Bonding Properties of a Novel Inorganometallic Complex, Ru(SnPh3)2(CO)2(iPr-DAB) (iPr-DAB = N,N′-Diisopropyl-1,4-diaza-1,3-butadiene), and its Stable Radical-Anion, Studied by UV-Vis, IR, and EPR Spectroscopy, (Spectro-) Electrochemistry, and Density Functional Calculations

Article abstract of DOI:10.1021/ic960042h

Ru(SnPh₃)₂(CO)₂(iPr-DAB) was synthesized and characterized by UV-vis, IR, ¹H NMR, ¹³C NMR, ¹¹⁹Sn NMR, and mass (FAB⁺) spectroscopies and by single-crystal X-ray diffraction, which proved the presence of a nearly linear Sn-Ru-Sn unit. Crystals of Ru(SnPh₃)₂(CO)₂(iPr-DAB)·3.5C ₆H₆ form in the triclinic space group P1? in a unit cell of dimensions a = 11.662(6) A?, b = 13.902(3) A?, c = 19.643(2) A?, α = 71.24(2)°, β = 86.91(4)°, γ = 77.89(3)°, and V = 2946(3) A°³. One-electron reduction of Ru(SnPh₃)₂(CO)₂(iPr-DAB) produces the stable radical-anion [Ru(SnPh₃)₂(CO)₂(iPr-DAB)]^?- that was characterized by IR, and UV-vis spectroelectrochemistry. Its EPR spectrum shows a signal at g = 1.9960 with well resolved Sn, Ru, and iPr-DAB (H, N) hyperfine couplings. DFT-MO calculations on the model compound Ru(SnH₃)₂(CO)₂(H-DAB) reveal that the HOMO is mainly of σ(Sn-Ru-Sn) character mixed strongly with the lowest π* orbital of the H-DAB ligand. The LUMO (SOMO in the reduced complex) should be viewed as predominantly π*(H-DAB) with an admixture of the σ(Sn-Ru-Sn) orbital. Accordingly, the lowest-energy absorption band of the neutral species will mainly belong to the σ(Sn-Ru-Sn)→π*(iPr-DAB) charge transfer transition. The intrinsic strength of the Ru-Sn bond and the delocalized character of the three-center four-electron Sn-Ru-Sn σ-bond account for the inherent stability of the radical anion.

Full text of DOI:10.1021/ic960042h

5468
Inorg. Chem. 1996, 35, 5468-5477
Bonding Properties of a Novel Inorganometallic Complex, Ru(SnPh3)2(CO)2(iPr-DAB)
(iPr-DAB ) N,N′-Diisopropyl-1,4-diaza-1,3-butadiene), and its Stable Radical-Anion,
Studied by UV-Vis, IR, and EPR Spectroscopy, (Spectro-) Electrochemistry, and Density
Functional Calculations
Maxim P. Aarnts,† Maikel P. Wilms,‡ Karin Peelen,† Jan Fraanje,§ Kees Goubitz,§
Frantisˇek Hartl,*,† Derk J. Stufkens,† Evert Jan Baerends,‡ and Anton´ın Vlcˇek, Jr.*,|
Anorganisch Chemisch Laboratorium, J. H. van ’t Hoff Research Institute, Universiteit van Amsterdam,
Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands, Amsterdam Institute for Molecular
Studies, Universiteit van Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam,
The Netherlands, Afdeling Theoretische Chemie, Vrije Universiteit, De Boelelaan 1083,
1081 HV Amsterdam, The Netherlands, J. Heyrovsky´ Institute of Physical Chemistry,
Academy of Sciences of the Czech Republic, Dolejsˇkova 3, 182 23 Prague, Czech Republic
ReceiVed January 12, 1996X
Ru(SnPh3)2(CO)2(iPr-DAB) was synthesized and characterized by UV-vis, IR, 1H NMR, 13C NMR, 119Sn NMR,
and mass (FAB+) spectroscopies and by single-crystal X-ray diffraction, which proved the presence of a nearly
linear Sn-Ru-Sn unit. Crystals of Ru(SnPh3)2(CO)2(iPr-DAB)‚3.5C6H6 form in the triclinic space group P1h in
a unit cell of dimensions a ) 11.662(6) Å, b ) 13.902(3) Å, c ) 19.643(2) Å, R ) 71.24(2)°, â ) 86.91(4)°,
γ ) 77.89(3)°, and V ) 2946(3) Å3. One-electron reduction of Ru(SnPh3)2(CO)2(iPr-DAB) produces the stable
radical-anion [Ru(SnPh3)2(CO)2(iPr-DAB)]•- that was characterized by IR, and UV-vis spectroelectrochemistry.
Its EPR spectrum shows a signal at g ) 1.9960 with well resolved Sn, Ru, and iPr-DAB (H, N) hyperfine couplings.
DFT-MO calculations on the model compound Ru(SnH3)2(CO)2(H-DAB) reveal that the HOMO is mainly of
σ(Sn-Ru-Sn) character mixed strongly with the lowest π* orbital of the H-DAB ligand. The LUMO (SOMO
in the reduced complex) should be viewed as predominantly π*(H-DAB) with an admixture of the σ(Sn-Ru-
Sn) orbital. Accordingly, the lowest-energy absorption band of the neutral species will mainly belong to the
σ(Sn-Ru-Sn)fπ*(iPr-DAB) charge transfer transition. The intrinsic strength of the Ru-Sn bond and the
delocalized character of the three-center four-electron Sn-Ru-Sn σ-bond account for the inherent stability of
the radical anion.
Introduction
originate either in metal d orbitals (MLCT),14-16 ligand-localized
orbital (LLCT),7,8,17,18 or a M-L σ-bonding orbital (σπ*).19-21
Excitation into low-energy MLCT, LLCT, or σπ* transitions
is closely related to a ligand-localized reduction of the same
complex.22 This is confirmed by the fact that many photo-
chemically initiated reactions also occur upon reduction of
the same complex.23-25 Well-known in this respect is the
photo- or electrocatalyzed reduction of CO2 to CO by Re(X)-
(CO)3(bpy) (X ) halide)4,26-28 or [Ru(X)2-n(CO)n(bpy)2]n+, n
) 1, 2.29,30
Mixed-ligand metal carbonyls (Cr-W, Re, Ru)1-9 containing
R-diimine ligands have been extensively studied because of their
interesting photophysical,2,10,11 photochemical,1 and redox4,12,13
properties. Many of these studies focus on the fundamental
aspects of the charge-transfer (CT) transitions, which are always
directed to the π* orbital of the R-diimine ligand and can
* To whom correspondence should be addressed.
† Anorganisch Chemisch Laboratorium, J. H. van ’t Hoff Research
Institute, Universiteit van Amsterdam.
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‡ Vrije Universiteit.
§ Amsterdam Institute for Molecular Studies, Universiteit van Amsterdam.
| Academy of Sciences of the Czech Republic.
X Abstract published in AdVance ACS Abstracts, August 15, 1996.
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S0020-1669(96)00042-0 CCC: $12.00 © 1996 American Chemical Society

Ru(SnPh3)2(CO)2(iPr-DAB)
Inorganic Chemistry, Vol. 35, No. 19, 1996 5469
directly added to a small excess of solid SnClPh3 (212 mg, 0.55 mmol)
and stirred for a few minutes. Further manipulations of the photolabile
product were performed under exclusion of light. After the solvent
had been removed by evaporation in Vacuo, the complex was purified
by column chromatography on activated Silica 60, using a hexane/
THF mixture in a 9:1 (v:v) ratio as an eluent. The solvents were
removed by evaporation in Vacuo. Ru(SnPh3)2(CO)2(iPr-DAB) was
obtained in 60% yield as a pink powder.
Figure 1. [Ru(SnPh3)2(CO)2(iPr-DAB)]0,•- complexes studied.
Characterization data for Ru(SnPh3)2(CO)2(iPr-DAB) follow. UV-
The photochemical and electrochemical properties of such
carbonyl-diimine complexes can be fine-tuned by varying the
nature of the R-diimine and axial ligands. In the case of the
Re(L)(CO)3(R-diimine)31,32 or Ru(L)2(CO)2(R-diimine)33,34 com-
plexes, their properties change profoundly when covalently
bonded axial ligands, usually an alkyl, benzyl, or another metal
fragment, are introduced. For M-C or M-M′ bonds, the
σ-bonding orbital is energetically high-lying and close to the
metal dπ and R-diimine π* orbitals. Hence, the σ-electrons
might well become involved in the excitation or redox processes.
