COMMUNICATION
four hydrogen atoms of the BH4 group are tetrahedrally
arranged around the boron. The Ni-H distances span a wide
range, 1.59(5)-1.83(5) Å, comparable to those in other
nickel hydride and borohydride complexes.16,18 Complex 2
is structurally very similar to its Co(I) analogue [(triphos)-
Co(η2-BH4)].19 However, the hapticity of the borohydride
ligand appears very sensitive to the oxidation state and nature
of the metal and its associated ligand set in late-transition-
metal borohydride complexes. Thus, BH4- adopts a tridentate
binding mode in the Ni(II) species [Tp*Ni(η3-BH4)]16 but a
monodentate mode in the Cu(I) analogue of 2, viz., [(triphos)-
Cu(η1-BH4)].20
The magnetic susceptibility of 2 in the solid state was
measured by the Gouy method and in solution in THF-d8
by the Evans technique.21 µeff values of 2.47 and 2.57 µB,
respectively, indicate the presence of a single unpaired
electron with an orbital contribution to the spin-only moment.
Attempts to obtain a reproducible electron paramagnetic
resonance spectrum have thus far been frustrated by sample
decomposition.
Figure 3. Representation of the model complex 2′ and its spin polarization.
Selected bond distances (Å) and angles (deg): Ni-P1, 2.230; Ni-P2, 2.224;
Ni-P3, 2.299; Ni-Ha, 1.699; Ni-Hb, 1.741; B-Ha, 1.279; B-Hb, 1.275;
B-Hc, 1.205; B-Hd, 1.205; P1-Ni-P2, 95.89; P1-Ni-P3, 90.16; P2-
Ni-P3, 89.60.
experimentally for complex 2. The computed total spin
densities for 2′ (Ni, 0.855; B, 0.138; P1, 0.001; P2, 0.004;
P3, 0.053; Ha, 0.000; Hb, -0.007; Hc, -0.008; Hd, -0.008)
clearly reveal the unpaired electron to be located predomi-
nantly on the metal, with only a minor presence on boron
(Figure 3). Negligible spin populations are found on the
phosphorus and bridging hydrogen atoms.
Density functional theory (DFT) calculations have been
carried out to explore the nature of the unpaired electron in
the model complex [{MeC(CH2PH2)3}Ni(η2-BH4)] (2′; Fig-
ure 3).22-26 The optimized geometry of this system, in which
the phenyl groups of the triphos ligand have been replaced
by hydrogen atoms, is in good agreement with that found
Although complex 2 is paramagnetic, it displays relatively
sharp resonances in its NMR spectra in THF-d8 between 25
and -70 °C, which are easily assigned to the triphos and
borohydride ligands (see the Supporting Information). For
example, the 11B NMR spectrum shows a quintet at -42.98
ppm (w1/2 ) 5 Hz), albeit measurably broader than that
(18) Alvarez, H. M.; Krawiec, M.; Donovan-Merkert, B. T.; Fouzi, M.;
Rabinovich, D. Inorg. Chem. 2001, 40, 5736.
(19) Dapporto, P.; Midollini, S.; Orlandini, A.; Sacconi, L. Inorg. Chem.
1976, 11, 2768.
(20) Ghilardi, C. A.; Midollini, S.; Orlandini, A. Inorg. Chem. 1982, 21,
4096.
-
obtained for free BH4 under the same conditions (-41.08
ppm; w1/2 ) 1.4 Hz). This sharpness is in accord with the
spin densities calculated for 2′ (qv). With a quartet centered
at -0.58 ppm in the 1H NMR spectrum (1JBH ) 82 Hz), the
borohydride ligand remains fluxional between 25 and -70
°C. The 31P{1H} NMR spectrum under the same conditions
shows a single resonance at 27.17 ppm.
In summary, the capability of the bulky triphos ligand to
switch from a bidentate to a tridentate binding mode, in
tandem with the reducing nature and size of the borohydride
ligand, promotes the change from square-planar 1 to the
unusual tetrahedral complex 2. Complex 2 is only the second
characterized stable borohydride of nickel and the first such
example in which this metal is in the 1+ oxidation state.
(21) Evans, D. F. J. Chem Soc. 1959, 2003.
(22) DFT calculations were performed with the Gaussian03 suite of
programs (ref 23) using the B3LYP functional (ref 24) and a 6-31G-
(d,p) basis set (ref 25). The geometry optimization was carried out
without imposing any symmetry constraint, and the reported structure
was found to be a true minima on the potential energy surface by
calculating analytical frequencies. The reported spin density values
are based on a Mulliken-type population analysis. The program
MOLEKEL (ref 26) was used for visualization (surface value: 0.0095).
(23) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin,
K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone,
V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G.
A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev,
O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P.
Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.;
Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas,
O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J.
B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;
Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,
W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian03, revision
B.05; Gaussian, Inc.: Pittsburgh, PA, 2003.
Acknowledgment. We are grateful to the University of
New Brunswick and NSERC of Canada for financial support,
to the DFG of Germany for a fellowship (to P.S.), and to
Dr. Larry Calhoun for his assistance with NMR measure-
ments.
(24) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang,
W.; Parr, R. G. Phys. ReV. B 1988, 37, 785.
Supporting Information Available: X-ray data in the form of
(25) (a) Hehre, W. J.: Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972,
56, 2257. (b) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973,
28, 213. (c) Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.;
Gordon, M. S.; DeFrees, D. J.; Pople, J. A. J. Chem. Phys. 1982, 77,
3654. (d) Rassolov, V.; Pople, J. A.; Ratner, M.; Windus, T. L. J.
Chem. Phys. 1998, 109, 1223.
1
a CIF for complexes 1 and 2; H, 31P, and 11B NMR spectra for
complex 2 in THF-d8 and Cartesian coordinates for the optimized
geometry of 2′. This material is available free of charge via the
(26) Flu¨kiger, P.; Lu¨thi, H. P.; Portmann, S.; Weber, J. MOLEKEL 4.0,
Swiss Center for Scientific Computing, Manno, Switzerland, 2000.
IC051541Y
8652 Inorganic Chemistry, Vol. 44, No. 24, 2005