Synthesis and structure of niobium and tantalum derivatives of bis(dicyclohexylphosphino)methane (dcpm)

Article abstract of DOI:10.1016/S0020-1693(99)00030-4

The sodium amalgam reduction (2 Na per M) of the Group 5 metal chlorides [M₂Cl₁₀] (M=Nb, Ta) in the presence of bis(dicyclohexylphosphino)methane (dcpm) leads to the binuclear compounds [(dcpm)Cl₂M(μ₂-Cl)₂MCl₂(dcpm)] (M=Nb, 1; Ta, 2). Solution NMR spectroscopic properties of 1 and 2 indicate that the dcpm ligand does not bridge (binucleate) the di-metal unit but is chelated to a single metal center. A single crystal X-ray diffraction study of 1 confirms this and shows two independent molecules both with an edge shared bis-octahedral geometry with Nb=Nb distances of 2.738(1) and 2.749(1) A. The addition of dcpm to the tantalum trichloride [Ta(OC₆H₃Prⁱ₂-2,6) ₂Cl₃]₂ initially leads to the adduct cis-mer-[Ta(OC₆H₃Prⁱ₂-2,6) ₂Cl₃(dcpm)] (3). The molecular structure of 3 is found to contain an η¹-bound dcpm ligand. In the ³¹P NMR spectrum of 3 well resolved doublets are present for the coordinated and 'dangling' phosphorus atoms. The sodium amalgam reduction of 3 in hydrocarbon solvents does not lead to any isolable binuclear compounds. Instead the reaction leads to the compound [Ta(OC₆H₃Prⁱ-η²-CMe=CH ₂)Cl(dcpm)] (4). Compound 4 contains an aryloxide ligand chelated to the metal via an η²-interaction with a vinyl group formed by the dehydrogenation of an ortho-isopropyl group. The structural parameters are consistent with a metallacyclopropane bonding description for this interaction.

Full text of DOI:10.1016/S0020-1693(99)00030-4

Inorganica Chimica Acta 288 (1999) 35–39
Synthesis and structure of niobium and tantalum derivatives of
bis(dicyclohexylphosphino)methane (dcpm)
Patrick N. Riley, Janet R. Clark, Phillip E. Fanwick, Ian P. Rothwell *
Department of Chemistry, Purdue Uni6ersity, 1393 Brown Building, West Lafayette, IN 49707-1393, USA
Received 21 May 1998; accepted 11 January 1999
Abstract
The sodium amalgam reduction (2 Na per M) of the Group 5 metal chlorides [M2Cl10] (M=Nb, Ta) in the presence of
bis(dicyclohexylphosphino)methane (dcpm) leads to the binuclear compounds [(dcpm)Cl2M(m2-Cl)2MCl2(dcpm)] (M=Nb, 1; Ta,
2). Solution NMR spectroscopic properties of 1 and 2 indicate that the dcpm ligand does not bridge (binucleate) the di-metal unit
but is chelated to a single metal center. A single crystal X-ray diffraction study of 1 confirms this and shows two independent
˚
molecules both with an edge shared bis-octahedral geometry with NbꢀNb distances of 2.738(1) and 2.749(1) A. The addition of
dcpm to the tantalum trichloride [Ta(OC6H3Pri2-2,6)2Cl3]2 initially leads to the adduct cis-mer-[Ta(OC6H3Pr2i -2,6)2Cl3(dcpm)] (3).
