Synthesis and characterization of the M(II) (M = Ge, Sn, or Pb) phosphinidene dimers {M(μ-PAr′)}2 (Ar′ = C 6H3-2,6-(C6H3-2,6-Pr i2)2)

Article abstract of DOI:10.1021/ic101068a

Reaction of M{N(SiMe₃)₂}₂ (M = Ge, Sn, or Pb) with the sterically encumbered primary phosphine Ar′PH₂ (2), Ar′ = C₆H₃-2,6-(C₆H ₃-2,6-Prⁱ₂), at ca. 200 °C afforded the highly colored phosphinidene dimers {M(μ-PAr′)}₂, M = Ge(3), Sn(4), or Pb(5), with disilylamine elimination. The compounds were characterized by single-crystal X-ray crystallography and heteronuclear NMR spectroscopy. The structures of 3, 4, and 5 featured similar M₂P ₂ ring cores, of which 4 and 5 have 50/50 P atom disorder, consistent with either a planar four-membered M₂P₂ arrangement with anti aryl groups or with an M₂P₂ ring folded along the M-M axis with syn aryl groups. A syn-folded structure was resolved for the Ge ₂P₂ ring in compound 3. The M-P distances resembled those in M(II) phosphido complexes and are consistent with single bonding. The coordination geometries at the phosphorus atoms are pyramidal. DFT calculations on the gas phase models {M(μ-PMe)}₂ (M = Ge, Sn, Pb) agreed with the syn (M-M folded) structural interpretation of the X-ray data. The synthesis of the bulky phosphine Ar′PH₂ 2 with the use of the aryl transfer agent Ar′MgBr(THF)₂ is also reported. This route afforded a significantly higher yield of product than that which was obtained using LiAr′, which tends to result in aryl halide elimination and the observation of insoluble red phosphorus.

Full text of DOI:10.1021/ic101068a

Inorg. Chem. 2010, 49, 8481–8486 8481
DOI: 10.1021/ic101068a
Synthesis and Characterization of the M(II) (M = Ge, Sn, or Pb) Phosphinidene
Dimers {M(μ-PAr0)}2 (Ar0 = C6H3-2,6-(C6H3-2,6-Pri2)2)
W. Alexander Merrill,† Eric Rivard,†,‡ Jeffrey S. DeRopp,† Xinping Wang,† Bobby D. Ellis,†,§ James C. Fettinger,†
Bernd Wrackmeyer,^ and Philip P. Power*,†
†Department of Chemistry, University of California, One Shields Avenue, Davis, California 95616,
‡Present address: Department of Chemistry, University of Alberta, 11227 Saskatchewan Drive, Edmonton, Alberta,
Canada T6G 2G2, §Present address: Department of Chemistry, Acadia University, Wolfville, Nova Scotia,
^
€
Canada B4P 2R6, and Laboratorium fu€r Anorganische Chemie, Universitat Bayreuth, D-95440 Bayreuth, Germany
Received May 27, 2010
Reaction of M{N(SiMe3)2}2 (M = Ge, Sn, or Pb) with the sterically encumbered primary phosphine Ar0PH2 (2), Ar0 =
C6H3-2,6-(C6H3-2,6-Pri2), at ca. 200 ꢀC afforded the highly colored phosphinidene dimers {M(μ-PAr0)}2, M = Ge(3),
Sn(4), or Pb(5), with disilylamine elimination. The compounds were characterized by single-crystal X-ray crystal-
lography and heteronuclear NMR spectroscopy. The structures of 3, 4, and 5 featured similar M2P2 ring cores, of which
4 and 5 have 50/50 P atom disorder, consistent with either a planar four-membered M2P2 arrangement with anti aryl
groups or with an M2P2 ring folded along the M-M axis with syn aryl groups. A syn-folded structure was resolved for
the Ge2P2 ring in compound 3. The M-P distances resembled those in M(II) phosphido complexes and are consistent
with single bonding. The coordination geometries at the phosphorus atoms are pyramidal. DFT calculations on the gas
phase models {M(μ-PMe)}2 (M = Ge, Sn, Pb) agreed with the syn (M-M folded) structural interpretation of the X-ray
data. The synthesis of the bulky phosphine Ar0PH2 2 with the use of the aryl transfer agent Ar0MgBr(THF)2 is also
reported. This route afforded a significantly higher yield of product than that which was obtained using LiAr0, which
tends to result in aryl halide elimination and the observation of insoluble red phosphorus.
Introduction
A few lower aggregated species that possess the potential for
π-bonding between a group 14 empty p-orbital and nitrogen
lone pairs are also known. These include the dimers {Ge(μ-
NR)}2 (R = Mes*= C6H2-2,4,6-But314 or R = C6H2-2,4,6-
(CF3)315) and the trimeric {Ge(μ-NC6H3-2,6-Pri2)}3.16 How-
ever, in these compounds, π-bonding is not extensive owing to
the considerable electronegativity and size differences between
these elements which reduce orbital overlap. Some correspond-
ing derivatives of the heavier pnictogens {M(μ-ER)}n (E=P,
As, Sb, or Bi; n = 4-7) are also known. The first well-
characterized example was the distorted hexagonal prismatic
{Sn(μ-PSiPri3)}6, which was reported by Driess in 1997.17 It
was prepared by the reaction of Sn{C6H2-2,4,6-(CF3)3}2 or
Sn{N(SiMe3)2}2 with RPH2 (R=SiPri3 or Si(Trip)Pri2, Trip=
C6H2-2,4,6-Pri3), and in the presence of SnCl2, the adduct
(SnPSi(Trip)Pri2)2(SnCl2) could also be isolated. As a result,
it was proposed that the dimeric phosphanediyl (SnPR)2 was a
The heavier group 14 elements Ge, Sn, and Pb form nume-
rous oligomeric metal(II) imides which display a wide variety of
structures.1 Their most common formula is {M(μ-NR)}4 (M=
Ge, Sn, or Pb; R = organic or silyl groups), which feature a hetero-
cubane arrangement of group 14 element and nitrogen atoms.2-13
*E-mail: pppower@ucdavis.edu.
(1) Veith, M. Coord. Chem. Rev. 1990, 90, 1.
(2) Veith, M.; Sommer, M.-L.; Jager, D. Chem. Ber. 1979, 112, 2581.
(3) Veith, M.; Lange, H. Angew. Chem., Int. Ed. Engl. 1980, 19, 401.