For Ru complexes, the simultaneous presence of two trans-
oriented M-L σ-bonds is expected21 to affect the bonding within
the Ru(R-diimine) fragment substantially and lead to new and
unusual spectroscopic and redox properties. More knowledge
of the special properties of these complexes is important because
of their potential use as luminophores, photosensitisers, or redox-
catalysts. Herein, we describe the first inorganometallic
complex of this type, Ru(SnPh3)2(CO)2(iPr-DAB) and its stable
radical-anion [Ru(SnPh3)2(CO)2(iPr-DAB)]•- (Figure 1), which
contain a nearly linear Sn-Ru-Sn structural moiety. We have
aimed at the understanding of the bonding in these complexes,
of effects on the electron-density distribution brought about by
the presence of two axial covalent Ru-Sn bonds, and of their
spectroscopic and redox consequences.
vis (THF): λmax ) 511 nm (ꢀ ) 6000 M-1 cm-1). IR (THF): ν(CO)
) 2005 vs, 1952 vs cm-1
.
1H NMR (300.13 MHz, C6D6): δ (ppm) )
3
3
7.58 (85% d, 15% dd, J(H,H) ) 6.3 Hz, J(117/119Sn,H) ) 76.4 Hz,
12H, Sn(o-C6H5)), 7.17 (m, 18H, Sn(m-/p-C6H5)), 6.90 (68% s, 30%
4
3
d, 2% t, J(117/119Sn,H) ) 26.1 Hz, 2H, imine H), 4.35 (sept, J(H,H)
3
) 6.5 Hz, 2H, (CH3)2CH), 0.71 (d, J(H,H) ) 6.5 Hz, 12H, (CH3)2-
CH). 13C NMR (75.46 MHz, C6D6): δ (ppm) ) 204.6 (2J(117/119Sn,13C)
) 56 Hz, CO), 147.3 (imine C), 143.4 (1J(117/119Sn,13C) ) 296 Hz,
SnC), 137.4 (2J(117/119Sn,13C) ) 33 Hz, SnCC), 128.0 (3J(117/119Sn,13C)
) 41 Hz, SnCCC), 127.7 (4J(117/119Sn,13C) ) 13 Hz, SnCCCC), 64.2
(C(CH3)2), 24.7 (C(CH3)2). 119Sn NMR (93.181 MHz, C6D6): δ (ppm)
) -53. Mass (FAB+): (m/z)+, (int %) ) 997 (3) [M]+, 647 (11) [M
- SnPh3]+, 619 (9) [M - SnPh3 - CO]+, 591 (3) [M - SnPh3 -
2CO]+, 570 (9) [M - SnPh3 - Ph]+, 351 (32) [SnPh3]+. Anal. Found
(calcd) C, 55.60 (55.39); H, 4.60 (4.65); N, 2.74 (2.81).
Crystal Structure Determination. The crystals were grown by
slow evaporation of the benzene solvent. A crystal with the ap-
proximate dimensions of 0.40 × 0.60 × 0.75 mm was used for data
collection on an Enraf-Nonius CAD-4 diffractometer with graphite-
monochromated Cu KR radiation and ω-2ϑ scan. A total of 8879
unique reflections was measured within the range -12 e h e 0, -15
e k e +15, -22 e l e +20. Of these, 6455 were above the
significance level of 2.5σ(I). The maximum value of (sin ϑ)/λ was
0.56 Å-1. Two reference reflections (2h01h, 2h01) were measured hourly
and showed a 6% decrease during the 100 h collection time, which
was corrected for. Unit-cell parameters were refined by a least-squares
fitting procedure using 23 reflections with 72° < 2ϑ < 93°. Corrections
for Lorentz and polarization effects were applied. The structure was
solved by the PATTY/ORIENT/PHASEX option of the DIRDIF91
program system.37,38 The hydrogen atoms were placed in calculated
positions. After isotropic refinement of the starting model, a ∆F
synthesis revealed a number of peaks (21) which could be interpreted
as three complete benzene molecules and one-half of a benzene
molecule (completed by the center of symmetry). Benzene was the
solvent used during the crystallization of the compound. Full-matrix
least-squares refinement on F was carried out, anisotropic for Ru and
Sn and isotropic for the remainder of the atoms, keeping the hydrogen
atoms fixed at their calculated position with U ) 0.05 Å2, and it
converged to R ) 0.121, Rw ) 0.180, and (∆/σ)max ) 0.10. The fact
that only the heavy atoms could be refined anisotropically and the rather
high R factor and residual electron density are probably due to a low
crystal quality, which is also reflected in the fact that there are 3.5
solvent molecules present in the asymmetric unit. A weighing scheme
Experimental Section
Materials. All chemicals were purchased from Janssen Chimica
unless stated otherwise. Solvents for spectroscopic experiments were
of analytical grade, distilled from sodium wire (THF, hexane) or CaH2
(CH3CN). The Silica 60 (Merck) used for the purification of the
complexes by column chromatography was activated by heating it
overnight in Vacuo at 180 °C, and it was stored under nitrogen. The
supporting electrolyte Bu4NPF6 (Aldrich) was dried overnight in Vacuo
at 80 °C. Ferrocene (Fc) (BDH) and SnClPh3 were used without further
purification.
Synthesis of Ru(SnPh3)2(CO)2(iPr-DAB). First, Ru(Cl)(SnPh3)-
(CO)2(iPr-DAB) was synthesized similarly to Ru(I)(Me)(CO)2(iPr-
DAB)7, by reaction of Ru3(CO)12 (Strem), iPr-DAB,35 and SnClPh3 in
hexane. To a solution of 342 mg (0.5 mmol) of Ru(Cl)(SnPh3)(CO)2-
(iPr-DAB) in 50 mL of THF, 0.6 mL of sodium-potassium (3:1) alloy
was added and the solution was stirred until the color changed from
blue green into deep red indicating that the reduction to [Ru(SnPh3)-
(CO)2(iPr-DAB)]- was completed.36 After removal of the excess of
NaK by filtration over a G4 frit, the extremely air-sensitive anion was
2
-1
w ) (5.3 + Fobs + 0.014Fobs
)
was used. An empirical absorption
correction (DIFABS)39 was applied, with coefficients in the range 0.37-
2.79. The secondary isotropic extinction coefficients40,41 were refined
to Ext ) 0.07(3). A final difference Fourier map revealed a residual
electron density between -4.2 and 4.0 e Å-3 in the vicinity of the
heavy atoms. Scattering factors were taken from Cromer.42,43 The
anomalous scattering of the Ru and Sn atoms was taken into account.44
(28) Christensen, P.; Hamnett, A.; Muir, A. V. G.; Timney, J. A. J. Chem.
Soc., Dalton Trans. 1992, 1455.
(29) Ishida, H.; Terada, T.; Tanaka, K.; Tanaka, T. Inorg. Chem. 1990,
29, 905.
(30) Tanaka, H.; Nagao, H.; Peng, S. M.; Tanaka, K. Organometallics 1992,
11, 1450.
(31) Rossenaar, B. D.; Kleverlaan, C. J.; van de Ven, M. C. E.; Stufkens,
D. J.; Oskam, A.; Goubitz, K.; Fraanje, J. J. Organomet. Chem. 1995,
493, 153.
(37) Smits, J. M. M.; Behm, H.; Bosman, W. P.; Beurskens, P. T. J.
Crystallogr. Spectrosc. Res. 1991, 18, 447.
(38) Motherwell, W. D. S.; Clegg, W. PLUTO. Program for Plotting
Molecular and Crystal Structures. University of Cambridge, U.K.,
1978.
(32) Rossenaar, B. D.; Kleverlaan, C. J.; Stufkens, D. J.; Oskam, A. J.
Chem. Soc., Chem. Commun. 1994, 63.
(39) Walker, N.; Stuart, D. Acta Crystallogr. 1983, A39, 158.
(40) Zachariasen, W. H. Acta Crystallogr. 1967, A23, 558.
(41) Larson, A. C. In Crystallographic Computing; Ahmed, F. R., Hall, S.
R., Huber, C. P., Eds.; Munksgaard: Copenhagen, 1969; pp 291-
294.
(42) Cromer, D. T.; Mann, J. B. Acta Crystallogr. 1968, A24, 321.
(43) Cromer, D. T.; Mann, J. B. International Tables for X-Ray Crystal-
lography; Kynoch Press: Birmingham, 1974; Vol. IV, p 55.
(44) Cromer, D. T.; Liberman, D. J. Chem. Phys. 1970, 53, 1891.
(33) Nieuwenhuis, H. A.; van de Ven, M. C. E.; Stufkens, D. J.; Oskam,
A.; Goubitz, K. Organometallics 1995, 14, 780.
(34) Nieuwenhuis, H. A.; van Loon, A.; Moraal, M. A.; Stufkens, D. J.;
Oskam, A.; Goubitz, K. J. Organomet. Chem. 1995, 492, 165.
(35) Bock, H.; tom Dieck, H. Chem. Ber. 1967, 100, 228.