The molecular structure of 3 is found to contain an h1-bound dcpm ligand. In the 31P NMR spectrum of 3 well resolved doublets
are present for the coordinated and ‘dangling’ phosphorus atoms. The sodium amalgam reduction of 3 in hydrocarbon solvents
does not lead to any isolable binuclear compounds. Instead the reaction leads to the compound [Ta(OC6H3Pri-h2-
CMeꢀCH2)Cl(dcpm)] (4). Compound 4 contains an aryloxide ligand chelated to the metal via an h2-interaction with a vinyl group
formed by the dehydrogenation of an ortho-isopropyl group. The structural parameters are consistent with a metallacyclopropane
bonding description for this interaction. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Crystal structures; Niobium complexes; Tantalum complexes; Diphosphinomethane complexes
more electron donating than dmpm [4]. In this paper
we report the chemistry of the ligand dcpm at low
valent Nb and Ta metal centers. This work was stimu-
lated by the expectation that the low valent derivatives
of these metals might lead to CH bond activation of the
cyclohexyl rings of dcpm. This could then lead to
partially dehydrogenated species that would give insight
into possible intermediates in the aryl–phosphine hy-
drogenation processes. Although CH bond activation
has been observed, unfortunately it is observed in the
dehydrogenation of ancillary aryloxide ligation.
1. Introduction
The bis(phosphine) compounds bis(diphenylphos-
phino)methane
(dppm)
and
bis(dimethylphos-
phino)methane (dmpm) have played an important role
in the chemistry of dinuclear transition metal com-
pounds [1]. The ‘binucleating’ characteristic of these
ligands has allowed the isolation and study of a wide
variety of binuclear compounds that are either un-
known or less stable with monodentate phosphine lig-
ands. Recently there has been increased research
interest in the chemistry of the more bulky phosphines
bis(di-tert-butylphosphino)methane and bis(dicyclo-
hexylphosphino)methane (dcpm) [2]. We have shown
that the ligand dcpm can be obtained by the niobium
aryloxide catalyzed hydrogenation of dppm [3]. Some
studies of the chemistry of dcpm at dimanganese car-
bonyl centers show that the bulky ligand is significantly
2. Results and discussion
Treatment of a mixture of dcpm with the Group 5
metal chlorides [M2Cl10] (M=Nb, Ta) with sodium
amalgam (2 Na per M) leads to deep purple solutions
from which can be isolated the binuclear compounds
[(dcpm)Cl2M(m2-Cl)2MCl2(dcpm)] (M=Nb, 1; Ta, 2)
* Corresponding author. Fax: +1-765-494 0239.
0020-1693/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(99)00030-4

36
P.N. Riley et al. / Inorganica Chimica Acta 288 (1999) 35–39
(Scheme 1). Previous studies of the reduction of
[M2Cl10] in the presence of bis(phosphines) have been
shown to lead to d2–d2 binuclear compounds which can
adopt different structures (see below) [5].
With binucleating ligands R2PCH2PR2 the structural
type formed appears to depend upon the steric size of
R. For R=Me and Et, structure [A] has been found
whereas for R=Ph, structure [B] was adopted. Interest-
ingly, for the compound [Ta2(m-Br)2Br4(depm)2] spec-
troscopic evidence was obtained by Messerle for both
structures in solution [5]. The solution spectroscopic
properties of 1 and 2 indicate the presence of structural
type [B]. Specifically the methylene protons of the dcpm
ligand appear as a sharp triplet and not as a virtual
pentet as found when binucleating ligands bridge the
two metal centers in a mutually trans configuration.
The molecular structure of 1 was confirmed by a single
crystal X-ray diffraction study (Fig. 1, Table 1). The
unit cell contains two independent molecules, both
containing an inversion center. The NbꢀNb distances of
Scheme 1.
2,6)(Cl)(dcpm)] (4). Considerable problems were encoun-
tered in obtaining an acceptable data set by X-ray
diffraction of compound 4. A solution of the best data
set obtained (Fig. 3, Table 4) shows the presence of an
aryloxide ligand chelated via an ortho-a-methyl vinyl
group. This group is presumably formed via dehydro-
genation of one of the aryloxide isopropyl groups.