(4) Veith, M.; Grossen, M. Z. Naturforsch. 1982, B37, 1375.
(5) Veith, M.; Schlemmer, G. Chem. Ber. 1982, 115, 2141.
(6) Veith, M.; Rechtenwald, O. Z. Naturforsch. 1983, B38, 1054.
(7) Chen, H.; Bartlett, R. A.; Dias, H. V. R.; Olmstead, M. M.; Power,
P. P. Inorg. Chem. 1991, 30, 3390.
€
(8) Grigsby, W. J.; Hascall, T.; Ellison, J. J.; Olmstead, M. M.; Power,
P. P. Inorg. Chem. 1995, 34, 3254.
(9) Allan, R. E.; Beswick, M. A.; Cromhout, N. L.; Paver, M. A.; Raithby,
P. R.; Teiner, A.; Trevithick, M.; Wright, D. S. Chem. Commun. 1996, 1501.
(10) Allan, R. E.; Beswick, M. A.; Edwards, A. J.; Paver, M. A.; Rennie,
M.-A.; Raithby, P. R.; Wright, D. S. J. Chem. Soc., Dalton Trans. 1995, 1991.
(11) Allan, R. E.; Beswick, M. A.; Coggan, G. R.; Raithby, P. R.;
Wheatley, A. E. H.; Wright, D. S. Inorg. Chem. 1997, 36, 5202.
(12) Benevelli, F.; Doyle, E. L.; Harron, E. A.; Feeden, N.; Quadrelli,
(14) Hitchcock, P. B.; Lappert, M. F.; Thorne, A. J. Chem. Commun.
1990, 1587.
(15) Ahlemann, J.-T.; Roesky, H. W.; Murugavel, R.; Parisini, E.;
Noltemeyer, M.; Schmidt, H.-G.; Muller, O.; Herbst-Irmer, R.; Markovskii,
L. N.; Shermolovich, Y. G. Chem. Ber. 1997, 130, 1113.
ꢀ
E. A.; Saez, D.; Wright, D. S. Angew. Chem., Int. Ed. 2000, 39, 1501.
(13) Armstrong, D. R.; Benevelli, F.; Bond, A. D.; Feeder, N.; Harron,
(16) Bartlett, R. A.; Power, P. P. J. Am. Chem. Soc. 1990, 112, 3660.
(17) Driess, M.; Martin, S.; Merz, K.; Pintchouk, V.; Pritzkow, H.;
ꢀ
E. A.; Hopkins, A. D.; McPartlin, M.; Moncrieff, D.; Saez, D.; Quadrelli,
€
Grutzmacher, H.; Kaupp, M. Angew. Chem., Int. Ed. Engl. 1997, 36, 1894.
E. A.; Woods, A. D.; Wright, D. S. Inorg. Chem. 2002, 41, 1482.
r
2010 American Chemical Society
Published on Web 08/24/2010
pubs.acs.org/IC

8482 Inorganic Chemistry, Vol. 49, No. 18, 2010
Merrill et al.
key intermediate which trimerized to form {Sn(μ-PR)}6 cage
structures. Several other related species with interesting structu-
ral arrangements have also been reported. They include the ar-
senic congener {Sn(μ-AsSiPri3)}6,18 the lead-arsenic hexamer
{Pb(μ-AsSiPri3)}6,19 the Sn-P tetramer {Sn(μ-PSiBut3)}4,20
stabilized by the bulkier SiBut3 substituent, together with the
mixed metallic cages such as {(THF)4Ba}2Sn2(μ-PSiBut3)4,20
the germanium-phosphorus hexamer {Ge(μ-PSiPri3)}6,21 and
the unique heptameric {Sn(μ-PSiPri3)}7.21 The common feature
of these compounds is that they have three-dimensional cage
structures in which the pnictogen is four-coordinate, as are the
Ge, Sn, or Pb atoms if their lone pairs are considered phantom
ligands. No lower-coordinate species, particularly the key
dimeric phosphanediides (MPR)2, which were proposed as
intermediates in their formation, have been characterized.
We now report that the use of sterically crowding terphenyl
ligand22,23 (Ar0 = C6H3-2,6-(C6H3-2,6-Pri2)2)24 as an organic
phosphine substituent permits the synthesis of the dimeric
phosphanediyls {M(μ-PAr0)}2 that were spectroscopically
and structurally characterized.
Nujol mulls between CsI plates on a Perkin-Elmer 1430 spectro-
photometer. UV-visible spectra were recorded as dilute hexane
solutions in 3.5 mL quartz cuvettes using a HP 8452 diode array
spectrophotometer. Melting points were determined on a Mel-
temp II apparatus using glass capillaries sealed with vacuum
grease and are uncorrected.
Ar0PX2 (X = Cl, Br) (1). Ar0MgBr(THF)2 (12.92 g, 20 mmol)
in ca. 150 mL of Et2O and 100 mL of THF was added dropwise
to PCl3 (2.75 g, 20 mml) in ca. 50 mL of Et2O at 0 ꢀC, affording a
yellow solution. The reaction mixture was stirred for 4 days to
yield a white precipitate, whereupon the solvents were removed
under reduced pressure. Benzene (200 mL), along with ca. 30 mL
of 1,4-dioxane, was added to effect MgX2 precipitation. The
solution was filtered via filter cannula, and the solvents were
removed to afford a pale yellow solid. This was dissolved in hot
hexane and concentrated to incipient crystallization to give two
crops of light yellow crystalline material identified by 31P NMR
as a mixture of halogenated species Ar0PX2 (X = Cl and/or Br).
Yield 7.0 g, 65% based on the average weight of the halogens.
1
31P, H, and 13C NMR spectroscopy showed that the product
was a mixture of Ar0PCl2, Ar0P(Cl)Br, and Ar0PBr2. 31P{1H}
NMR (C6D6, 25 ꢀC): δ 158.77 (Ar0PCl2, ca. 50%), 153.97
(Ar0P(Cl)Br, ca. 40%), 146.30 (Ar0PBr2, ca. 10%).
Ar0PH2 (2). To LiAlH4 (2.0 g, 53 mmol) in ca. 100 mL of Et2O
at 0 ꢀC was added Ar0PX2 (7.0 g, ca. 13 mmol) in ca. 150 mL of
Et2O. The mixture was stirred overnight, whereupon the solvent was
removed under reduced pressure. The residue was extracted with ca.