(36) Aarnts, M. P.; Peelen, K.; Hartl, F.; Stufkens, D. J. To be submitted
for publication.

5470 Inorganic Chemistry, Vol. 35, No. 19, 1996
Aarnts et al.
Table 1. Crystallographic Data for Ru(SnPh3)2(CO)2(iPr-DAB)
procedure to obtain accurate Coulomb and exchange potentials in each
SCF-cycle, by the accurate and efficient numerical integration51,52 of
the Hamiltonian matrix elements and the possibility to freeze core
orbitals. The LSD exchange correlation potential was used,53 with the
Vosko-Wilk-Nusair54 parametrization of the electron gas data for the
local density approximation of the correlation energy. Becke’s nonlocal
corrections55,56 to the exchange energy and Perdew’s nonlocal correc-
tions57,58 to the correlation energy were used. A double-ú STO basis
set for H, C, N, and O was used, and a triple-ú STO basis set for Ru
and Sn was employed. The calculations will be referred to, in the
remainder of this paper, as “MO calculations” since the Kohn-Sham
formulation of density functional theory leads to molecular orbitals with
a good physical basis that can be used very well in MO theoretical
considerations.59 All bases were augmented with one polarization
function. Transition dipole moments were calculated using the program
Dipole.60 EPR parameters for the radical anion were calculated using
the program GATENQ61 and included Fermi contact terms and dipolar
interactions.
formula
Mr
cryst syst
C46H46N2O2RuSn2‚3.5C6H6 V (Å3)
2946(3)
2
1.62
997.8
Z
triclinic
Dx (gcm-3
)
space group P1h
λ(Cu KR) (Å)
µ(Cu KR) (cm-1
F(000)
1.5418
141.0
1286
193
a (Å)
11.662(6)
)
b (Å)
13.902(3)
19.643(2)
71.24(2)
86.91(4)
77.89(3)
c (Å)
T (K)
R (deg)
â (deg)
γ (deg)
no. of obsd
reflcns
6455
Ra
Rw
0.121
0.18
b
a R ) ||Fohbs| - k|Fhcalc||/∑h|Fohbs|. b Rw
)
w ∆F2/ w |Fhobs| .
2
∑
∑
∑
h
h
h
h
h h
All calculations were performed with XTAL,45 unless stated otherwise.
Attempts to refine all non-hydrogen atoms anisotropically led to
nonpositive definite temperature factors. A summary of the crystal-
lographic data is given in Table 1.
Spectroscopic and (Spectro-) Electrochemical Measurements. All
sample manipulations were performed under nitrogen atmosphere using
Schlenk techniques. Elemental analyses were performed by the
Microanalytisches Laboratorium of Dornis und Kolbe, Mu¨lheim a. d.
Ruhr, Germany. Fast atom bombardment (FAB) mass spectrometry
was carried out using a JEOL JMS SX/SX102A four-sector mass
spectrometer, coupled to a JEOL MS-MP7000 data system. The
samples were loaded in a matrix solution (nitrobenzyl alcohol) onto a
stainless steel probe and bombarded with xenon atoms with an energy
of 3 KeV. Electronic absorption spectra were recorded on a Perkin-
Elmer Lambda 5 UV-vis spectrophotometer provided with a Model
3600 data station. IR spectra were measured on a BioRad FTS-7
spectrometer. X-band EPR spectra were recorded at room temperature
on a Bruker ECS 106 spectrometer with a field modulation of 100
KHz. The frequency was measured with a HP5350B microwave
frequency counter. The magnetic field was calibrated with an AEG
magnetic field meter. The microwave power incident to the cavity
was measured with a HP432B powermeter. The EPR measurement
was carried out in a gastight EPR tube attached to a reaction vessel, of
ca. 10 mL. In this vessel, a ca. 10-3 M solution of the parent complex
in THF was chemically reduced by 1% sodium amalgam. After the
completion of the reduction (as judged by the color change from red
to green), the sample was carefully decanted into the attached EPR
tube. The program “ESR-simulations”46 was used to simulate the
spectrum.
Electrochemical measurements were performed with a PA4 (EKOM)
potentiostat. Cyclic voltammetry was carried out at a Pt-disk electrode
of 0.56 mm2 apparent surface area. An Ag wire and a Pt gauze were
used as pseudoreference and auxiliary electrodes, respectively. The
Fc/Fc+ redox couple served as an internal standard for determination
of the reduction potential and the electrochemical reversibility of the
reduction step.47 Concentrations of 10-1 M supporting electrolyte (Bu4-
NPF6) and 10-3 M Ru(SnPh3)2(CO)2(iPr-DAB) in THF were used. IR
and UV-vis spectroelectrochemical data were obtained with an OTTLE
(optically transparent thin-layer electrochemical) cell equipped with a
Pt-minigrid working electrode (32 wires per cm).48 NaCl windows were
used for IR (OTTLE) experiments. For UV-vis (OTTLE) measure-
ments, CaF2 or quartz windows were employed. The working electrode
was carefully masked to avoid spectral interference from the nonelec-
trolyzed solution.
Results
Synthesis and Characterization. The Ru(SnPh3)2(CO)2(iPr-
DAB) complex was prepared according to the reactions in (1),
following a procedure discribed elsewhere.36
Ru(Cl)(SnPh3)(CO)2(iPr-DAB) NaK8
Na(K)[Ru(SnPh3)(CO)2(iPr-DAB)] + Na(K)Cl
(1)
SnClPh3
Na(K)[Ru(SnPh3)(CO)2(iPr-DAB)]
8
Ru(SnPh3)2(CO)2(iPr-DAB) + Na(K)Cl
The pink product is soluble in most common aprotic solvents
(hexane, THF, CH3CN, etc.). It is slightly photosensitive in
solution but hardly photosensitive in the solid state. It is air
stable both in solution and in the solid state. Its characterization
1
by H, 13C, and 119Sn NMR, IR, UV-vis, and FAB+ mass
spectroscopies pointed to the trans,cis-Ru(SnPh3)2(CO)2(iPr-
DAB) formulation. Corresponding data are summarized in the
Experimental Section.
1
Beside the expected features, the H NMR spectrum shows
two interesting effects. First, the signals due to the methyl
groups of the isopropyl substituents are shifted approximately
0.5 ppm to higher field with respect to the same signals in Ru-
(Cl)(Me)(CO)2(iPr-DAB)7 since they are in the vicinity of the
shielding cones of the phenyl groups of the SnPh3 ligands which
are umbrella-like placed above and under the iPr-DAB ligand.
Second, the imine proton signals are shifted ca. 0.4 ppm upfield
in Ru(SnPh3)2(CO)2(iPr-DAB) compared with those found62 for
Ru(Cl)(SnPh3)(CO)2(iPr-DAB) in CDCl3. Moreover, the 4J(117,119
Sn,H) coupling strongly depends on the axial ligands (e.g. 7.0
-
(51) Boerrigter, P. M.; te Velde, G.; Baerends, E. J. Int. J. Quantum Chem.
1988, 33, 87.
(52) te Velde, G.; Baerends, E. J. J. Comput. Phys. 1992, 99, 84.
(53) Parr, R. G.; Yang, W. Density Functional Theory of Atoms and
Molecules; Oxford University Press: New York, 1989.
(54) Vosko, S. H.; Wilk, L.; Nusair, M. J. Can. J. Phys. 1980, 58, 1200.
(55) Becke, A. D. J. Chem. Phys. 1986, 84, 4524.
(56) Becke, A. D. Phys. ReV. 1988, A38, 3098.
(57) Perdew, J. P. Phys. ReV. 1986, B33, 8822.
(58) Perdew, J. P. Phys. ReV. 1986, B34, 7406.
(59) Baerends, E. J.; Gritsenko, O. V.; van Leeuwen, R. In Chemical
Applications of Density-Functional Theory; Laird, B., Ross, R., Ziegler,
T., Eds.; ACS Symposium Series 629; American Chemical Society:
Washington, DC, 1996; p 20.
Computational Details. All calculations were performed using the
Amsterdam density functional program package ADF.49,50 The com-
putational scheme is characterized by the use of a density fitting
(45) Hall, S. R.; Stewart, J. M. XTAL3.0 User’s Manual; Universities of
Western Australia and Maryland: Perth, Australia, and College Park,
MD, 1990.
(46) Bruns, W.; Schulz, A. ESR-Simulation; Universita¨t Stuttgart: Stuttgart,
Germany, 1991.
(47) Gagne´, R. R.; Koval, C. A.; Lisensky, G. C. Inorg. Chem. 1980, 19,
2854.
(48) Krejcˇ´ık, M.; Daneˇk, M.; Hartl, F. J. Electroanal. Chem. 1991, 317,
179.
(60) Wilms, M. P. Internal Report; Free University Amsterdam: Amster-
dam 1995.