Previously we have shown that the reduction
of the niobium dichloride [Nb(OC6H3Pri2-2,6)3Cl2] in
thf solution leads to
a
dehydrogenation pro-
duct [Nb(OC6H3Pri-h2-CMeꢀCH2)(OC6H3Pr2i -2,6)2(thf)]
whose chemistry has been explored [14]. Furthermore,
addition of olefins to the di-hydride [Ta(OC6H3Pri2-
2,6)2Cl(H)2(L)2] (L=PMePh2) has been shown to lead to
the dehydrogenation products Ta(OC6H3Pri-h2-CMeꢀ
CH2)(OC6H3Pri2-2,6)Cl(L)2] which are directly analogous
to 4 [15]. The structural parameters for 4 strongly support
a tantalacyclopropane bonding picture. For example the
Ta–C distances are comparable to those found for alkyl
groups bound to this metal, while the C–C distance of
˚
2.738(1) and 2.749(1) A are comparable to those re-
ported for the related d2–d2 binuclear derivatives of
niobium and tantalum (Table 2) [6–12]. The adoption
of structural type [B] by 1 and 2 is presumably a
consequence of the steric size of the cyclohexyl
substituent.
The tantalum trichloride compound [Ta(OC6H3Pri2-
2,6)2Cl3]2 has been shown to possess a dimeric structure
in the solid state with bridging chloride groups and
terminal aryloxide ligands trans to the m-Cl groups [13].
The addition of dcpm to this trichloride results in the
formation of an adduct complex [Ta(OC6H3Pri2-
2,6)2Cl3(dcpm)] (3). The solution NMR spectroscopic
properties of 3 show that the dcpm ligand is h1-bound
through a single phosphorus atom. The solid state
molecular structure of 3 (Fig. 2, Table 3) shows the
presence of a six-coordinate tantalum metal center with
a cis, mer arrangement of ligands. There is a distortion
from a regular octahedral geometry for 3 with the three
chloride ligands and the phenoxide bound through O(4)
bent towards the phosphine donor atom. This type of
distortion has been observed for related adducts of Nb
and Ta and rationalized in terms of both p- and
s-bonding effects [13].
When compound 3 was reduced with sodium amalgam
(2 Na per Ta) a reaction mixture is produced from which
can be isolated, in low yield, the new organometallic
Fig. 1. Molecular structure of molecule
Cl)2NbCl2(dcpm)] (1).
1 of [(dcpm)Cl2Nb(m2-
compound
[Ta(OC6H3Pri-h2-CMeꢀCH2)(OC6H3Pri2-

P.N. Riley et al. / Inorganica Chimica Acta 288 (1999) 35–39
37
Table 1
˚
Selected bond distances (A) and angles (°) for [(dcpm)Cl2Nb(m2-
Cl)2NbCl2(dcpm)] (1)
Molecule 1
Molecule 2
˚
Bond distances (A)
Nb(1)–Nb(1)
Nb(1)–Cl(13)
Nb(1)–Cl(12)
Nb(1)–Cl(11)
Nb(1)–Cl(11)
Nb(1)–P(2)
2.738(1) Nb(2)–Nb(2)
2.407(2) Nb(2)–Cl(22)
2.427(1) Nb(2)–Cl(23)
2.452(2) Nb(2)–Cl(21)
2.457(1) Nb(2)–Cl(21)
2.658(2) Nb(2)–P(3)
2.674(2) Nb(2)–P(4)
2.749(1)
2.399(2)
2.414(2)
2.451(1)
2.456(1)
2.663(2)
2.674(1)
Nb(1)–P(1)
Bond angles (°)
Cl(13)–Nb(1)–Cl(12) 165.36(5) Cl(23)–Nb(2)–Cl(22) 165.87(5)
Cl(13)–Nb(1)–Cl(11)
Cl(13)–Nb(1)–Cl(11)
Cl(12)–Nb(1)–Cl(11)
Cl(12)–Nb(1)–Cl(11)
P(2)–Nb(1)–P(1)
Cl(13)–Nb(1)–P(2)
Cl(13)–Nb(1)–P(1)
Cl(12)–Nb(1)–P(2)
Cl(12)–Nb(1)–P(1)
94.21(5) Cl(23)–Nb(2)–Cl(21)
93.61(5) Cl(23)–Nb(2)–Cl(21)
94.83(5) Cl(22)–Nb(2)–Cl(21)
93.60(5) Cl(22)–Nb(2)–Cl(21)
65.84(5) P(3)–Nb(2)–P(4)
87.18(5) Cl(22)–Nb(2)–P(3)
86.45(5) Cl(22)–Nb(2)–P(4)
81.24(5) Cl(23)–Nb(2)–P(3)
80.66(5) Cl(23)–Nb(2)–P(4)
93.80(5)
94.51(5)
94.00(5)
93.49(5)
66.32(4)
86.37(5)
86.56(5)
81.88(5)
81.56(5)
Fig. 2. Molecular structure of [Ta(OC6H3Pri2-2,6)2Cl3(h1-dcpm)] (3).