150 mL of benzene and filtered over a Celite padded frit. Removal of
the solvent afforded an off-white analytically pure powder in 87%
yield (4.85 g), mp 105 ꢀC. Anal. Calcd for C30H39P: C, 83.68; H,
9.13. Found: C, 83.11; H 9.41. IR 2300 ν(P-H). 1H NMR (C6D6,
Experimental Section
General Procedures. All manipulations were performed with
the use of modified Schlenk techniques under argon or in a
Vacuum Atmospheres drybox under N2. Solvents were dried
and collected using a Grubbs-type solvent purification system
(Glass Contour)25 and were degassed immediately prior to use
by sparging with dry Ar or N2 for 10 min. Unless otherwise
noted, all chemicals were obtained from commercial sources and
used without further purification. PCl3 was obtained from
3
25 ꢀC): δ 1.08 (d, 12H, CH(CH3)2, JHH = 6.6 Hz), 1.29 (d,
12H, CH(CH3)2, 3JHH = 6.6 Hz), 2.78 (sept, 2H, CH(CH3)2,
˚
Aldrich and distilled onto 4 A molecular sieves prior to use.
GeCl2(dioxane) was synthesized according to the literature
1
3JHH = 7.2 Hz), 3.27 (d, 2H, ArPH2, JHP = 212 Hz), 7.07
(m, 6H, m- and p-Dipp), 7.19 (d, 2H, m-C6H3, 3JHH = 8.4 Hz),
procedure26 using GeCl4 and Bun SnH. M{N(SiMe3)2}2 (M =
3
3
7.32 (t, 1H, p-C6H3, JHH = 8.4 Hz). 13C{1H} NMR (75.45
Ge, Sn, Pb) was prepared by literature procedures27 from the
corresponding lithium amide and GeCl2(dioxane), SnCl2, or
PbCl2. Ar0MgBr(THF)2 was prepared as described previously.28
All physical measurements were obtained under strictly
anaerobic and anhydrous conditions. 1H and 13C NMR spectra
were acquired on a Varian Mercury 300 MHz instrument and
referenced internally either to residual protiobenzene or trace
silicone vacuum grease, δ 0.29 ppm in C6D6. 31P NMR spectra
were recorded on a Varian Mercury 300 MHz instrument and
referenced externally to 85% H3PO4 in D2O. 119Sn NMR
spectra were acquired a Varian Inova 600 MHz (at 223.7 MHz)
MHz, C6D6, 25 ꢀC): δ 23.68, 25.23 (CH(CH3)2), 31.07 (CH-
(CH3)2), 122.86, 123.50, 127.16, 128.73, 128.95, 139.58, 144.69,
146.47 (Ar-C). 31P NMR (121.4 MHz, C6D6, 25 ꢀC) δ -139.4
(t, Ar0PH2, 1JPH = 212 Hz).
{Ge(μ-PAr0)}2 (3). Ar0PH2 (0.861 g, 2 mmol) and Ge{N-
(SiMe3)2}2 (0.865 g, 2.2 mmol) were combined in a Schlenk flask
and heated at 170 ꢀC in an oil bath with stirring. A deep red melt
resulted, and the evolution of vapor was observed. Heating was
continued over ca. 30 min, up to a temperature of 230 ꢀC, with
intermittent reduced pressure applied, to afford a deep red solid. All
volatile materials were removed by distillation. The residue was
extracted with 30 mL of hexanes, and the solution was filtered via
cannula. Concentration and storage at 7 ꢀC afforded deep red,
diamond-shaped X-ray quality crystals of 3•n-hexane (0.240 g, 21%
based on Ge), mp 310 ꢀC. UV-vis, nm (ε, M-1 cm-1): 368 (1650),
instrument and were referenced externally to Bun Sn in C6D6 (δ
4
-11.7 ppm). 207Pb NMR spectra were recorded on a Bruker
500 MHz instrument (at 105.3 MHz), externally referenced to
Pb(NO3)2 in D2O (δ -2984.4 ppm). IR spectra were recorded as
1
518 (7100). H NMR (C6D6, 25 ꢀC) δ 0.89 (t, 8H, n-hexane
(18) Westerhausen, M.; Makropoulos, N.; Piotrowski, H.; Warchhold,
CH3), 1.00 (d, 24H, CH(CH3)2, 3JHH = 6.6 Hz), 1.10 (d, 24H,
CH(CH3)2, 3JHH = 6.6 Hz), 1.24 (m, 8H, n-hexane CH2), 2.84
€
M.; Noth, H. J. Organomet. Chem. 2000, 614-615, 70.
€
(19) Von Hanisch, C.; Nikolova, D. Z. Anorg. Allg. Chem. 2004, 630, 345.
3
(20) Westerhausen, M.; Krofta, M.; Schneiderbauer, S.; Piotrowski, H. Z.
(sept, 8H, CH(CH3)2, JHH = 7.2 Hz), 7.11-7.29 (m, 18H,
Anorg. Allg. Chem. 2005, 631, 1391.
ArH). 13C{1H} NMR (C6D6, 70 ꢀC): δ 14.18 (n-hexane CH3),
22.98 (n-hexane CH2), 23.78 (CH(CH3)2), 25.54 (CH(CH3)2),
31.25 (CH(CH3)2), 31.93 (n-hexane CH2), 124.14, 126.00,
129.14, 129.49, 141.42, 142.63, 143.98, 147.63 (Ar-C). 31P
NMR (121.4 MHz, C6D6, 70 ꢀC) δ þ270.2 (singlet/quintet,
€
(21) Nikolova, D.; von Hanisch, C.; Adolf, A. Eur. J. Inorg. Chem. 2004,
2321.
(22) Clyburne, J. A. C.; McMullen, N. Coord. Chem. Rev. 2000, 210, 73.
(23) Twamley, B. T.; Haubrich, S. T.; Power, P. P. Adv. Organomet.
Chem. 1999, 44, 1.
(24) Schiemenz, B.; Power, P. P. Angew. Chem., Int. Ed. Engl. 1996, 35,
2150.
(25) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmens, F. J. Organometallics 1996, 15, 518.
(26) Fjeldberg, T.; Haaland, A.; Schilling, B. E. R.; Lappert, M. F.;
Thorne, A. J. J. Chem. Soc., Dalton Trans. 1986, 8, 1551.
(27) Harris, D. H.; Lappert, M. F. J. Chem. Soc., Chem. Commun. 1974,
895.