(61) Belanzoni, P.; Baerends, E. J.; Van Asselt, S.; Langewen, P. B. J.
Phys. Chem. 1995, 99, 13094.
(49) Baerends, E. J.; Ellis, D. E.; Ros, P. Chem. Phys. 1973, 2, 52.
(50) Baerends, E. J.; Ros, P. Int. J. Quantum Chem. 1978, S12, 169.
(62) Aarnts, M. P.; Stufkens, D. J.; Oskam, A.; Fraanje, J.; Goubitz, K.
Inorg. Chim. Acta, in press.

Ru(SnPh3)2(CO)2(iPr-DAB)
Inorganic Chemistry, Vol. 35, No. 19, 1996 5471
Table 2. Selected Bond Distances (Å) and Angles (deg) for
Ru(SnPh3)2(CO)2(iPr-DAB)a
Bond Distances
Ru-Sn(1)
Ru-Sn(2)
Ru-C(9)
Ru-C(10)
Ru-N(1)
Ru-N(2)
C(1)-C(2)
2.686(2)
2.691(2)
1.80(2)
1.87(2)
2.08(2)
2.01(2)
1.39(3)
C(1)-N(1)
C(2)-N(2)
C(3)-N(1)
C(6)-N(2)
C(9)-O(9)
C(10)-O(10)
1.34(2)
1.34(3)
1.47(3)
1.55(3)
1.25(2)
1.16(3)
Bond Angles
Sn(1)-Ru-C(9)
Sn(1)-Ru-C(10)
Sn(1)-Ru-N(1)
Sn(1)-Ru-N(2)
Sn(2)-Ru-C(9)
Sn(2)-Ru-C(10)
Sn(2)-Ru-N(1)
86.7(6)
89.3(5)
96.0(4)
90.0(4)
87.6(5)
87.4(5)
87.9(4)
Sn(2)-Ru-N(2)
C(9)-Ru-C(10)
N(1)-Ru-N(2)
C(9)-Ru-N(1)
C(10)-Ru-N(2)
Sn(1)-Ru-Sn(2)
96.0(4)
89.6(9)
77.8(7)
96.8(8)
96.0(8)
173.45(7)
a Standard deviations in parentheses.
Figure 2. ORTEP drawing of the X-ray structure of Ru(SnPh3)2(CO)2-
(iPr-DAB).
Hz for Ru(Cl)(SnPh3)(CO)2(iPr-DAB) and 26.0 Hz for Ru-
(SnPh3)2(CO)2(iPr-DAB)). The δ value of -53 ppm for the
119Sn NMR signal of Ru(SnPh3)2(CO)2(iPr-DAB) lies in the
range +20 to -100 ppm, which is typical for this type of
tetrahedrally surrounded Sn atom (M-SnPh3).63,64
In the CO-stretching region of the IR spectrum, Ru(SnPh3)2-
(CO)2(iPr-DAB) exhibits two bands of approximately equal
intensity at 2004 and 1951 cm-1, respectively (see Figure 3),
which is characteristically for cis-dicarbonyls.65 They have been
assigned to νs(CO) and νas(CO), respectively.7 Their wave-
numbers are rather small compared with those of related
complexes (e.g. Ru(I)(Me)(CO)2(iPr-DAB): ν˜(CO) 2022, 1959
cm-1 in THF),7 due to coordination of the two strongly
σ-donating and weakly π-accepting66 SnPh3 ligands. This leads
to an increased π-back-donation to the CO ligands.
Crystal Structure. An ORTEP drawing of the crystal
structure of Ru(SnPh3)2(CO)2(iPr-DAB) is presented in Figure
2. Selected bond distances and bond angles are collected in
Table 2. The complex has a slightly distorted octahedral
geometry in which the SnPh3 ligands occupy axial positions.
The Ru-Sn bond lengths of 2.686(2) and 2.691(2) Å lie in the
range which is typical for Ru-Sn bonds (2.55-2.69Å).67 The
two SnPh3 ligands, which form a nearly linear (173.45(7)°) Sn-
Ru-Sn configuration, are in a staggered conformation with
respect to their phenyl groups. Interestingly, the central C(1)-
C(2) bond (1.39(3) Å) of the iPr-DAB ligand appears to be
shorter than the same bond in closely related Ru(Cl)(SnPh3)-
(CO)2(iPr-DAB) (1.435(6) Å).62 At the same time, the C(1)-
N(1) and C(2)-N(2) bond distances (1.34(2) and 1.34(3) Å)
are elongated with respect to those of the latter complex (1.279-
(6) and 1.303(6) Å).
Figure 3. IR spectral changes in the carbonyl stretching region
monitored during the electrochemical reduction of Ru(SnPh3)2(CO)2-
(iPr-DAB) in THF at room temperature, using the OTTLE cell.48
perature. The complex is reversibly reduced in an one-electron
step at E1/2 ) -1.86 V Vs Fc/Fc+. The chemical reversibility
is documented by the peak current ratio Ip,a/Ip,c ) 1. The
electrochemical reversibility is revealed by identical values of
∆Ep (85 mV) for both the complex and the Fc/Fc+ redox couple
(at comparable concentrations), which was used as an internal
standard.47 The reduction step is diffusion-controlled, as Ip,c
depends linearly on the square root of the scan rate in the range
20 mV/s e V e 500 mV/s. Comparison of the cathodic peak
current of Ru(SnPh3)2(CO)2(iPr-DAB) with the anodic peak
current of ferrocene oxidation points to a transfer of a single
electron in the reduction step.
Electrochemical reversibility indicates that the reduction of
Ru(SnPh3)2(CO)2(iPr-DAB) is not accompanied by any major
structural change, apparently producing the [Ru(SnPh3)2(CO)2-
(iPr-DAB)]•- radical anion. Chemical reversibility points to
its inherent stability, which made it possible to generate and
study this species spectroscopically.
Cyclic Voltammetry. The cyclic voltammogram of Ru-
(SnPh3)2(CO)2(iPr-DAB) was recorded in THF at room tem-
(63) Harris, R. K.; Kennedy, J. D.; McFarlane, W. In NMR and the Periodic
Table.; Harris, R. K., Mann, B. E., Eds.; Academic Press: London,
1978; p 342.
(64) Andre´a, R. R.; de Lange, W. G. J.; Stufkens, D. J.; Oskam, A. Inorg.
Chim. Acta 1988, 149, 77.
(65) Braterman, P. S. Metal Carbonyl Spectra; Academic Press: London,
1975.
(66) Ugo, R.; Cenini, S.; Banati, F. Inorg. Chim. Acta 1967, 451.
(67) Holt, M. S.; Wilson, W. L.; Nelson, J. H. Chem. ReV. 1989, 89, 11.
Formation and Spectroscopic Properties of [Ru(SnPh3)2-
(CO)2(iPr-DAB)]•-. Spectroelectrochemistry. Upon the one-
electron reduction, the two ν(CO) bands of the parent compound
(2004, 1951 cm-1) shift isosbestically to 1975, 1910 cm-1
(Figure 3), with a nearly 100% recovery of the original band
intensities upon reoxidation. The UV-vis spectra monitored

5472 Inorganic Chemistry, Vol. 35, No. 19, 1996
Aarnts et al.
simulated multiplet are slightly sharper than the experimental
ones since the very small 12 aH couplings of the four methyl
groups of the iPr moieties were not included in the simulation.
Furthermore, the hyperfine couplings arising from the two
117/119Sn nuclei were also not considered in this simulation
procedure because the program used did not correct for the
above-mentioned second-order effects. As a result, the central
line of the triplet, which is hidden under the left branch of the
central multiplet, is not taken into account. Nevertheless, the
simulated spectrum fits very accurately with the experimental
one; see Figure 5. The EPR spectrum thus fully confirmed the
proposed composition of the [Ru(SnPh3)2(CO)2(iPr-DAB)]•-
species, in particular the symmetry equivalence of the two Sn
nuclei.
MO Calculations. In order to understand the electronic
structure and bonding properties of the parent complex as well
as of the radical-anionic product, MO calculations were
performed on the model compound Ru(SnH3)2(CO)2(H-DAB)
and its radical anion. The bond distances and bond angles of
this model complex, in which the SnPh3 ligands are replaced
by SnH3, and isopropyl substituents of iPr-DAB by hydrogen
atoms, were taken from the crystallographic data of the parent
complex, except for the N-H and Sn-H bond distances, for
which values of 1.01 and 1.7 Å, respectively, were used.70 The
results of these calculations are presented in Tables 4-6. They
were used to construct a qualitative MO scheme shown in Figure
6, which gives a more pictorial insight into the electronic
structure and which will be used as a basis for further discussion.
