,
R.; Angermund, K.; Kru¨ger, C. Z. Naturforsch. 1985, 40b, 199-209.
(19) (a) Woska, D.; Prock, A.; Giering, W. P. Organometallics 2000,
19, 4629-4638. (b) Landis, C. R.; Feldgus, S.; Uddin, J .; Wozniak, C.
E. Organometallics 2000, 19, 4678-4886.

1836 Organometallics, Vol. 20, No. 9, 2001
Berning et al.
-
F igu r e 4. Plots of ∆GH (open circles) and ν(Ni-H) (solid
diamonds) for [HNi(diphosphine)2]X (X ) BF4 or PF6) vs
the potential of the E(II/I) couples for the corresponding
[Ni(diphosphine)2](BF4)2 complexes in acetonitrile.
F igu r e 3. Plot of pKa values for [HNi(diphosphine)2]X (X
) BF4 or PF6) complexes in acetonitrile vs the potentials
of the E(I/0) couples of the corresponding [Ni(diphosphine)2]-
(BF4)2 complexes in acetonitrile.
2
2
is the antibonding dx -y orbital. As this is the acceptor
orbital for hydride ligands in hydride transfer reactions,
it is also reasonable that the energy of this orbital would
correlate with the hydride acceptor ability of the [Ni-
(diphosphine)2]2+ complexes. The energy of this orbital
is affected by the electron donor ability of the substit-
uents on the diphosphine ligands and by a tetrahedral
distortion of these square-planar complexes. As the
substituents on the phosphorus atoms become more
electron donating, the energy of the LUMO increases.
That is, the potentials of the (II/I) couple become more
negative, and this leads to poorer hydride acceptors or
(depe)2 (-1.29 V), and Ni(dmpp)2 (-1.33 V), which have
very similar (I/0) potentials, as indicated.9 The value of
-1.08 V found for the (I/0) couple of Ni(dedpe)2 is nearly
the average of the (I/0) values of Ni(depe)2 and Ni-
(dppe)2, as expected.
Table 3 lists the half-wave potentials of the (II/I) and
(I/0) couples for a number of [Ni(diphosphine)2]2+ or Ni-
(diphosphine)2 complexes in acetonitrile vs ferrocene.
Measured pKa values for these complexes are also shown
with the free energies calculated for reactions 2 and 3
from pKa data and redox potentials. A plot of the pKa
values vs the potentials of the (I/0) couples is shown in
Figure 3. It can be seen from Figure 3 that the pKa
values exhibit a linear dependence on the (I/0) couple,
with the best-fit line given by eq 7.
2
2
better hydride donors. The energy of the dx -y orbital
also becomes more positive (lower) as the Ni complexes
undergo a tetrahedral distortion. This distortion is
observed for [Ni(dmpp)2]2+ and [Pt(dmpp)2]2+, and it
decreases the antibonding interaction between the σ
pKa ) -20.8E1/2(I/0) - 3.6
(7)
2
2
orbital of the phosphorus atoms and the metal dx -y
orbital. As a result, these complexes are better hydride
acceptors than might be expected based only on a
consideration of the electron-donating ability of the
methyl substituents. A more extensive treatment of the
relationship between this distortion and molecular
orbital energies has been presented previously.9,10
Typically M-H stretching frequencies have been
interpreted in terms of M-H bond strengths, the
homolytic cleavage of the M-H bond.20 However, ionic
cleavage of the M-H bond may also serve as a measure
of bond strength, particularly if the M-H bond is
This result is similar to those of Angelici and co-workers,
who demonstrated a linear correlation between the heat
of protonation and the oxidation potentials of several
series of complexes.7 To a first approximation, the
potential of the (I/0) couple is a measure of the energy
of the highest occupied molecular orbital of the Ni-
(diphosphine)2 complexes, and it is reasonable that this
energy would correlate well with the basicity of these
complexes. Although the correlation shown in Figure 3
is not perfect (r ) 0.986), it appears that an accurate
measurement of the potentials of the (I/0) couples for
Ni(diphosphine)2 or [Ni(diphosphine)2]2+ complexes
should enable the pKa values for the corresponding
[HNi(diphosphine)2]+ complexes to be predicted within
approximately 1 pKa unit.