Ar0PGe, 1J
73Ge = 553 Hz).
31P-
{Sn(μ-PAr0)}2 (4). Ar0PH2 (0.861 g, 2 mmol) and Sn{N-
(SiMe3)2}2 (0.967 g, 2.2 mmol) were combined in a Schlenk
flask and heated from 170 ꢀC up to 220 ꢀC in an oil bath with
stirring. A deep violet melt resulted, and the evolution of vapor
was observed. Heating was continued over ca. 5 min, with
intermittent reduced pressure applied, to yield a violet solid.
All volatile materials were removed by distillation. The crude
solid product was extracted with 35 mL of hexanes, which was
(28) Zhu, Z.; Brynda, M.; Wright, R. J.; Fischer, R. C.; Merrill, W. A.;
Rivard, E.; Wolf, R.; Fettinger, J. C.; Olmstead, M. M.; Power, P. P. J. Am.
Chem. Soc. 2007, 129, 10847.

Article
Inorganic Chemistry, Vol. 49, No. 18, 2010 8483
heated to a brief reflux and filtered. The solution was concen-
trated to ca. 25 mL and stored at 7 ꢀC, which yielded deep violet,
block-shaped X-ray quality crystals of 4•n-hexane in two crops.
Yield: 0.354 g, 32% based on Sn, mp 332 ꢀC. UV-vis, nm
(ε, M-1 cm-1): 402 (5114), 548 (5525). IR, cm-1: 460, 390, 330
ν(P-Sn). 1H NMR (C6D6, 25 ꢀC) δ 0.89 (t, 6H, n-hexane CH3),
1.10 (overlapped d’s, 48H, CH(CH3)2, 3JHH = 6.6 Hz), 1.24 (m,
8H, n-hexane CH2), 2.89 (sept, 8H, CH(CH3)2, 3JHH = 7.2 Hz),
3.27 (d, 2H, ArPH2, 1JHP = 212 Hz), 7.05-7.27 (m, 18H, ArH).
13C{1H} NMR (C6D6, 70 ꢀC): δ 14.18 (n-hexane CH3), 22.97
(n-hexane CH2), 24.28 (CH(CH3)2), 25.44 (CH(CH3)2), 31.23
(CH(CH3)2), 31.92 (n-hexane CH2), 124.44, 124.75, 128.55,
129.33, 129.47, 142.70, 142.81, 147.21 (Ar-C). 31P NMR
Table 1. Selected Crystallographic Data for Compounds 3-5
compound
formula
fw
3•n-C6H14
4•n-C6H14
5•n-C6H14
C66H88P2Ge2
1088.48
red, rod
C66H88P2Sn2
1180.68
violet, rod
C66H88P2Pb2
1357.68
deep
violet, block
monoclinic
I2/m
11.846(3)
20.612(7)
12.737(3)
90
98.466(4)
90
color, habit
crystal system
space group
monoclinic
I2/m
12.0005(5)
19.9995(8)
12.7349(5)
90
98.9790(10)
90
3019.09(2)
2
monoclinic
C2/m
16.0315(16)
20.514(2)
11.8910(12)
90
128.261(2)
90
3070.6(5)
2
0.40 ꢀ
0.20 ꢀ 0.20
90(2)
1.277
0.902
˚
a, A
˚
b, A
˚
c, A
R, deg
β, deg
γ, deg
(121.4 MHz, C6D6, 25 ꢀC) δ þ255.61 (s/d, Ar0PSn, 1J
=
1463 Hz). 119Sn{1H} NMR (223.7 MHz, C6D6, 70 ꢀC) δ þ966.6
31P-117/119Sn
3
˚
V, A
Z
crystal
3076.2(15)
2
(t, ArPSn,1J
119Sn = 1464 Hz).
31P-
0.40 ꢀ 0.35 ꢀ
0.40 ꢀ
{Pb(μ-PAr0)}2 (5). Ar0PH2 (0.861 g, 2 mmol) and Pb{N-
(SiMe3)2}2 (1.161 g, 2.2 mmol) were reacted in a similar manner
to that described for 3 and 4 combined in a Schlenk flask and heated
from 190 ꢀCupto210ꢀC in an oil bath with stirring. The crude solid
product was extracted with 35 mL of hexanes, and the resultant
solution was heated to a brief reflux and filtered. The solution was
concentrated to ca. 25 mL and stored at 7 ꢀC to afford dark violet,
block-shaped X-ray quality crystals of 5•n-hexane in two crops
(0.450 g, 31% based on Pb), mp 274 ꢀC. UV-vis, nm (ε, M-1
cm-1): 406 sh (2500), 518 (3300), 650 (1500). IR, cm-1: 460, 390,
270 ν(P-Pb). 1HNMR(C6D6, 70ꢀC) δ0.88 (t, 6H, n-hexane CH3),
1.09 (overlapped d’s, 48H, CH(CH3)2, 3JHH = 6.6 Hz), 1.25 (m,
8H, n-hexane CH2), 2.88 (sept, 8H, CH(CH3)2, 3JHH = 7.2 Hz),
dimension, mm
T, K
0.13
0.38 ꢀ 0.33
90(2)
1.197
1.086
90(2)
1.466
5.555
dcalc, g/cm3
abs. coefficient
μ, mm-1
θ range, deg
reflections
collected
1.91 to 27.50
3565
1.90 to 28.70
21241
1.89 to 27.50
16047
obs reflections
[I > 2σ(I)]
data/restraints/
parameters
R1, observed
reflections
wR2, all
2943
3041
2587
3565/0/183
0.0394
4084/0/174
0.0574
3598/0/175
0.0904
0.1055
0.1178
0.2027
1
3.27 (d, 2H, ArPH2, JHP = 212 Hz), 6.77-7.41 (m, 18H,ArH).
13C{1H} NMR (C6D6, 70 ꢀC): δ 13.89 (n-hexane CH3), 23.38
(n-hexane CH2), 24.72 (CH(CH3)2), 25.00 (CH(CH3)2), 31.23
(CH(CH3)2), 31.64 (n-hexane CH2), 123.25, 124.04, 124.92,
128.41, 141.86, 146.40, 147.45, 148.59 (Ar-C). 31P NMR (121.4
package,32a and the geometries were generated with the Chem-
craft program.32b
MHz, C6D6, 70 ꢀC), þ302.48 (s, d, Ar0PPb, 1J 207Pb = 2070 Hz).