Before discussing the characters of the orbitals, it is useful to
describe separately the orbitals of the SnH3 and H-DAB
fragments. The relevant orbitals of SnH3 are the 3a1 and 3e
orbitals. The 3a1 is the singly occupied “sp3” hybrid; the 3e is
a combination of the Sn-H σ bonding orbitals. For the H-DAB
ligand, the 2b1 is the lowest unoccupied π* orbital and the 1a2
is the highest occupied π orbital.
Figure 4. UV-vis spectral changes monitored during the spectro-
electrochemical reduction of Ru(SnPh3)2(CO)2(iPr-DAB) in THF at
room temperature, using the OTTLE cell.48
during the spectroelectrochemical reduction (Figure 4) also show
a clean, isosbestic conversion. The relative intensities of the
IR bands of the reduction product and their wavenumber
difference of ∆ν˜ ) 65 cm-1 are very similar to those of the
parent complex, ∆ν˜ ) 53 cm-1. This implies that the structure
of the complex has hardly changed upon reduction.
Thus, the spectroelectrochemical results clearly show that Ru-
(SnPh3)2(CO)2(iPr-DAB) is reduced to an intrinsically stable
trans,cis-[Ru(SnPh3)2(CO)2(iPr-DAB)]•- radical anion which
retains the structure of the parent complex. An identical product
is formed by reduction with 1% sodium amalgam in THF, as
was checked by IR and UV-vis spectroscopies. In fact, Ru-
(SnPh3)2(CO)2(iPr-DAB) is the first complex of the [Ru(L)-
(L′)(CO)2(R-diimine)]0,+ family12,68 that may be reduced to its
radical anion without losing one of the axial ligands.
EPR. In order to understand the bonding properties and the
unpaired electron localization in the [Ru(SnPh3)2(CO)2(iPr-
DAB)]•- radical-anionic complex, its EPR spectrum was studied
in detail. The EPR signal, obtained in THF solution, is centered
at g )1.9960. It conists of singlet, doublet, and triplet features
due to the hyperfine coupling with 0, 1, and 2 117/119Sn nuclei,
respectively, each of them being further split by the hyperfine
coupling with one 99/101Ru nucleus, two equivalent 14N nuclei,
and two pairs of equivalent 1H nuclei, the latter corresponding
to the pairs of imine and iPr hydrogens, respectively, (Figure
5, Table 3). The intensity ratios between the singlet, doublet,
and triplet components arising from the 117/119Sn couplings
correspond reasonably well to the calculated values: experi-
mental, 100.00:18.26:0.87; calculated, 100.00:17.38:0.77. The
tin satellite lines in the spectra are found to be asymmetrically
placed about the central line. This is because the hyperfine
splitting is sufficiently large to invalidate the high-field ap-
proximation in the Breit-Rabit formula.69 This second-order
effect results in a field- (H-) dependent 117/119Sn splitting
constant. In order to obtain the experimental aSn value,
corrections to the measured coupling constants were calculated
(one iteration cycle) using the correction factors taken from ref
69. To obtain the experimental aN, aH, and aRu values, the
central multiplet was computer-simulated. The lines of the
The symmetric (sp3 + sp3) combination of the SnH3 orbitals
mixes with the Ru 4dz orbital while their antisymmetric (sp3
2
- sp3) combination interacts with the 5pz metal orbital. Hence,
a delocalized three-center, four-electron Sn-Ru-Sn σ-bond is
formed. MO calculations show that the HOMO of the complex
is the Ru(pz) + Sn(sp3-sp3) combination (10b1) which mixes
also with the lowest π* orbital of H-DAB(2b1). The 11b1
LUMO orbital is dominated (61%) by the contribution from
the 2b1 H-DAB π* orbital with a characteristic admixture of
the metal dyz orbital (11%). However, even the LUMO contains
a large contribution from the Sn (sp3-Sp3) combination (27%)
that again results from the σ-π* mixing. The σ-π* interaction
actually gives rise to a strong delocalization of the electron
density from the Sn-Ru-Sn σ-bond to the H-DAB ligand that
occurs together with of the usual π-back-bonding via the
dyz-π* mixing in the occupied 9b1 and empty 11b1 (LUMO)
orbitals.
Calculations on the [Ru(SnH3)2(CO)2(H-DAB)]•- radical
anion confirmed that the extra electron enters the 11b1 LUMO,
as expected (Tables 4 and 5). The large positive shift of all
the calculated orbital energies is due to the uncompensated effect
of the mononegative charge of the radical anion in the vacuum
that, however, does not affect the energy differences between
the molecular orbitals. These may be readily compared by
perusal of the MO-diagrams shown in Figure 7 where the 10b1
HOMO of the parent complex and of the radical anion are
placed, for clarity, at the same energy. It may be seen that there
are only small differences in the relative energies of the orbitals
(68) Nieuwenhuis, H. A. Thesis, University of Amsterdam, 1994.
(69) Goodman, B. A.; Raynor, J. B. AdV. Inorg. Chem, Radiochem. 1970,
13, 135.
(70) Huheey, J. E. Inorganic Chemistry, Principles of Structure and
ReactiVity, 3rd ed.; Harper and Row: New York, 1983.

Ru(SnPh3)2(CO)2(iPr-DAB)
Inorganic Chemistry, Vol. 35, No. 19, 1996 5473
Figure 5. EPR spectrum of [Ru(SnPh3)2(CO)2(iPr-DAB)]•- in THF at room temperature generated by in situ reduction of the parent complex with
1% Na amalgam. Sn* ) 117/119Sn isotopes (modulation amplitude 0.4 G; attenuation 10 dB).
Table 3. EPR Parameters of [Ru(SnPh3)2(CO)2(iPr-DAB)]•-
obtained in THF solution (g ) 1.9960 ( 0.0002)
The results of the density functional calculations on the [Ru-
(SnH3)2(CO)2(H-DAB)]•- model compound have been also used
to calculate the hyperfine tensor and the isotropic hyperfine
splitting constants, using the second order perturbation theory.61
The splitting constants are shown in Table 3. Even though the
calculation somewhat underestimates the values of aRu and
overestimates the aH and aN values, the generally good cor-
respondence with the experiment indicates that the conclusions
from the MO calculations on the model compound are well
applicable to the complex actually studied.
splitting const (G) deviation
natural
nucleus no. abundance (%) spin
exptl
calcdc absolute
%
5
99Ru
101Ru
117Sn
119Sn
14N
1
1
2
2
2
2
2
12.7
17.1
7.6
/
/
/
/
/
/
/
5.7a
6.4a
4.14 -1.56 27
4.65 -1.75 27
2
2
2
2
1
2
2
5
1
1
1
1
1
317.0b
332.0b
8.20a
344
360
+27
+28
8
8
8.6
99.6
99.9
99.9
11.59 +3.39 29
1He
3.55a,g
3.25a/g
4.32 +0.77 18
1Hf
d
d
d
Electronic Absorption Spectra and Their Assignment.
The parent complex shows a narrow strong absorption band at
511 nm (ꢀ ) 6000 M-1 cm-1) in THF, shown in Figure 4, which
will mainly belong to the HOMOfLUMO transition. This
assignment is fully supported by the results of the MO
calculations of the transition probabilities which show that the
HOMOfLUMO transition is 6.96 and 9.79 times more probable
than the energetically close 6a2 (dxz)fLUMO and 9b1
(dyz)fLUMO MLCT transitions respectively. (The probabilities
of the electronic transitions were obtained by calculating the
transition dipole matrix element. The transition probability is
directly proportional to the square of this matrix element.) As
follows from the characters of HOMO and LUMO (Table 4),
the 10b1f11b1 transition may be described qualitatively as σ-
(Sn-Ru-Sn)fπ*. The delocalized nature of the orbitals
involved is responsible for a rather small charge-separation in
the excited state, which is manifested by a relatively small
solvatochromism (710 cm-1; λmax ) 508 nm in CH3CN and
527 nm in hexane). For comparison, the typical MLCT
absorption band of Ru(Cl)(SnPh3)(CO)2(iPr-DAB) exhibits
a Based on the simulated values. b Determined from the spectra using
the correction factors taken from ref 69. c Calculated from the theoretical
spin density distribution in (Ru(SnH3)2(CO)2(H-DAB) obtained from
the DFT calculations. d No iPr-H in the model compound. e Imine
proton (CHdN(iPr)). f iPr proton (CH(CH3)2). g We assume that also
in the case aH(CHdN(iPr) > aH(CH(CH3)2), in agreement with the data
collected in Table 8.
below the HOMO but that the energy differences between the
HOMO and higher orbitals are clearly different (Tables 4 and
5 and Figure 7). Thus, the calculated energy difference between
the HOMO and SOMO of the reduced complex (2.090 eV) is
larger than that between the HOMO and LUMO of the parent
complex (1.940 eV). The energy difference between the SOMO
of the radical anion and its higher-lying orbitals is smaller,
compared with that between the LUMO and the higher
unoccupied orbitals of the parent complex. Data in Tables 4
and 5 also show that the characters of the 10b1 HOMO and
11b1 LUMO of the parent complex change only little upon
reduction, while the unoccupied 20a1 and 19a1 orbitals undergo
significant redistribution. Notably, a 7% contribution of the
H-DAB nitrogen σ-lone-electron pairs (6a1) in the radical-anion
suggest even some axial-equatorial σ-delocalization. To
understand the charge redistribution in the Ru(SnH3)2(CO)2(H-
DAB) molecule upon its one-electron reduction, the differences
in the valence-electron densities between Ru(SnH3)2(CO)2(H-
DAB) and [Ru(SnH3)2(CO)2(H-DAB)]•- were calculated. The
results, summarized in Table 6, clearly show that the electron
density at Ru hardly changes. The extra electron density in
the radical anion is mainly accepted by the SnH3 3a1 and H-DAB
2b1 orbitals, while a smaller part is spread over the CO ligands
via the normal π-back-donation mechanism.
solvatochromism ca. 2.5 times larger, i.e. 1710 cm-1 62
It should
.
be noted, however, that the shielding effect of the two bulky
SnPh3 groups may also diminish the solvatochromism.