Dri-Lab or by standard Schlenk techniques. Hydrocar-
bon solvents were dried by distillation from sodium/ben-
zophenone and stored under dry nitrogen. The dcpm
ligand was prepared by the hydrogenation of dppm [3].
The compound [Ta(OC6H3Pri2-2,6)2Cl3]2 was obtained by
previously reported procedures [13]. The 1H and 31P
NMR spectra were recorded on Varian Associates Gem-
ini 200 and General Electric QE-300 spectrometers and
were referenced to protio impurities of commercial
benzene-d6 or to 85% H3PO4. Microanalyses were ob-
tained in-house at Purdue University.
Cl(11)–Nb(1)–Cl(11) 112.19(4) Cl(21)–Nb(2)–Cl(21) 111.88(4)
Cl(11)–Nb(1)–P(2)
Cl(11)–Nb(1)–P(2)
Cl(11)–Nb(1)–P(1)
Cl(11)–Nb(1)–P(1)
90.53(5) Cl(21)–Nb(2)–P(3)
157.11(5) Cl(21)–Nb(2)–P(3)
156.33(5) Cl(21)–Nb(2)–P(4)
91.35(5) Cl(21)–Nb(2)–P(4)
90.87(5)
157.16(4)
157.16(5)
90.88(4)
1.56(3) considerably exceeds (although clearly there is a
large standard deviation) the value of 1.337(2) found for
ethylene. The 31P NMR spectrum of 4 consists of an AB
pattern showing that the two ends of the chelating dcpm
ligand are non-equivalent. This is consistent with the
observed solid state structure and indicates that dissoci-
ation and rearrangement of the dcpm ligand is slow on
the NMR time scale.
3.1. Preparation of [(dcpm)Cl2Nb(v2-Cl)2NbCl2(dcpm)]
(1)
A mixture of [NbCl5] (1.50 g, 5.55 mmol) and dcpm
(3.37 g, 8.33 mmol) in benzene (150 ml) was stirred over
a sodium amalgam (0.27 g, 11.7 mmol of Na) for 2 h.
3. Experimental
All operations were carried out under a dry nitrogen
atmosphere or in vacuo either in a Vacuum Atmosphere
Table 3
i
˚
Selected bond distances (A) and angles (°) for cis, mer-[Ta(OC6H3Pr2-
2,6)2Cl3(h1-dcpm)] (3)
Table 2
˚
Bond distances (A)
Structurally characterized compounds [M2(m-Cl)2Cl4(P–P)2]a
Ta–O(3)
Ta–Cl(2)
Ta–Cl(1)
1.855(7)
2.343(3)
2.394(3)
Ta–O(4)
Ta–Cl(3)
Ta–P(1)
1.889(6)
2.378(3)
2.788(3)
˚
M
P–P
Structural type
M–M (A)
Ref.