31P-
Results and Discussion
207Pb NMR (105.28 MHz, C6D6, 70 ꢀC): δ þ6150 (t, Ar0PPb,
1JPb-P = 2068 Hz).
Synthesis. The phosphinidene compounds 3-5 were
synthesized by amine elimination reactions between the
silylamides M{N(SiMe3)2}2 (M = Ge, Sn, or Pb) and
Ar0PH2 (2) (eq 1).
X-ray Crystallography. Crystals of appropriate quality for
X-ray diffraction studies were removed from a Schlenk tube
under a stream of nitrogen and immediately covered with a thin
layer of hydrocarbon oil (Paratone-N). A suitable crystal was
selected, attached to a glass fiber, and quickly placed in a low-
temperature stream of nitrogen (90(2) K).29 Data for all com-
pounds were obtained on a Bruker SMART 1000 or SMART
MfNðSiMe3Þ2g2 þ Ar0PH2 f 1=2ðMPAr0Þ2 þ 2HNSiMe3
ð1Þ
˚
APEX instrument using Mo KR radiation (λ = 0.71073 A) in
The primary phosphine Ar0PH2 was prepared by a modi-
fied route involving the reaction of the Grignard reagent
Ar0MgBr(THF)228 with PCl3 to give 1 and the subsequent
reduction of 1 with LiAlH4 to afford 2. We had observed
previously that the synthetic approach to bulky terphenyl
phosphine derivatives using LiAr# (Ar#=C6H3-2,6-Mes2)23
or LiAr* (Ar*=C6H3-2,6-(C6H2-2,4,6-Pri3)2)33 as ter-
phenyl transfer agents led to undesirable side products
and low yields. The direct reaction of LiAr0 and PCl3 gave
an approximate 50/50 mixture of the desired Ar0PCl2 and
Ar0Cl, which co-crystallized from hexane or ether ex-
tracts. An unidentified insoluble red material was also
observed. These results had similarities to those reported
by Hey-Hawkins and co-workers for the LiMes* (Mes*=
C6H2-2,4,6-But3)-based synthesis of Mes*PCl2.34 How-
ever, the use of the Ar0MgBr(THF)2 transfer agent in-
stead of LiAr0 afforded “Ar0PX2” exclusively and in
good yield with essentially no aryl halide elimination.
conjunction with a CCD detector. The collected reflections were
corrected for Lorentz and polarization effects by using Bles-
sing’s method as incorporated into the program SADABS.30a,b
The structures were solved by direct methods and refined with
the SHELXTL v.6.1 software package.31a,b Refinement was by
full-matrix least-squares procedures with all carbon-bound
hydrogen atoms included in calculated positions and treated
as riding atoms. A summary of crystallographic and data
collection parameters for 3-5 is given in Table 1.
Theoretical Methods. The model compounds {M(μ- PAr0)}2
(30-50) were calculated as gas phase dimers at the B3LYP/6-
31G* level for M = Ge and at the B3LYP/cc-pVTZ-PP level for
M = Sn or Pb. All calculated structures represent true minima.
DFT calculations were performed with the Gaussian 03
(29) Hope, H. Prog. Inorg. Chem. 1995, 41, 1.
(30) (a) Blessing, R. H. Acta Crystallogr. 1995, A51, 33–38. (b) Sheldrick,
€
€
€
G. M. SADABS; Universitat Gottingen: Gottingen, Germany, 2008.
(31) (a) Sheldrick, G. M. SHELXTL, version 6.1; Bruker AXS Inc.:
Madison, WI, 2002. (b) Sheldrick, G. M. SHELXS97 and SHELXL97;
€
€
€
Universitat Gottingen: Gottingen, Germany, 1997.
(32) (a) Frisch, M. J., et al. Gaussian 03, revision A.1; Gaussian, Inc.:
Pittsburgh, PA, 2003 (full reference can be found in the Supporting Information).
(b) Zhurko, G. A. Chemcraft.
(33) Twamley, B.; Sofield, C. D.; Olmstead, M. M.; Power, P. P. J. Am.
Chem. Soc. 1999, 121, 3357.
(34) Kurz, S.; Hey-Hawkins, E. Organometallics 1992, 11, 2729.

8484 Inorganic Chemistry, Vol. 49, No. 18, 2010
Merrill et al.
Figure 1. Thermal ellipsoid (30%) drawing of 3. Hydrogen atoms,
isopropyl groups, and disordered and minor twin component core atoms
˚
˚
are not shown. Ge Ge 3.392 A; Ge-P 2.313 A (average); P(1)-
3 3 3
Ge(1)-P(1A) 85.67ꢀ.
Figure 2. Thermal ellipsoid (30%) drawing of 4. Hydrogen atoms are
not shown. Phosphorus atoms have 50% occupancies for the P(1)/P(1A)
˚
˚
or P(1B)/P(1C) positions. Sn Sn 3.572 A; Sn-P 2.466 A (average);
3 3 3
This material proved suitable for reduction to the arylphos-
phine Ar0PH2 by reaction with an excess of LiAlH4.
When the reactions were carried out with 2 equiv of
H2PAr0 per M{N(SiMe3)2}2, the corresponding phos-
phinidenes 3, 4, or 5 were also the only pure products
isolated from solution. It is possible that any “M{P-
(H)Ar0}2” formed in this reaction readily eliminates
H2PAr0 to form the phosphinidene dimer (eq 2).
P(1)-Sn(1)-P(1A) 87.20ꢀ.
MfPðHÞAr0g2 / 1=2ðMPAr0Þ2 þ Ar0PH2
ð2Þ
Other attempts to synthesize diphosphides M{P(H)Ar0}2
(M = Sn, Pb) by the reaction of 2 equiv of LiP(H)Ar0 with
SnCl2 or PbBr2 at -78 ꢀC resulted in grey precipitates and
weakly colored solutions upon warming to room tempera-
ture. Treatment of GeCl2(dioxane) with 2 equiv of LiP-
(H)Ar0 did not afford isolable quantities of Ge{P(H)Ar0}2.