•
strongly polarized. As can be seen from Table 3, ∆G°H
varies by less than 4 kcal/mol for the Ni complexes,
-
while ∆G°H varies by nearly 18 kcal/mol and the pKa
values by 11.1 units (15 kcal/mol). These data suggest
that it is the ionic component of the Ni-H bond that
varies the most in this series of complexes. If the Ni-H
bond is polarized with negative charge on the hydrogen
atom and positive charge on nickel, then the Ni-H
stretching frequencies may be more closely related to
the hydride transfer reaction than to the homolytic bond
dissociation energy. The heterolytic cleavage of the
-
Figure 4 shows a linear correlation for ∆G°H vs the
potential of the (II/I) couple. The best-fit line is given
by eq 8 (r ) 0.988).
∆G°H- ) 17.5E1/2(II/0) + 76.0
(8)
This represents the first correlation of the hydicity of
metal complexes with redox potentials. The potential
of this couple can be interpreted as a measure of the
energy of the lowest unoccupied molecular orbital
(LUMO) of the [Ni(diphosphine)2]2+ complexes, which
(20) (a) Pleune, B.; Morales, D.; Meunier-Prest, R.; Richard, P.;
Collange, E.; Fettinger, J . C.; Poli, R. J . Am. Chem. Soc. 1999, 121,
2209-2225. (b) For Si-H bonds: Chatgilialoglu, C.; Guerrini, A.;
Lucarini, M. J . Org. Chem. 1992, 57, 3405-3409. (c) For C-H bonds:
McKean, D. C. Int. J . Chem. Kinet. 1989, 21, 445.

Proton and Hydride Donor Abilities
Organometallics, Vol. 20, No. 9, 2001 1837
[HNi(diphosphine)2]+ h
[Ni(diphosphine)2]2+ + H- ∆G°H-
Ni-H bond can be regarded as a very long hydride
stretch; that is, the positive and negative charges are
effectively moved to infinity. For strongly acidic metal
hydrides, the M-H bond strength may correlate with
the acidity or pKa values of the metal hydrides, because
the M-H bond may be polarized in the M-H+ direction.
For the Ni complexes studied in this work the correla-
tion of the Ni-H stretching frequency with the Ni(I/0)
couple is poor (r ) 0.78). This is the redox couple that
shows a strong correlation with the pKa values. Simi-
larly, the correlation of the Ni-H stretching frequency
(10)
BH+ h B + H+
- + H+ h H2
1.37pKBH+ (11)
H
-76.0
(12)
net: [HNi(diphosphine)2]+ + BH+
[Ni(diphosphine)2]2+ + H2 + B
h
∆G°H
(13)
2
•
with ∆G°H for the Ni-H bond is poor (r ) 0.57). For
these Ni complexes, the Ni-H stretching frequencies
correlate best with the (II/I) couples of the Ni(diphos-
phine)2 complexes (r ) 0.998) or the hydride donor
abilities of the [HNi(diphosphine)2]+ complexes.
Reaction 10 is the hydride donor reaction, and the free
-
energy associated with this reaction is of course ∆G°H
.
Reaction 11 is the deprotonation of BH+ to form B with
+
a free energy of 1.37pKBH . Reaction 12 is the reverse
Using the plot in Figure 3 or eq 7 and the potentials
of the (I/0) couples, the pKa values of the corresponding
[HNi(diphosphine)2]+ complexes can be predicted with
reasonable accuracy. This is seen by a comparison of
the pKa values calculated using eq 7 (shown in paren-
theses in Table 3) with the experimentally measured
values. The agreement is typically within 1 pKa unit.
Similarly, from the plots in Figure 4 or eq 8 and the
(II/I) potentials, the hydride donor abilities of the
corresponding [HNi(diphosphine)2]+ complexes can be
of the heterolytic cleavage of hydrogen in acetonitrile.
The ∆G° value for this reaction (-76.0 kcal/mol) can be
calculated from the free energy for the homolytic cleav-
age of H2 in acetonitrile (103.6 kcal/mol)6 and the free
energies for the reduction of H+ (53.6 kcal/mol see
reaction 2)6,9 and H• (-26.0 kcal/mol)6 in acetonitrile.
The sum of reactions 10-12 is the protonation of a [HNi-
(diphophine)2]+ complex to form hydrogen, reaction 13.