Nb
Ta
Ta
Nb
Nb
Nb
Ta
Nb
Ta
dmpm
dmpm
depm
dppm
dcpm
dppe
dmpe
depe
depe
A
A
A
B
B
B
B
B
B
2.711(3)
2.692(2)
[6]
[7]
[5]
Bond angles (°)
O(3)–Ta–O(4)
O(3)–Ta–Cl(1)
O(3)–Ta–Cl(2)
O(3)–Ta–Cl(3)
O(3)–Ta–P(1)
O(4)–Ta–Cl(1)
O(4)–Ta–Cl(2)
O(4)–Ta–Cl(3)
O(4)–Ta–P(1)
97.0(3)
92.5(2)
96.9(3)
94.2(2)
173.0(2)
170.6(2)
93.5(2)
93.0(2)
88.0(2)
Cl(1)–Ta–Cl(2)
Cl(1)–Ta–Cl(3)
Cl(1)–Ta–P(1)
Cl(2)–Ta–Cl(3)
Cl(2)–Ta–P(1)
Cl(3)–Ta–P(1)
Ta–O(3)–C(31)
Ta–O(4)–C(41)
85.3(1)
86.3(1)
82.60(9)
166.4(1)
87.7(1)
80.6(1)
170.5(8)
179.8(6)
2.698(1), 2.711(1)
2.696(1)
2.738(1), 2.749(1)
2.721(2), 2.738(3)
2.710(1)
[8]
a
[9]
[10]
[11]
[12]
2.741(3)
2.737(1), 2.724(1)
a This work.

38
P.N. Riley et al. / Inorganica Chimica Acta 288 (1999) 35–39
over a sodium amalgam (0.21 g, 9.13 mmol of Na) for
8 h. The initially yellow solution became intensely
purple colored within 15 min. The suspension was
filtered and the purple filtrate was concentrated and
then layered with hexane to induce the formation of
the product. An additional recrystallization was per-
formed with benzene/hexane (1:1) to obtain pure
product. Yield: 0.38
g (6.6%). Anal. Calc. for
C50H92Cl6P4Ta2: C, 43.15; H, 6.66; Cl, 15.28; P, 8.90.
Found: C, 46.93; H, 6.56; Cl, 13.33; P, 6.49%. 1H
NMR (C6D6, 30°C): l 0.81–2.32 (m, 88H, cyclohexyl
resonances), 2.47 (t, 4H, PCH2P). 31P NMR (C6D6,
30°C): l −8.61.
3.3. Preparation of [Ta(OC6H3Pri2-2,6)2Cl3(p1-dcpm)] (3)
Fig. 3. Molecular structure of [Ta(OC6H3Pri2-2,6)(OC6H3Pr2i -h2-
CMeꢀCH2)(dcpm)Cl] (4).
To a benzene solution (20 ml) of [Ta(OC6H3Pri2-
2,6)2Cl3]2 (1.00 g, 1.94 mmol) was added dcpm (0.87
g, 2.13 mmol). The yellow solution was stirred vigor-
ously for 48 h then evaporated in vacuo. The resulting
yellow solid was extracted with hexane and after 24 h
yellow crystals of the product precipitated out of solu-
tion. The crystals were washed with hexane and dried
in vacuo. Yield: 0.35 g (17.2%). Anal. Calc. for
C49H80Cl3O2P2Ta: C, 56.03; H, 7.68; Cl, 10.13; P,
5.78. Found: C, 55.88; H, 8.00; Cl, 10.32; P, 5.78%.
1H NMR (C6D6, 30°C): l 1.07–1.77 (m, 44H, cyclo-
hexyl); 1.34 (d, 24H, CHMe); 4.61 (septet, 2H,
CHMe); 6.82–7.19 (m, aromatics). 31P NMR (C6D6,
30°C): −10.02 (d), 23.21 (d), 2J(31P–31P)=51.3 Hz.
The initially red solution became intensely purple col-
ored within 5 min. The solution was decanted from
the mercury pool, concentrated and layered with hex-
ane to induce the formation of crystals of the product.