Structures. Each of the phosphinidenes 3-5 crysta-
llized with one-quarter of the dimer comprising the asym-
metric unit (crystallographic data collection parameters are
given in Table 1). The structures (Figures 1-3) have similar
M2P2 ring cores, which in the case of 4 and 5 have 50/50
disorder of the pyramidally coordinated phosphorus atoms
over two positions, consistent with either a planar four-
membered ring with aryl P-substituents that are anti with
respect to each other or with an M2P2 ring folded along the
Figure 3. Thermal ellipsoid (30%) drawing of 5. Hydrogen atoms,
isopropyl groups, and disordered Pb atom positions (above and below
Pb(1) and Pb(1A), respectively) are not shown. Phosphorus atoms have
50% occupancies for the P(1)/P(1C) or P(1A)/P(1B) positions. Pb Pb
3 3 3
˚
˚
3.487(3) A; Pb-P 2.621(3) A (average); P(1)-Pb(1)-P(1C) 86.40ꢀ.
37
(Trip)(But) F}]2,36 as well as 2.738 A in Sn{P(C4Et4)}2.
˚
The most relevant structurally characterized molecular lead-
(II) phosphorus compounds are the phosphido derivatives
36
Pb[P(SiPri3){Si(Trip)(But)F}]2 (Pb-P 2.654(4) A) and
M
disorder was resolved, however, in 3 (see Supporting In-
M vector with syn-disposed aryl P-substituents. Such
˚
3 3 3
[Pb{μ-P(SiMe3)2}P(SiMe3)2]2 (Pb-P 2.696(7), terminal, to
2.796(7) A, bridging),38 which have average Pb-P distances
formation), which has a Ge2P2 core folded along the Ge
˚
3 3 3
Ge axis and clearly syn-oriented aryl substituents. The fold-
ing between the two M2P planes in the syn structures
increased in the order 3 < 4 < 5 (45.4ꢀ for Ge, 59.7ꢀ for
Sn, and 63.0ꢀ for Pb, cf. Table 2), which may be steric in ori-
gin. The internal ring angles at the metals average 85.67(5)ꢀ
(3), 87.20ꢀ (4), and 86.40ꢀ (5), and those at phosphorus
average 94.33ꢀ (3), 92.78ꢀ (4), and 89.86ꢀ (5). The Ge-P and
Sn-P bond lengths are considerably shorter than those
observed in the three-dimensional cage species or phosphide
˚
that are longer than the 2.621(3) A found for 5. Overall, the
M-P bond lengths in 3-5 are consistent with single bond
distances predicted from the sums of the single bond radii:
39
˚
˚
˚
Ge-P (2.32 A), Sn-P (2.51 A), and Pb-P (2.55 A).
The M2P2 cores of 3-5, which have two-coordinate
metal atoms, are unprecedented and are comparable only
with the less sterically encumbered congeneric imido
14
germanium compounds {Ge(μ-NMes*)}2 and {Ge(μ-
N(C6H2-2,4,6-(CF3)3)}2.15 Although the M-P distances
˚
derivatives. The M-P distances average 2.312(10) A in 3
and 2.466(18) A in 4; cf. 2.416(2) to 2.487(2) A in the
˚
˚
(36) Driess, M; Jonoshek, R.; Pritzkow, H.; Rell, S.; Winkler, U. Angew.
Chem., Int. Ed. Engl. 1995, 34, 1614.
germanium-phosphinidene cage {Ge(μ-PSiPri3)}6,21 2.490
A in {Ge(Br)PBu 2}2, and 2.626(3) to 2.665(3) A in the tin
t
35
˚
˚
€
(37) Westerhausen, M.; Ossberger, M. W.; Keilbach, A.; Guckel, C.;
i
species {Sn(μ-PSiPri3)}6,17 2.567(1) A in Sn[P(SiPr3){Si-
€
Piotrowski, H.; Suter, M.; Noth, H. Z. Anorg. Allg. Chem. 2003, 629, 2398.
˚
(38) Goel, S. C.; Chiang, M. Y.; Rauscher, D. J.; Buhro, W. E. J. Am.
Chem. Soc. 1993, 115, 1650.
€
(39) Pyykko, P.; Matsumi, S. Chem.;Eur. J. 2009, 15, 12770.
(35) Jones, P. G.; Ruthe, F. Private communication 2006.

Article
Inorganic Chemistry, Vol. 49, No. 18, 2010 8485
˚
Table 2. Selected Interatomic Distances (A) and Angles (deg) for Compounds 3, 4, and 5. Calculated Values for the Corresponding Model Compounds {M(μ-PMe)}2, M =
Ge (30), Sn (40), Pb (50) Are Shown in Brackets
{Ge(μ-PAr0)}2 (3)
Ge(1) Ge(1A)
3 3 3
3.392
P(1)-Ge(1)-P(1A)
Ge(1)-P(1)-Ge(1A)
C(1)-P(1)-Ge(1)
P(1)-C(1)-C(4)
Ge2P2 dihedral angle
Σ angles at P(1)
85.67 [81.6]
94.33 [78.5]
120.40 [111.1]
161.03
134.63 [115.1]
335.38
Ge(1)-P(1)
2.3104(10) [2.354]
2.3153(9) [2.354]
2.902
Ge(1A)-P(1)
P(1) P(1A)
P(1)-C(1)
3 3 3
1.865(2) [1.886]
{Sn(μ-PAr0)}2 (4)
Sn(1) Sn(1A)
3 3 3
3.572
P(1)-Sn(1)-P(1A)
Sn(1)-P(1)-Sn(1A)
C(1)-P(1)-Sn(1)
P(1)-C(1)-C(4)
Sn2P2 dihedral angle
Σ angles at P(1)
87.20 [77.8]
92.78 [84.2]
120.82 [114.3]
158.10
130.26 [115.7]
329.49
Sn(1)-P(1)
2.4134(18) [2.562]
2.5192(17) [2.562]
1.424
3.091
1.908(5) [1.884]
Sn(1A)-P(1)
P(1) P(1B)
P(1) P(1A)
3 3 3
3 3 3
P(1)-C(1)
{Pb(μ-PAr0)}2 (5)
Pb(1) Pb(1A)
3 3 3
3.487
P(1)-Pb(1)-P(1C)
Pb(1)-P(1)-Pb(1A)
C(1)-P(1)-Pb(1)
P(1)-C(1)-C(4)
Pb2P2 dihedral angle
Σ angles at P(1)
86.40 [77.5]
89.86 [84.2]
111.8 [114.4]
157.13
117.03 [115.1]
318.79
Pb(1)-P(1)
2.616(3) [2.653]
2.625(3)
1.482(8)
3.166(10)
1.905(10) [1.887]
Pb(1A)-P(1)
P(1) P(1B)
P(1)---P(1C)
3 3 3
P(1)-C(1)
in 3-5 are consistent with single bonding, with little
π-donation from P to M,16,38 it is noteworthy that the
coordination geometry at phosphorus becomes increas-
ingly flattened with decreasing atomic number of M: ΣꢀP
(Ge > Sn > Pb) = 335.38ꢀ (3), 329.49ꢀ (4), and 318.79ꢀ
(5). A similar trend can be seen in the X-ray structures of
[M{P(SiMe3)2}2]2 (M = Zn, Cd, Hg, Sn, Pb), where the
degree of pyramidalization at the terminal phosphido
ligand can also be correlated with increasing atomic
number of M.38
7.8%) is apparently unique, and it cannot be compared to
other values at present. The couplings to the tin and lead
nuclei are discussed further below.