Figure 4 reveals that, upon reduction of Ru(SnPh3)2(CO)2-
(iPr-DAB), the strong band at 511 nm is replaced by two bands
of the radical anion at 454 (ꢀ ) 3300 M-1 cm-1) and 652 nm
(ꢀ ) 1900 M-1 cm-1), respectively. On the basis of the
similarly narrow band shape, the former, high-energy band is
assigned to the 10b1f11b1 HOMOfSOMO transition that is
the counterpart of the HOMOfLUMO transition of the parent
species. The low-energy band will then belong to the excitation

5474 Inorganic Chemistry, Vol. 35, No. 19, 1996
Aarnts et al.
CO
Table 4. Characters (%)a and Energies of the Relevant MO’s of Ru(SnH3)2(CO)2(H-DAB)b
orbital
20a1
19a1
13b2
11b1
10b1
6a2
description
ꢀ (eV)
Ru
SnH3
46% 3a1
20% 3a1
H-DAB
2
σ* Ru-Sn
σ* Ru-Sn
2π CO
π*
σ Ru-Sn
dt2g
-1.400
-1.802
-2.075
-3.984
-5.924
-6.509
-7.009
-7.407
19% dz
23% 2π, 5% 5σ
41% 2π
62% 2π
2
2
10% dx -y
12% px, 9% dxy
11% dyz
15% pz
9% 4b2
61% 2b1
27% 2b1
13% 1a2
6% 2b1
27% 3a1
42% 3a1
12% 3e
27% 3e
9% 2π
63% dxz
11% 2π
9b1
18a1
dt2g
dt2g
53% dyz
7% 2π
2
2
67% dx -y
9% 1π, 17% 2π
12b2, 5a2
17a1, 8b1
16a1
Sn-H (4×)
≈-7.7
-7.820
-9.061
≈5%
11% 5s, 53% dz
16% dxz
≈85%
≈4%
}
2
σ Ru-Sn
7% 2a1, 34% 3a1
5% 3e
4a2
π
73% 1a2
a Based on Mulliken population analyses per MO. b The HOMO (10b1) and LUMO (11b1) orbitals are printed in boldface type.
Table 5. Characters (%)a and Energies of the Relevant MO’s of [Ru(SnH3)2(CO)2(H-DAB)]•- b
orbital
20a1
19a1
description
ꢀ (eV)
2.224
2.107
1.896
Ru
SnH3
18% 3a1
32% 3a1
H-DAB
CO
c
c
2
σ* Ru-Sn
σ* Ru-Sn
2π CO
π*
σ Ru-Sn
dt2g
dt2g
dt2g
σ Ru-Sn
6% dz , 42% 6s
7% 6a1
7% 6a1
7% 4b2
66% 2b1
24% 2b1
19% 1a2
4% 2b1
17% 2π
27% 2π
68% 2π
2
2
2
11% dz , 7% dx -y
10% px, 6% dxy
9% dyz
13b2
11b1
10b1
6a2
9b1
18a1
0.300
25% 3a1
49% 3a1
5 % 3e
-1.790
-2.319
-2.887
-3.284
-3.656
12% pz
62% dxz
11% 2π
12% 2π
7% 1π, 20% 2π
60% dyz
68% dx -y
11% 3e
2
2
2
17a1
10% 5s, 43% dz
4% 2a2, 40% 3a1
12b2, 5a2,
8b1, 16a1
4a2
Sn-H (4×)
π
≈-3.9
-4.714
≈5%
19% dxz
≈85%
19% 3e
≈4%
56% 1a2
}
a Based on Mulliken population analyses per MO. b The HOMO (10b1) and SOMO (11b1) orbitals are printed in boldface type. c Symmetric
combination of nitrogen lone electron pairs.
Table 6. Changes in Valence-Electron Density (∆q) of Ru(SnH3)2(CO)2(H-DAB) upon One-Electron Reductiona
2
2
2
orbitals
Ru 5s
-0.04
Ru 4py
-0.02
Ru 4px
-0.01
Ru 4pz
-0.05
Ru 4dz
-0.05
Ru 4dx -y
-0.05
Ru 4dxy
+0.01
Ru 4dyz
+0.07
Ru 4dxz
-0.05
∆qb
orbitals
SnH3 3a1
+0.59
SnH3 e
+0.07
DAB a1
-0.05
DAB a2
-0.02
DAB 2b1
+0.51
DAB b2
-0.04
CO σ
CO π
+0.12
∆qb
a Based on Mulliken population analysis. b Change in valence-electron density upon reduction.
of the unpaired electron from the SOMO to the higher empty
orbitals, most probably the σ*(Sn-Ru-Sn) 19a1 and/or 20a1.
(Note that the transition to the 13b2 orbital is symmetry
forbidden.) This assignment is qualitatively supported by the
MO calculations which show that the energy difference between
the HOMO and SOMO in the radical anion of the model
compound is larger by 0.283 eV (2280 cm-1) than the difference
between the 19a1 orbital and the SOMO (11b1). Also, the
energy difference between the HOMO and SOMO in the radical
anion is larger by 0.15 eV (1210 cm-1) than between the HOMO
and LUMO of the parent complex, in qualitative agreement with
the high-energy shift of the 10b1f11b1 transition by some 2460
cm-1 observed experimentally; see Figure 4.
axial ligands, appear to be responsible for most of the unusual
properties of Ru(SnPh3)2(CO)2(iPr-DAB) and its radical anion.
Thus, the donation of electron density from the Sn-Ru-Sn
unit to the π*-DAB orbital manifests itself already in the
molecular structure of Ru(SnPh3)2(CO)2(iPr-DAB) which shows
longer CdN and shorter C(1)-C(2) bonds of the iPr-DAB
ligands than analogous complexes not containing trans-σ-bonded
ligands, like Ru(Cl)(SnPh3)(CO)2(iPr-DAB) and Ru(I)(Me)-
(CO)2(iPr-DAB).62,71 These differences in bond-lengths are in
accordance with the CdN antibonding and C(1)-C(2) bonding
character of the DAB-2b1 π* orbital.
Also, the ν(CO) wavenumbers of Ru(SnPh3)2(CO)2(iPr-DAB)
are relatively low, reflecting a larger electron density on the
Ru atom. The σ-π* interaction also changes the character of
the electronic transition responsible for the visible absorption
band. For most of the carbonyl-diimine complexes studied
previously, such a band belongs to MLCT transitions. In Ru-
(SnPh3)2(CO)2(iPr-DAB) and also [Ru(SnPh3)2(CO)2(iPr-DAB)]•-,
the HOMOfLUMO (SOMO) transition occurs between the
delocalized 10b1 and 11b1 orbitals that both contain significant
SnPh3 contribution. This change in character of the electronic
transition is manifested by a higher extinction coefficient and
smaller solvatochromism of the absorption band, as compared,
e.g., with Ru(Cl)(SnPh3)(CO)2(iPr-DAB).62
Discussion
In the preceding chapters, the synthesis, structure, spectro-
scopic, and (spectro-) electrochemical properties of an unprec-
edented inorganometallic complex containing a nearly linear
Sn-Ru-Sn unit oriented perpendicular to the Ru(DAB) chelate
ring have been described. The MO calculations on the Ru-
(SnH3)2(CO)2(H-DAB) model compound and its radical anion
revealed the most important aspects of the electronic structure
of these two complexes, namely the strong electron donation
from the SnPh3 ligands and the extensive mixing of the σ(Sn-
Ru-Sn) and DAB π* orbitals in both the HOMO and LUMO
(SOMO). These unique electronic features, which are absent
in carbonyl-diimine complexes that lack strongly σ-bonded
The up-field shift of the imine-proton in the 1H NMR
spectrum, and the increased value of the 4J(117/119Sn,H) coupling
(71) Kleverlaan, C. J.; et al. Unpublished results.