The purple crystals were washed with hexane and
dried in vacuo. Yield: 2.10 g (62%). Crystals suitable
for X-ray diffraction were obtained from toluene/hex-
ane solutions. Anal. Calc. for C50H92Cl6P4Nb2 · C7H8:
C, 52.35; H, 7.71; Cl, 16.26; P, 9.47. Found: C, 53.85;
1
H, 7.91; Cl, 15.18; P, 9.36%. H NMR (C6D6, 30°C):
l 0.80–2.27 (m, 88H, cyclohexyl resonances), 2.49 (t,
4H, PCH2P). 31P NMR (C6D6, 30°C): l 2.81.
3.2. Preparation of [(dcpm)Cl2Ta(v2-Cl)2TaCl2(dcpm)] (2)
3.4. Preparation of [Ta(OC6H3Pri2-2,6)(OC6H3Pr2i -p2-
CMeꢀCH2)(dcpm)Cl] (4)
A mixture of [TaCl5] (1.48 g, 4.13 mmol) and dcpm
(2.19 g, 5.36 mmol) in benzene (150 ml) was stirred
A mixture of [Ta(OC6H3Pri2-2,6)2Cl3]2 (2.00 g, 3.88
mmol) and dcpm (1.27 g, 3.11 mmol) in benzene (150
ml) was stirred over a sodium amalgam (0.19 g, 8.26
mmol) for 18 h. The initially yellow solution turned
green–brown after 5 min. The solution was filtered
and the filtrate stripped in vacuo yielding a brown
solid. Recrystallization from hexane produced brown
crystals of poor quality and in very low yield, which
were washed with pentane and dried in vacuo. 31P
Table 4
Selected bond distances (A) and angles (°) for [Ta(OC6H3Pri2-
˚
2,6)(OC6H3Pri2-h2-CMeꢀCH2)(dcpm)Cl] (4)
˚
Bond distances (A)
Ta–O(1)
Ta–C(121)
Ta–Cl
1.96(1)
2.21(2)
2.468(5)
2.736(5)
Ta–O(2)
Ta–C(122)
Ta–P(3)
1.92(1)
2.21(2)
2.772(5)
1.56(3)
Ta–P(4)
C(121)–C(122)
2
NMR (C6D6, 30°C): l −4.71, 1.38 (AB pattern, JP–
P=110.8 Hz, 1 P, PCH2P).
Bond angles (°)
O(2)–Ta–O(1)
O(2)–Ta–Cl
O(2)–Ta–P(3)
O(2)–Ta–P(4)
O(2)–Ta–C(121)
O(2)–Ta–C(122)
O(1)–Ta–Cl
O(1)–Ta–P(3)
O(1)–Ta–P(4)
O(1)–Ta–C(121)
O(1)–Ta–C(122)
Ta–O(2)–C(21)
110.4(5)
87.0(3)
91.5(4)
149.4(4)
119.0(6)
89.0(6)
158.1(4)
81.8(4)
83.4(4)
119.0(6)
75.8(7)
161.5(11)
Cl–Ta–P(3)
Cl–Ta–P(4)
84.7(2)
75.1(2)
91.2(5)
119.1(6)
62.7(1)
149.0(5)
156.2(6)
86.5(5)
Cl–Ta–C(121)
Cl–Ta–C(122)
P(3)–Ta–P(4)
P(3)–Ta–C(121)
P(3)–Ta–C(122)
P(4)–Ta–C(121)
P(4)–Ta–C(122)
C(121)–Ta–C(122)
Ta–O(1)–C(11)
4. Supplementary material
Description of the experimental procedures for
X-ray diffraction studies, tables of thermal para-
meters, bond distances and angles, intensity data,
torsion angles, and multiplicities are available on
request from the author.
121.3(6)
41.2(7)
119.4(11)

P.N. Riley et al. / Inorganica Chimica Acta 288 (1999) 35–39
39
W. Hiller, M. Heckel, J. Riede, M.U. Schmidt, J. Organomet.
Chem. 490 (1995) 51. (l) R. Ficker, W. Hiller, C.T. Regius, P.