Although a 73Ge NMR spectrum was not observed,
119Sn and 207Pb NMR spectroscopy afforded signals that
display coupling to the phosphorus nuclei. Initially, we
were unable to find a 119Sn NMR signal for 4 in C6D6 in
the region deemed most probable on the basis of scarce
published 119Sn NMR data for Sn(II)-P compounds,42
some of which are shown for comparison in Table 3.
However,at elevated temperature (70 ꢀC), sufficient solu-
bility of 4 in C6D6 was achieved and a signal with a 1:2:1
intensity pattern due to coupling to two equivalent 31P
NMR and Electronic Spectroscopy. The 31P NMR
chemical shifts observed at þ270.2 andþ255.6 ppm for 3
and 4 differed by only ca. 15 ppm. The 31P signal for the lead
compound 5was observed somewhat further downfield than
these at þ302.5 ppm. The þ255.6 ppm shift for the Ge
derivative 3 is considerably downfield of the þ13.8 and
-36.8 ppm chemical shifts reported for the terminal and
bridging P atoms in the three-coordinate Ge phosphido
compound {Ge(PPri2)μ-PPri2}2.40 The 31P signal of the tin
derivative 4 is well downfield of the þ0.1 ppm reported for
the dark green tin phosphide Sn{P(Ph)Ar#}2 (Ar# = C6H3-
2,6-Mes2).41 The 31P NMR chemical shift of the lead com-
pound 5 (þ302.5 ppm) is also downfield of the few chemical
shifts that have been reported for Pb-P bonded compounds,
-88.8 (terminal P) and -95.6 ppm (bridging P) in Pb[P-
(SiPri3){Si(Trip)(But) F}]236 (see Table 3). In general, these
downfield shifts are typical for metalated derivatives relative
to the upfield shifts of their parent phosphines (cf. Ar0PH2
31P NMR δ=-139.4 ppm). The 31P NMR spectra of com-
pounds 3-5 also displayed marked 1J coupling to the group
14 element spin-active nuclei. The coupling constants are
nuclei (J
31
119Sn- P = 1465 Hz) was observed at þ966.6 ppm.
This signal is farther downfield than the δ = þ499.5 ppm
observed for the Sn-P doubly bonded species Trip2Snd
PMes* (Trip=C6H2-2,4,6-Pri3, Mes*=C6H2-2,4,6-But3),
a phosphinidene Sn(IV) derivative,43 although it is upfield
of the þ1101 ppm reported for the sterically encum-
bered Sn(II) phosphide Sn{P(Ph)Ar#}2 (Ar#=C6H3-2,6-
Mes2).41 The chemical shift of 4 lies further downfield than
the þ738.9 ppm observed for the related species {Sn(μ-
NAr#)}2,44 which is the opposite of what is expected on the
basis of σ-inductive effects. However, the {Sn(μ-NAr#)}2
displays closer average aryl(centroid) Sn approaches
˚
than those in 4 (ca. 3.17 vs 3.32 A) and may be responsible
3 3 3
for the shift differences. The 1J
31P coupling found that
119Sn-
4 is essentially the same as that determined from the 31P
NMR spectrum and is almost twice that found for the
hexameric tin triisopropylsilylphosphanediide of Driess
and co-workers.17
1
1
1J
31P-73Ge =553 Hz, J 119Sn =1464 Hz, and J
=
31P-207Pb
31P-
2068 Hz. The value obtained for coupling to 73Ge (I = 9/2,
(42) Wrackmeyer, B. Annu. Rep. NMR Spectrosc. 1999, 38, 203.
ꢀ
ꢀ
(40) Druckenbrodt, C.; du Mont, W.-W.; Ruthe, F.; Jones, P. G. Z.
Anorg. Allg. Chem. 1998, 624, 590.
(43) Ranaivonjatovo, H.; Escudie, J.; Couret, C.; Satge, J. J. Chem. Soc.,
Chem. Commun. 1992, 1047.
(41) Rivard, E.; Sutton, A. D.; Fettinger, J. C.; Power, P. P. Inorg. Chim.
Acta 2007, 60, 1278.
(44) Merrill, W. A.; Wright, R. J.; Stanciu, C. S.; Olmstead, M. M.;
Fettinger, J. C.; Power, P. P. Inorg. Chem. 2010, 49, 7097.

8486 Inorganic Chemistry, Vol. 49, No. 18, 2010
Merrill et al.
Table 3. Selected NMR Parameters for Some Ge-P, Sn-P, and Pb-P Bonded
Compounds
M δ,
ppm
31P δ,
ppm
JP-M,
Hz
compound
reference
3
þ270.2
þ13.8t,
-36.8br
þ255.6
-184.2
553
this work
40
Figure 4. Calculated structure of the P-methyl gas-phase model 5.
{Ge(PPri2)μ-PPri2}2
Density Functional Theory Results. Structural para-
meters for the gas-phase model dimers {M(μ-PMe2)}2
(M=Ge (30), Sn (40), Pb(50)) were calculated at the B3LYP/
cc-pVTZ-PP (40, 50) and B3LYP/6-31G* levels (30). As
observed in the experimental crystal structures, the geome-
tries calculated for the model compounds displayed folding
4
Li(THF)4-
{Sn2(μ3-PC6H11)3}2
{Sn(μ-PSiPri3}6
Sn[{P(SiPri3)-
þ996.6
-139.5
1464
550
this work
9
-475.2
-102.5
708
1682
17
36
þ1551
†
{Si(Trip)2F}]2
Sn{P(Ph)Ar#}2
Trip2SndPMes*a,b
5
þ1101
þ499.5
þ6150
þ0.1
891
2208
2068
39
42
along their M M axes (Sn model shown in Figure 4).