Ru(SnPh3)2(CO)2(iPr-DAB)
Inorganic Chemistry, Vol. 35, No. 19, 1996 5475
Table 7. EPR Parameters of Selected Radical Compounds
Containing Ru or Sn Hyperfine Splitting Constants
99 101
compound
a( Ru) (G)
/
g
conditions
ref
80
[RuCp(CO)2N(O)R]•
5.04/5.70 2.0078 toluene,
room temp
2.0048 toluene, 313 K 81
[Ru(SiMe3)(CO)4N(O)R]•
[Ru(bpy)(CN)4]3-
≈5
3.0
1.999 DMF,
room temp
4.58/5.14 1.9934 MeCN,
room temp
2.002 CH2Cl2,
82
76
83
[Ru(bpz)(CN)4]3-
[Ru(PPh3)2(CO)2(o-O2C6Cl4)•+
3.7
room temp
117
a(
119
a(
)
Sn
Sn)
compound
(G)
(G)
g
conditions
ref
Sn•(N(SiMe3)2)3
3176
3426
1776
2.0094
1.9912
C6H6, 293 K 84
C6H6, 293 K 84
Sn•(CH(SiMe3)2)3 1698
(Me3)SnCH2C•H2
(Me3)SnC•H2
467.7
132.5
488.9 2.00205 172 K
137.0 203 K
85
86
a
a Not reported.
(CO)2(iPr-DAB) is reduced considerably more positively (E1/2
) -1.55 V Vs Fc/Fc+ in THF)12,68,74 in accordance with its
lower LUMO energy relative to Ru(SnPh3)2(CO)2(iPr-DAB).
This comparison reflects the difference between the bonding
properties of the two complexes where the LUMO of Ru(I)(Me)-
(CO)2(iPr-DAB) has a major contribution from the lowest π*
orbital of iPr-DAB whereas the LUMO of the Ru(SnPh3)2(CO)2-
(iPr-DAB) possesses a significantly mixed iPr-DAB/SnPh3
character. The Ru(Cl)(SnPh3)(CO)2(iPr-DAB) complex is also
reduced much more positively (E1/2 ) -1.48 V Vs Fc/Fc+ in
THF)36 than Ru(SnPh3)2(CO)2(iPr-DAB). This comparison
clearly shows that it is the simultaneous presence of two axial
covalent Ru-Sn bonds that leads to the σ-π* interaction and,
hence, to a rise in the LUMO energy and drop in the reduction
potential. Interestingly, rather negative reduction potentials were
also found for other DAB complexes that contain σ-bonded
ligands like Re(Me)(CO)3(iPr-DAB) (E1/2 ) -1.74 V Vs Fc/
Fc+ in nPrCN)73 and Pt(Me)4(c-hexyl-DAB) (E1/2 ) -1.93 V
Vs Fc/Fc+ in CH3CN).21
Figure 6. Qualitative MO scheme of Ru(SnH3)2(CO)2(H-DAB) and
chosen orientation of the axes.
One of the most spectacular features of Ru(SnPh3)2(CO)2-
(iPr-DAB) is its reduction to intrinsically stable [Ru(SnPh3)2-
(CO)2(iPr-DAB)]•-. Analogous Ru(L)(L′)(CO)2(R-diimine)12,68
complexes undergo facile ligand dissociation upon reduction,
whereas σ-bonded Re and Pt species (e.g. Re(SnPh3)(CO)3(1,-
10-phenanthroline),75 Pt(Me)4(c-hexyl-DAB)21) are also revers-
ibly reduced to rather stable radical anions. The results of the
MO calculations clearly show that the remarkable stability of
[Ru(SnPh3)2(CO)2(iPr-DAB)]•- originates in the large capacity
of the SnPh3 and DAB ligands and, to a lesser extent, of the
CO ligands to accommodate the extra electron density conferred
upon the Ru(SnPh3)2(CO)2(iPr-DAB) molecule by its one-
electron reduction. This effect is well documented by the
calculated differences of the Mulliken valence-electron densities
in inidividual orbitals between the Ru(SnH3)2(CO)2(H-DAB)
model compound and its radical anion (Table 6). It is also
evidenced by the negative shift of ν(CO) wavenumbers (by
about 34 cm-1) upon reduction, that is caused by the increased
RufCO π-back-bonding and, most of all, by the hyperfine
splitting constants determined from the EPR spectrum of [Ru-
(SnPh3)2(CO)2(iPr-DAB)]•-. These splitting constants reflect
the localization of the spin density on individual atoms in the
molecule, i.e. the localization of the SOMO wavefunction.
Assuming that the SOMO of [Ru(SnPh3)2(CO)2(iPr-DAB)]•-
and the LUMO of Ru(SnPh3)2(CO)2(iPr-DAB) have similar
Figure 7. MO diagrams of Ru(SnH3)2(CO)2(H-DAB) and [Ru(SnH3)2-
(CO)2(H-DAB)]•-, based on DFT calculations. The HOMO’s are
positioned at the same energy level.
constants also reflect this delocalized character of the Ru-DAB
chelate ring.72
The value of the reduction potential (E1/2 ) -1.86 V Vs Fc/
Fc+ in THF), which is more negative than those found for most
of known DAB-complexes,3,12,68,73,74 is also in line with the
strong σ-π* interaction which changes the character of LUMO
and increases its energy because of a mixing between the σ-
(Sn-Ru-Sn) and π*-DAB orbitals. For comparison, Ru(I)(Me)-
(72) tom Dieck, H.; Renk, I. W.; Franz, K. D. J. Organomet. Chem. 1975,
94, 417.
(73) Rossenaar, B. D.; Hartl, F.; Stufkens, D. J. Organometallics, submitted
for publication.
(74) tom Dieck, H.; Rohde, W.; Behrens, U. Z. Naturforsch. 1989, 44B,
(75) Luong, J. C.; Faltynek, R. A.; Wrighton, M. S. J. Am. Chem. Soc.
1979, 101, 1597.
158.

5476 Inorganic Chemistry, Vol. 35, No. 19, 1996
Aarnts et al.
Table 8. EPR Parameters of Some Largely R-DAB Localized Radicals Related to [Ru(SnPh3)2(CO)2(iPr-DAB)]•-
Hyperfine splitting constants (G)
a
b
c
compound
aM
aN
aH
aH
ax
g
conditions
Et2O, 293 K
THF
ref
[iPr-DAB]•-
5.47
5.6
4.87
5.60
6.84
6.96
6.8
7.34
7.4
7.48
8.20
8.20
4.79
4.3
5.87
5.6
4.1
3.95
4.0
5.03
4.0
4.35
4.20
3.55
3.74
0.15
2.0034
2.0035
e
2.0024
2.0039
g
87
88
[tBu-DAB]•-
d
[Zn(Et)(tBu-DAB)]•
0.48, aH
0.58, aCl
T > 223 K
89,90
[Zn(Cl)(tBu-DAB)]• f
4.4
THF, 213-303 K
THF, 298 K
DMF, 293 K
DMF, 293 K
cyclohexane, 343 K
DME, 253 K
toluene, 203 K
THF, 295 K
THF, 293 K
88
3
77
77
91
3
91
21
[Mo(CO)4(iPr-DAB)]•-
[Mo(CO)3(PBu3)(tBu-DAB)]•-
[Mo(CO)3(PBu3)2(tBu-DAB)]•-
[Re(CO)3(tBu-DAB)]•
[Cr(CO)4(iPr-DAB)]•-
[Mn(CO)3(tBu-DAB)]•
[Pt(Me)4(cHexcyl-DAB)]•-
[Ru(SnPh3)2(CO)2(iPr-DAB)]•-
1.52
2.95
3.1
35.55
44.1, ap
45.0, ap
g
2.0046
2.0031
2.0043
1.9945
1.9960
1.65
3.25
8.47
61.2
6.40
≈325, aSn
this work
a The hyperfine splitting of the CHdNR proton of the R-DAB ligand. b Additional hyperfine splitting of the C(CH3)3 protons in tBu-DAB
complexes, and the CH(CH3)2 protons in iPr-DAB complexes. c Additional splitting. d RH of the ethyl group. e Not reported. f 67Zn enriched. g g
value between 2.000 and 2.010.
characters, as is substantiated by the MO calculations (Tables
4 and 5), the EPR spectrum also affords indirect information
on the LUMO of the Ru(SnPh3)2(CO)2(iPr-DAB) parent. To
put the measured hyperfine splitting constants (Table 3) in a
broader perspective, a comparison is made with previously
reported hyperfine splitting constants of some radical species
containing Sn or Ru atoms and/or R-diimine ligands (see Tables
7 and 8).