Hofmann, Z. Kristallogr. 211 (1996) 58. (m) R. Ficker, W.
Hiller, C.T. Regius, P. Hofmann, Z. Kristallogr. 211 (1996) 62.
[3] J.S. Yu, I.P. Rothwell, J. Chem. Soc., Chem. Commun. (1992)
632.
Acknowledgements
We thank the National Science Foundation (Grant
CHE-9700269) for financial support of this research.
[4] C.A. Morgan, P.E. Fanwick, I.P. Rothwell, Inorg. Chim. Acta
224 (1994) 105.
[5] L. Messerle, Chem. Rev. 88 (1988) 1229.
References
[6] F.A. Cotton, S.A. Duraj, L.R. Falvello, W.J. Roth, Inorg.
Chem. 24 (1985) 4389.
[7] A.R. Chakravarty, F.A. Cotton, M.P. Diebold, D.B. Lewis, W.J.
Roth, J. Am. Chem. Soc. 108 (1986) 971.
[8] F.A. Cotton, W.J. Roth, Inorg. Chem. 22 (1983) 3654.
[9] F.A. Cotton, W.J. Roth, Inorg. Chim. Acta 71 (1983) 175.
[10] F.A. Cotton, L.R. Falvello, R.C. Najjar, Inorg. Chem. 22 (1983)
375.
[11] J.A.M. Canich, F.A. Cotton, Inorg. Chem. 26 (1987) 4236.
[12] F.A. Cotton, M.P. Diebold, W.J. Roth, Inorg. Chem. 26 (1987)
4130.
[13] J.R. Clark, A.L. Pulverenti, P.E. Fanwick, M. Sigalis, O. Eisen-
stein, I.P. Rothwell, Inorg. Chem. 36 (1997) 3623.
[14] J.S. Yu, L. Felter, M.C. Potyen, J.R. Clark, V.M. Visciglio, P.E.
Fanwick, I.P. Rothwell, Organometallics 15 (1996) 4443.
[15] J.R. Clark, P.E. Fanwick, I.P. Rothwell, J. Chem. Soc., Chem.
Commun. (1995) 553.
[1] F.A. Cotton, R.A. Walton, Multiple Bonds Between Metal
Atoms, 2nd ed., Oxford University Press, New York, 1993.
[2] (a) C. Kruger, Y.-H. Tsay, Acta Crystallogr., Sect. B 28 (1972)
1941. (b) P. Hofmann, H. Heiss, G. Muller, Z. Naturforsch., Teil
B 42 (1987) 395. (c) P. Hofmann, C. Meier, U. Englert, M.U.
Schmidt, Chem. Ber. 125 (1992) 353. (d) P. Hofmann, L.A.
Perez-Moya, O. Steigelmann, J. Riede, Organometallics 11
(1992) 1167. (e) F.L. Joslin, J.R. Mague, D.M. Roundhill,
Polyhedron 10 (1991) 1713. (f) M.J. Irwin, L.M. Rendina, J.J.
Vittal, R.J. Puddephatt, J. Chem. Soc., Chem. Commun. (1996)
1281. (g) T. Nickel, R. Goddard, C. Kruger, K.-R. Porschke,
Angew. Chem., Int. Ed. Engl. 33 (1994) 879. (h) P. Hofmann,
L.A. Perez-Moya, M.E. Krause, O. Kumberger, G. Muller, Z.
Naturforsch., Teil B 45 (1990) 897. (i) R. Ficker, W. Hiller, T.W.
Gerl, P. Hofmann, Z. Kristallogr. 211 (1996) 951. (j) P. Hof-
mann, H. Heiss, P. Neiteler, G. Muller, J. Lachmann, Angew.
Chem., Int. Ed. Engl. 29 (1990) 880. (k) P. Hofmann, C. Meier,
.