The free energy of this reaction (∆G°H ) is the sum of
2
the free energies associated with reactions 10-12. If
-
+
estimated (∆G°H values shown in parentheses for Table
∆G°H is set to 0, then the value of pKHB for which
2
3). The agreement with the experimentally determined
value is generally within 1.5 kcal/mol. The estimated
pKa can also be used to calculate ∆G°H for reaction 2
reaction 13 will be at equilibrium can be calculated (eq
14).
•
using the corresponding equation and a measured (I/0)
potential. These values are listed in Table 3 as the first
member of the ordered pair shown in parentheses
pKBH+ ) (76.0 - ∆G°H-)/1.37
(14)
These values are shown for each complex in Table 3
(pKBH ). Acids having pKa values less than the pKBH
values shown in Table 3 are thermodynamically capable
of protonating the corresponding [HNi(diphosphine)2]+
complexes to form hydrogen. [HNi(diphosphine)2]+ com-
plexes are expected to be thermodynamically stable to
•
•
following the experimental value of ∆G°H . ∆G°H for
+
+
reaction 2 can also be calculated from the Ni(II/I) couple,
-
an estimated ∆G°H from eq 8, and eq 9.
∆G°H• ) ∆G°H- - 23.06E1/2(II/I) - 26.0
(9)
+
acids having pKa values greater than the pKBH values
shown in Table 3 and to bases whose pKBH values are
These values are shown as the second member of the
ordered pair shown in parentheses following the experi-
mental value. These values are also generally within
1.5 kcal of the experimental values. These results
indicate that all three free-energy values associated with
the Ni-H bond can be determined with reasonable
accuracy from the potentials of the (II/I) and (I/0)
couples of the Ni(diphosphine)2 complexes.
+
less than the pKa values shown in Table 3. For example,
in Table 3 the pKa value of [HNi(dppv)2]+ is 13.2 and
+
the pKBH value is 7.0. This means that protonated
p-bromoaniline (p-BrC6H4NH3+), whose pKBH value is
+
9.6, should protonate Ni(dppv)2 to form [HNi(dppv)2]+,
but this hydride should not be protonated by p-BrC6H4-
+
NH3 to form hydrogen and [Ni(dppv)2]2+. Thus a
The ability to predict these thermodynamic param-
eters can be extremely valuable for predicting and
understanding reactions of these complexes. For ex-
ample, the protonation of Ni(diphosphine)2 complexes
can lead to the formation of [HNi(diphosphine)2]+
complexes or to [Ni(diphosphine)2]2+ complexes and H2
depending on the strength of the acid used. Plots similar
to those in Figures 3 and 4 (or eqs 7 and 8) can be used
to predict the pKa range over which [HNi(diphos-
phine)2]+ complexes will be stable. Only acids with pKa
values lower than the pKa values listed in Table 3 will
be capable of protonating the Ni(diphosphine)2 com-
plexes to form a hydride. Similarly, the hydride donor
knowledge of the potentials of the (I/0) and (II/I) couples
can be used to select acids for protonating Ni(diphos-
phine)2 complexes without resulting in a second proto-
nation to form hydrogen and Ni(II). This can be useful
for designing reactions to prepare the desired hydrides
or for selecting reaction conditions for which the hy-
drides will be stable.
Su m m a r y a n d Con clu sion s
A linear free-energy relationship is shown to exist
between the half-wave potentials of the (II/I) couples of
Ni(diphosphine)2 complexes and the hydride donor
abilities or ∆G°H values of [HNi(diphosphine)2]+ can
ability (∆G°H ) of the corresponding [HNi(diphos-
-
-
be used to calculate the free energy associated with
phine)2]+ complexes. This is the first quantitative cor-
relation of transition metal hydricities with an easily
measured spectroscopic or electrochemical parameter.
A similar correlation also exists between the half-wave
protonation of the [HNi(diphosphine)2]+ complexes to
2
form hydrogen (∆G°H ), as shown by the thermodynamic
cycle given by eqs 10-13.

1838 Organometallics, Vol. 20, No. 9, 2001
Berning et al.
Successive Fourier maps were used to locate all carbon atoms
for two more ligands. Site occupancies were refined against
those of the original ligands. Final values were 0.9018(9) and
0.0982(9) for the original ligands and the new component.