The value of the Ru hyperfine splitting constant, aRu ) 6.4
G, measured for [Ru(SnPh3)2(CO)2(iPr-DAB)]•- is only slightly
larger than the aRu values of RuII complexes with coordinated
ligand-localized radicals collected in Table 7. This is in
accordance with the proposed dominant localization of the 11b1
SOMO on the iPr-DAB and SnPh3 ligands together with the
positive contribution from the σ-π* mixing that would not
occur in complexes lacking σ-bonded ligands.
The coupling constants collected in Table 8 provide informa-
tion about the SOMO distribution over the M-(DAB) chelate
ring of the radical anion. In view of the fact that aN and aH
constants of iPr-DAB•- and tBu-DAB•- closely resemble one
another, a comparison is made with related tBu-DAB complexes.
It is obvious from Table 8 that large values of aN are
accompanied by small values of aH and Vice Versa. This reflects
the variable spin density in the π* SOMO on the carbon or the
nitrogen atoms. The large aN and small aH values for [Ru-
(SnPh3)2(CO)2(iPr-DAB)]•- demonstrate that a large part of the
spin density on the DAB skeleton is localized on the nitrogen
atoms, reflecting thus the σfπ* donation of the spin density.
In contrast to the g values of most of the DAB complexes
listed in Table 8, the g value of 1.9960 found for [Ru(SnPh3)2-
(CO)2(iPr-DAB)]•- is considerably smaller than the free electron
value, ge ) 2.0023. It lies much closer to the g value of [Pt-
(Me)4(cHexyl-DAB)]•-.21 This small value is due to the spin-
orbit interaction that arises from the presence of three heavy
atoms. The negative sign of the g deviation from the ge value
indicates that the spin-orbit coupling dominantly involves the
admixture of higher unoccupied orbitals78,79 with a significant
contribution from the Ru and Sn orbitals (19a1, 20a1), rather
than of lower, doubly occupied ones, into the SOMO. This
observation is in full accordance with the results of the MO
calculations which show that the HOMO-SOMO energy gap
is larger than the separation between the SOMO and the next
two unoccupied orbitals by 0.49 and 0.28 eV respectively. The
visible absorption spectrum of [Ru(SnPh3)2(CO)2(iPr-DAB)]•-
points to the same conclusion, showing the HOMOfSOMO
transitions at a significantly higher energy than the SOMOf19a1/
20a1 transition. In a forthcoming article it will be demonstrated
Comparing the 117/119Sn hyperfine splitting of [Ru(SnPh3)2-
(CO)2(iPr-DAB)]•- (317 and 332 G, respectively) with some
data taken from the literature (Table 7), we see that the radicals,
in which the unpaired electron is dominantly localized on the
Sn atom, exhibit only 5-10 times larger Sn hyperfine splitting
constant. The splitting constant aSn rapidly decreases when the
unpaired electron is more localized on the R-carbon atom as in
Me3SnC•H2, and increases again when localized on the â-carbon
atom (Table 7). The still rather large aSn is a very remarkable
feature of the EPR spectrum of [Ru(SnPh3)2(CO)2(iPr-DAB)]•-.
It may be readily explained by the σπ* delocalization, i.e. the
large (25 %) participation of the Sn sp3 orbital in the SOMO
predicted by the MO calculations. Further information on the
relative magnitude of this effect, in relation to other radical
complexes, can be obtained from the a/Aiso ratios.76 The ratio
117
117
119
Sn)
119Sn)
of 0.044 obtained for both a( Sn)/Aiso(
and a( Sn)/Aiso(
(78) Kaim, W. Coord. Chem. ReV. 1987, 76, 187.
in [Ru(SnPh3)2(CO)2(iPr-DAB)]•-, is considerably larger than
(79) Kaim, W.; Kohlmann, S. Inorg. Chem. 1990, 29, 2909.
(80) Sostero, S.; Rehorek, D.; Polo, E.; Traverso, O. Inorg. Chim. Acta
1993, 209, 171.
31
31P)
the ratio of a( P)/Aiso(
(0.012) in [Mo(PBu3)2(CO)2(tBu-
) -7268, Aiso(
DAB)]•-77 (Aiso(
) -7603, Aiso(
)
117Sn)
119Sn)
31P)
3676);69 see Table 8. The much larger spin density on Sn in
the former complex compared to that on P in the latter one may
be related to the higher energy of the a1 (sp3) σ-donor orbital
of SnPh3 in comparison with that of PBu3, giving rise to a
stronger σ-π* mixing in [Ru(SnPh3)2(CO)2(iPr-DAB)]•- than
in [Mo(PBu3)2(CO)2(tBu-DAB)]•-. These observations fully
support the above conclusions on the high capacity of the SnPh3
ligands to accommodate the extra electron density in the radical
anion.
(81) Hudson, A.; Lappert, M. F.; Lednor, P. W.; MacQuitty, J. J.; Nicholson,
B. K. J. Chem. Soc., Dalton Trans. 1981, 2159.
(82) Samuels, A. C.; DeArmond, M. K. Inorg. Chem. 1995, 34, 5548.
(83) Connelly, N. G.; Manners, I.; Protheroe, J. R. C.; Whiteley, M. W. J.
Chem. Soc., Dalton Trans. 1984, 2713.
(84) Cotton, J. D.; Cundy, C. S.; Harris, D. H.; Hudson, A.; Lappert, M.
F. J. Chem. Soc., Chem. Commun. 1974, 651.
(85) Kawamura, T.; Kochi, J. K. J. Am. Chem. Soc. 1972, 94, 648.
(86) Hudson, A.; Hussain, H. A. J. Chem. Soc. 1969, B, 793.
(87) de Klerk-Engels, B.; Hartl, F.; Vrieze, K. Inorg. Chim. Acta, in press.
(88) Clopath, P.; von Zelewsky, A. HelV. Chim. Acta 1972, 55, 52.
(89) Jastrzebski, J. T. B. H.; Klerks, J. M.; van Koten, G.; Vrieze, K. J.
Organomet. Chem. 1981, 210, C49.
(76) Waldho¨r, E.; Poppe, J.; Kaim, W.; Cutin, E. H.; Gar´ıa Posse, M. E.;
Katz, N. E. Inorg. Chem. 1995, 34, 3093.
(77) tom Dieck, H.; Franz, K. D.; Hohmann, F. Chem. Ber. 1975, 108,
163.
(90) Klerks, J. M.; Jastrzebski, J. T. B. H.; van Koten, G.; Vrieze, K. J.
Organomet. Chem. 1982, 224, 107.
(91) Andre´a, R. R.; de Lange, W. G. J.; van der Graaf, T.; Rijkhoff, M.;
Stufkens, D. J.; Oskam, A. Organometallics 1988, 7, 1100.

Ru(SnPh3)2(CO)2(iPr-DAB)
Inorganic Chemistry, Vol. 35, No. 19, 1996 5477
that this delocalized character of HOMO and LUMO has also
a dramatic effect on the excited state properties of the neutral
parent complex.
the highly delocalized character of the SOMO (i.e. LUMO of
the parent complex) which allows the accommodation of the
extra electron density on the SnPh3, iPr-DAB, and, to a lesser
extent, CO ligands.
Conclusions
Acknowledgment. The Netherlands Foundation for the
Chemical Research (SON) and the Netherlands Organisation
for Advancement of Pure Research (NWO), together with COST
D4 Action and European Scientific Network Program, are
thanked for financial support. We also thank Dr. S. P. J.
Albracht of the E. C. Slater Institute for Biochemical Research
for his assistance with the EPR measurements.
The simultaneous presence of two trans-oriented Ru-Sn
σ-bonds influences profoundly the bonding within the Ru-
(SnPh3)2(CO)2(iPr-DAB) molecule and leads to an electronic
structure that is unprecedented among substituted metal carbonyl
diimine complexes known. Its main features are a delocalized
3-center, 4-electron Sn-Ru-Sn σ-bond and an extensive mixing
of the Ru-d(π), Ru-5pz, Sn-sp3, and DAB-π* characters in
both the HOMO and LUMO, which results in a delocalization
of the σ-electron density on the iPr-DAB ligand. Reduction of
Ru(SnPh3)2(CO)2(iPr-DAB) affords a radical anion that has
approximately the same molecular and electronic structure as
the parent molecule. The remarkable stability of the radical
anion stems from the strength of the Sn-Ru-Sn bond and from
Supporting Information Available: Listing of the fractional
coordinates and thermal parameters for Ru(SnPh3)2(CO)2(iPr-DAB) and
the hydrogen atom parameters, bond distances, and bond angles (11
pages). Ordering information is given on any current masthead page.
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