þ170.7
þ302.5
-88.8,
-95.6
3 3 3
this work
36
However, in these much less hindered MeP-bridged models
of all three elements Ge, Sn, and Pb, the calculated dihedral
angle was found to be similar (ca. 115ꢀ) in 30-50. This is in
contrast to the experimental values from the crystal struc-
tures of 3-5, {134.6ꢀ (3), 130.3ꢀ (4), and 117.0ꢀ (5)}. This
difference can be attributed to ligand-ligand opposing steric
pressure in the real compounds which forces open their
Pb[P(SiPri3)-
1995, 1979
{Si(Trip)(But) F}]2
Li(THF){PbPBut2}3
{Pb(PBut2)μ-PBut2}2
þ71.5
1770
2452, 1100
48
49
þ90.8t,
þ65.3br
a Trip = C6H2-2,4,6-Pri3, Mes* = C6H2-2,4,6-But3. b Sn(IV). t Term-
inal. br Bridging.
M
M “hinge” in comparison to the simple P-Me models
3 3 3
which experience no such interligand opposition. As ob-
served in the sequence of fold angles for the real compounds
3-5, this steric pressure effect is more pronounced in the
order Ge > Sn > Pb;the smaller the metal, the closer the
interligand approach. The calculated bond lengths and
angles for the {M(μ-PMe2)}2 models 30-50 are shown in
brackets for comparison in Table 2.
The 207Pb NMR signal for 5 was also initially difficult to
locate in the region expected for a two-coordinate Pb(II)
species (ca. 2000-5000 ppm).46a,b Upon repeated data
collections in a variety of spectral windows, and reduction
of the relaxation, or recycle, delay time to the minimum
allowed by the instrument, a well-resolved 31P-coupled triplet
(1J207Pb- P=2025 Hz) was observed at 6150 ppm47 (cf. δ=
31
Although the syn structure of the M2P2 core was
resolved for the Ge compound 3 (Figure 1), 50/50 up/
down P atom disorder seen in the crystal structures
determined for 4 and 5 (Figures 2 and 3) prohibits crystal-
þ5019 ppm for Pb(μ-NAr#)}2.44 1
J 31 coupling con-
207Pb- P
stants are rare for Pb(II)-P compounds.46 Some examples
are given in Table 3. These include the “ate” complex
Li(THF){PbPBut2}348 (1JPb-P=1770 Hz) and the lead tert-
butylphosphanediide {Pb(μ-PBut2)PBut2}2 (no δ 207Pb re-
ported).49 The 2068 Hz 1JPb-P found for 5 lies between the
2452 Hz reported for the bridging PBut2 groups in {Pb(μ-
PBut2)PBut2}2 and about twice that assigned to its terminal
PBut2 groups (1100 Hz).
lographic distinction between a (M M) folded (syn) or
3 3 3
flat M2P2 (anti) core structure. However the DFT-calcu-
lated geometries for 40 and 50 featuring a folded rhomboid
M2P2 core (PMe groups syn-disposed) suggest that this
geometry is preferred in single molecules 4 and 5 in the
crystalline state, rather than a planar (PAr0 groups anti-
disposed) configuration.
The UV-visible spectra of 3-5 in hexane solution reveal
λmax absorptions similar to the metal lone pair to empty
p-orbital (n f p) transitions reported for V-shaped mono-
meric amido germylenes, -stannylenes, and -plumbylenes.50
The absorption maxima attributable to these (n f p) transi-
tions were observed at 518 nm (3), 548 nm (4), and 650 nm
(5). These absorptions are all at lower energies than the
corresponding (n f p)-related maxima recorded for the
nitrogen-containing ring congeners {M(μ-NAr#)}2 (M =
Ge, Sn, Pb; Ar# = C6H3-2,6-Mes2).44 This bathochromic
shift is to be expected given the lower electronegativitiy of P
versus N which results in a higher energy for the lone pair
orbital as a result of the weaker σ-withdrawing P ligands. As
expected, the energy of this HOMO f LUMO transition
decreases in the sequence Ge > Sn > Pb for the series 3-5.
Conclusions
Compounds 3-5 are dimers of the putative monomeric
“Ar0PdM:” (M = Ge, Sn, Pb) fragments which remain un-
known as stable species. The large size of the Ar0 ligand has
allowed the stabilization of a stable homologous series of
dimeric heavier group 14 phosphinidenes that are precedented
only by congeneric M2N2 rings in the compounds{M(μ-NR)}2
(M = Ge, R = Mes*,14 or C6H2-2,4,6-(CF3)3;15 M = Ge, Sn,
Pb, R=Ar#44). Furthermore, 31P, 119Sn, and 207Pb NMR spec-
troscopy has allowed rare heteronuclear shifts and JP-M
coupling constants to be recorded. The data indicate that there
is minimal delocalization of the phosphorus lone pair into the
valence p-orbitals of the tetrel atoms.
(45) Merrill, W. A.; Fettinger, J. C.; Reiff, W. M.; Power, P. P. Unpub-
lished work.
Acknowledgment. We thank the National Science
Foundation (CHE-0948417) for financial support and
NSERC Canada for Postdoctoral Fellowship (B.D.E.
and E.R.).
(46) (a) Wrackmeyer, B.; Horchler, K. Annu. Rep. NMR Spectrosc. 1989,
22, 249. (b) Wrackmeyer, B. Annu. Rep. NMR Spectrosc. 2002, 47, 1.
(47) Wrackmeyer, B. Personal communication 2009.
(48) Arif, A. M.; Cowley, A. H.; Jones, R. A.; Power, J. M. J. Chem. Soc.,
Chem. Commun. 1986, 1446.
(49) Cowley, A. H.; Goodland, D. M.; Jones, R. M.; Nunn, C. M.; Power,
J. M. Polyhedron 1988, 7, 1909.
Supporting Information Available: Crystallographic informa-
tion files (CIF) for 1-6. This material is available free of charge
via the Internet at http://pubs.acs.org.
(50) Gynane, M. J. S.; Harris, D. H.; Lappert, M. F.; Power, P. P.;
ꢁ
ꢁ
Riviere, P.; Riviere-Baudet, M. J. Chem. Soc., Dalton Trans. 1977, 2004.