Neither hydrogens nor anisotropic thermal parameters were
modeled for the minor component of this disorder. The angle
between the normals to the planes P(7),C(91),C(92),P(8) and
P(1′),C(39′),C(40′),P(2′) is 96°. The disorder may be described
as a 96° rotation of the molecule. After modeling this disorder,
the final difference map was essentially featureless, except for
a single 2.05 e/Å3 peak located 0.749 Å from Ni(2). This peak
may be a result of the disorder.
Electr och em ica l Stu d ies. All electrochemical experiments
were carried out under an atmosphere of N2 in 0.3 M Bu4NBF4
in benzonitrile or 0.3 M Et4NBF4 in acetonitrile. Cyclic
voltammetry experiments were carried out on a Cypress
Systems computer-aided electrolysis system. The working
electrode was a glassy carbon disk (2 mm diameter), and the
counter electrode was a glassy carbon rod. A platinum wire
immersed in a permethylferrocene/permethylferrocenium solu-
tion was used as a pseudoreference electrode to fix the
potential. Ferrocene was used as an internal standard, and
all potentials are referenced to the ferrocene/ferrocenium
couple.
Syn t h eses. Bis(1,5-cyclooctadiene)nickel(0), vinyldiphe-
nylphosphine, diethylphosphine, and cis-1,2-bis(diphenylphos-
phino)ethylene were purchased from Strem Chemical Co. and
used without further purification. Benzonitrile, acetonitrile,
2,2′-azobis(methylproprionitrile) (AIBN), CD3CN, ammonium
hexafluorophosphate, and triethylamine were purchased from
Aldrich Chemical Co. and used as received. Anisidine was
purchased from Aldrich Chemical Co. and sublimed. Tetrahy-
drofuran was purchased from Aldrich Chemical Co. and
distilled over Na/benzophenone prior to use.
potentials of the (I/0) couples of Ni(diphosphine)2 com-
plexes and the pKa values of the corresponding [HNi-
(diphosphine)2]+ complexes. As a result, it is possible
to use the potentials of these two couples to predict the
free energies of all three Ni-H bond cleavage reactions,
+
•
-
∆G°H , ∆G°H , and ∆G°H , for this class of nickel
complexes.
Exp er im en ta l Section
1
P h ysica l Mea su r em en ts a n d Gen er a l P r oced u r es. H
and 31P NMR spectra were recorded on a Varian Unity 300
MHz spectrometer at 299.95 and 121.42 MHz, respectively.
1H chemical shifts are reported relative to tetramethylsilane
using residual solvent protons as a secondary reference. 31P
chemical shifts are reported relative to external phosphoric
acid. In experiments where accurate integrations of 31P
resonances were required, spectra were collected using a 52°
pulse and a repetition rate of 11.96 s. This is sufficient to allow
complete relaxation of nuclei with T1 < 27 s. Measured T1
values for Ni(dedpe)2 and Ni(dedpe)2H+ were 1.1 and 3.5 s,
respectively. Infrared spectra were recorded on a Nicolet 510P
spectrometer as Nujol mulls. Elemental analyses were per-
formed by Galbraith Laboratories, Inc., Knoxville, TN. All
syntheses were carried out using Schlenk and drybox tech-
niques.
X-r a y Diffa ction Stu d ies. Crystals were manipulated
under a light hydrocarbon oil. The datum crystal was affixed
with a small amount of silicone stopcock grease to a thin glass
fiber attached to a tapered copper mounting-pin. This assembly
was transferred to the goniometer of a Bruker SMART CCD
diffractometer equipped with an LT-2a low-temperature ap-
paratus operating at 143 K.
Et2P CH2CH2P P h 2, d ed p e. Diethylphosphine (0.62 g, 6.8
mmol), vinyldiphenylphosphine, (1.45 g, 6.8 mmol), and 2,2′-
azobis(methylproprionitrile) (0.1 g) were placed in a Schlenk
flask and irradated overnight in a Rayonet photoreactor using
lamps of 2537, 3000, and 3500 Å wavelengths. Volatile
materials were removed in vacuo at 80 °C to produce a clear,
Ni(d ed p e)2. Cell parameters were determined using reflec-
tions harvested from a series of three orthogonal sets of 20
0.3° ω scans. Final cell parameters were refined using 7820
reflections with I > 10σ(I) chosen from the entire data set. All
data were corrected for Lorentz and polarization effects, as
well as for absorption.
Structure solution in centrosymmetric space group Pbca
revealed the heavy atom positions and many of the carbon
positions. Additional carbon sites were located after four cycles
of least-squares refinement followed by difference Fourier
synthesis. Hydrogen atoms were placed at calculated geom-
etries and allowed to ride on the position of the parent atom.
All non-hydrogen atoms were refined with anisotropic param-
eters for thermal motion. Thermal parameters for hydrogen
atoms were set to 1.2 times the equivalent isotropic U of the
parent. The final difference map was essentially flat and
featureless.
Ni(d p p v)2. Cell parameters were determined using reflec-
tions harvested from a series of three orthogonal sets of 20
0.3° ω scans. Final cell parameters were refined using 6896
reflections with I > 10σ(I) chosen from the entire data set. All
data were corrected for Lorentz and polarization effects, as
well as for absorption.
Solution of the structure in the centrosymmetric space group
P1h revealed two crystallographically independent molecules.
Additional atoms of these molecules were located via least-
squares refinement followed by difference Fourier synthesis.
Hydrogen atoms were placed at calculated positions, which
were allowed to ride on the position of the parent atom. Non-
hydrogen atoms were refined with anisotropic parameters for
thermal motion; hydrogen thermal motion was modeled at 1.2
times the equivalent isotropic U of the parent atom.
Fourier maps calculated near the final refinement indicated
residual electron density near Ni(2). These 1.5 e/Å3 peaks were
within bonding range for phosphorus. After adding these peaks
to the model as phosphorus atoms with 0.1 site occupancy,
additional peaks consistent with the ligands began to appear.
1
air-sensitive oil (1.10 g, 59%). H NMR (toluene-d8): 7.4, 7.1,
7.0 ppm (m’s, C6H5); 2.1, 1.4 ppm (m’s, PCH2CH2P); 1.15 ppm
3
3
(q, J HH ) 7.8 Hz, PCH2CH3); 0.89 ppm (dt, J PH ) 7.5 Hz,
PCH2CH3).21 31P NMR (toluene-d8): -11.25 ppm (d, J PP ) 25
Hz, PPh2); -17.46 ppm (d, PEt2).
3
Ni(d ed p e)2. A 100 mL Schlenk flask was charged with
dedpe (1.20 g, 4.42 mmol) in THF (50 mL). Ni(COD)2 (0.60 g,
2.18 mmol) was added slowly at room temperature. The orange
reaction mixture was stirred for 2 h and the solvent removed
in vacuo. The yellow product was washed with acetonitrile (3
× 40 mL) and dried in vacuo (0.96 g, 72%). Anal. Calcd for
1
C
36H48NiP4: C, 65.2; H, 7.3. Found: C, 64.3; H, 7.5. H NMR
(toluene-d8): 7.0-7.8 ppm (m, PC6H5); 0.76 and 1.23 ppm (m,
PCH2CH3); 2.2, 1.68, 1.42, and 1.23 ppm (m, PCH2CH2P and
PCH2CH3). 31P NMR (toluene-d8): AA′XX′ spectrum; 49.3 ppm
(m, N ) 80 Hz, PPh2); 41.7 ppm (m, PEt2). X-ray quality
crystals of Ni(dedpe)2 were grown by using excess triethy-
lamine to deprotonate [HNi(dedpe)2](BF4) in acetonitrile. Yel-
low crystals form slowly at room temperature over a period of
several hours.
Ni(d p p v)2. This compound was prepared using a literature
procedure.14 1H NMR (toluene-d8): 6.82-7.35 (m’s, Ph and
ethylene protons). 31P NMR (toluene-d8): 51.5 (s). X-ray quality
crystals were obtained by protonating Ni(dppv)2 with [p-C6H4-
(OCH3)(NH3)]BF4 in acetonitrile and back-titrating with p-C6H4-
(OCH3)(NH2). Yellow crystals form slowly at room temperature
over a period of several hours.
[HNi(d ed p e)2](BF 4). A 100 mL Schlenk flask was charged
with Ni(dedpe)2 (1.0 g, 1.66 mmol) in THF (50 mL). p-(OCH3)-
2
3
(21) Generally the J PH coupling is smaller than the J PH coupling
in ethyl groups attached to phosphorus (see ref 9).

Proton and Hydride Donor Abilities
Organometallics, Vol. 20, No. 9, 2001 1839
C6H4(NH3)]BF4 (0.36 g, 1.70 mmol) was added at room tem-
perature, and the reaction mixture stirred for 30 min. The
solvent was removed from the reaction mixture in vacuo, and
the resulting yellow solid was washed with toluene (3 × 30
mL) and dried in vacuo (1.01 g, 81%). Anal. Calcd for C36H45P4-
BF4Ni: C, 57.9; H, 6.1. Found: C, 57.1; H, 6.6. 1H NMR
(acetonitrile-d3): 7.1-7.6 ppm (m, PC6H5); 1.18 and 0.88 ppm
(m, PCH2CH3); 1.4-2.4 ppm (m, PCH2CH2P and PCH2CH3);
31P NMR signals. The same ratios were observed within
experimental error from 15 min to 1 day. Similar experiments
were performed in deuteroacetonitrile. The latter experiments
were accompanied by the formation of red crystals of Ni(dppv)2.
However, sufficient Ni(dppv)2 remained in solution to permit
accurate integration.
p Ka of [HNi(d ed p e)2]P F 6 in Ben zon itr ile. This experi-
ment was performed by preparing a 1.41 × 10-2 M solution of
[Ni(dedpe)2H]PF6 in benzonitrile in a drybox. Various amounts
of triethylamine in benzonitrile (1.40 M) were added to 0.75
mL of the [Ni(dedpe)2H]PF6 followed by benzonitrile to give a
constant volume of 1.00 mL. The mixtures were equilibrated
for at least 30 min, and the relative concentrations of the
species present were calculated by integration of the 31P NMR
signals.
2
-13.1 (tt, J PH ) 25 and 15 Hz, Ni-H). 31P NMR (acetonitrile-
d3): AA′XX′ spectrum; 48.6 ppm (m, PPh2); 47.5 ppm (m, PEt2).
IR Nujol: ν(Ni-H) 1934 cm-1
.
[HNi(d p p v)2](BF 4). This complex can be prepared from Ni-
(dppv)2 and [p-BrC6H4(NH3)]BF4 in 90% yield using the
procedure for [HNi(dedpe)2](PF6). 1H NMR (acetonitrile-d3):
7.08 and 7.26 ppm (m’s, Ph and ethylene protons); -11.87
(quintet, 2J PH ) 10 Hz, Ni-H). 31P NMR (acetonitrile-d3): 56.71
ppm (s). No band could be unambiguously assigned to the
Ni-H stretch in the IR spectrum.
p Ka of [HNi(d p p v)2]BF 4 in Ben zon itr ile a n d Aceton i-
tr ile. These experiments were performed by first preparing
two solutions in a drybox. The first solution contained Ni-
(dppv)2 (1.41 × 10-2 M) and [p-C6H4(OCH3)(NH3)]BF4 (1.90 ×
10-2 M) in benzonitrile. Full conversion to the metal hydride
was determined by 31P NMR spectroscopy. A second solution
of p-C6H4(OCH3)(NH2) (1.40 M) was also prepared in benzoni-
trile. Various amounts of the second solution were added to
0.5 mL aliquots of the first solution followed by benzonitrile
to give a constant volume of 0.75 mL. A total of five samples
were prepared in this fashion. The samples were allowed to
equilibrate for at least 30 min, and the relative concentrations
of the species present were calculated by integration of the
Ack n ow led gm en t. This work was supported by the
U.S. Department of Energy, Office of Science, Chemical
Sciences Division. M.C.R.D. would like to acknowledge
the support of the National Science Foundation.
Su p p or tin g In for m a tion Ava ila ble: Tables of crystal
data, data collection parameters, structure solution and refine-
ment, atomic coordinates and equivalent isotropic displace-
ment parameters, bond lengths, bond angles, anisotropic
thermal parameters, and hydrogen coordinates and isotropic
displacement parameters for Ni(dedpe)2, and Ni(dppv)2. This
information is available free of charge via the Internet at
http://pubs.acs.org.
OM0100582