Synthesis, structure and bonding situation of [(dcpe)Pt(InCp*)2] and {((dcpe)Pt[GaC(SiMe3)3]2} - Two novel examples of platinum complexes of low valent group 13 metal species

Article abstract of DOI:10.1016/S0277-5387(01)01027-0

In analogy to the corresponding Cp*Al- and Cp*Ga-compounds two further bis(phosphine)platinum complexes [(dcpe)Pt(InCp*)₂] (1(In)) and {((dcpe)Pt[GaC(SiMe₃)₃]₂} (2(Ga)) containing monovalent Group 13 metal species as ligands are accessible by thermal activation of [(dcpe)Pt(H)(CH₂t-Bu)] (dcpe = bis(dicyclohexylphosphino)ethane) followed by the reaction with 2 equiv. of InCp* (Cp* = pentamethylcyclopentadienyl) or GaC(SiMe₃)₃. The crystal structure analysis reveals a distorted tetrahedral coordination of the platinum center for both compounds. The Pt-In distances in 1(In) amount to 2.569(1) and 2.556(1) ?. The Pt-Ga distances in 2(Ga) are exceptionally short and amount to 2.315(1) and 2.318(1) ?. Comparative theoretical investigations have been performed on this type of complexes and allow a deeper insight in the bonding situation. The NBO analysis reveals a significantly larger Pt → Ga π-back-donation for the model compound {(dhpe)Pt[GaC(SiH₃)₃]₂} (2M(Ga)) (0.44 e; dhpe = 1,2-diphosphinoethane) in comparison with the related model compound [(dhpe)Pt(GaCP)₂] (1M(Ga)) (0.29 e) bearing a strong π-donating organic ligand at the Ga center. A similar trend is observed for the Pt-Ga bond dissociation energies (D_e= 33.0 kcal mol^-1 for 2M(Ga), D_e = 18.3 kcal mol^-1 for 1M(Ga)). For the model compound [(dhpe)Pt(InCp)₂] (1M(In)) a value of D_e= 19.1 kcal mol^-1 has been calculated.

Full text of DOI:10.1016/S0277-5387(01)01027-0

Polyhedron 21 (2002) 535–542
www.elsevier.com/locate/poly
Synthesis, structure and bonding situation of [(dcpe)Pt(InCp*)2]
and {(dcpe)Pt[GaC(SiMe3)3]2}—two novel examples of platinum
complexes of low valent Group 13 metal species
Dana Weiß a, Manuela Winter a, Klaus, Merz a, Annegret Knu¨fer a,
Roland A. Fischer a,*, Nikolaus Fro¨hlich b, Gernot Frenking b,*
a Chemische Fakulta¨t, Ruhr-Uni6ersita¨t Bochum, Uni6ersita¨tsstraße 150, 44780 Bochum, Germany
b Fachbereich Chemie, Philipps-Uni6ersita¨t Marburg, Hans-Meerwein-Straße, 35032 Marburg, Germany
Received 8 August 2001; accepted 27 November 2001
Abstract
In analogy to the corresponding Cp*Al- and Cp*Ga-compounds two further bis(phosphine)platinum complexes
[(dcpe)Pt(InCp*)2] (1(In)) and {(dcpe)Pt[GaC(SiMe3)3]2} (2(Ga)) containing monovalent Group 13 metal species as ligands are
accessible by thermal activation of [(dcpe)Pt(H)(CH2t-Bu)] (dcpe=bis(dicyclohexylphosphino)ethane) followed by the reaction
with 2 equiv. of InCp* (Cp*=pentamethylcyclopentadienyl) or GaC(SiMe3)3. The crystal structure analysis reveals a distorted
tetrahedral coordination of the platinum center for both compounds. The PtꢀIn distances in 1(In) amount to 2.569(1) and 2.556(1)
,
,
A. The PtꢀGa distances in 2(Ga) are exceptionally short and amount to 2.315(1) and 2.318(1) A. Comparative theoretical
investigations have been performed on this type of complexes and allow a deeper insight in the bonding situation. The NBO
analysis reveals a significantly larger Pt“Ga p-back-donation for the model compound {(dhpe)Pt[GaC(SiH3)3]2} (2M(Ga)) (0.44
e; dhpe=1,2-diphosphinoethane) in comparison with the related model compound [(dhpe)Pt(GaCp)2] (1M(Ga)) (0.29 e) bearing
a strong p-donating organic ligand at the Ga center. A similar trend is observed for the PtꢀGa bond dissociation energies
(De=33.0 kcal mol−1 for 2M(Ga), De=18.3 kcal mol−1 for 1M(Ga)). For the model compound [(dhpe)Pt(InCp)2] (1M(In)) a
value of De=19.1 kcal mol−1 has been calculated. © 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Gallium; Indium; Platinum; Heterobimetallic complexes; X-ray structure analysis; Quantum chemical calculations
1. Introduction
Si(t-Bu)3 or sterically demanding aryl groups) as well as
salt elimination and insertion reactions [2]. In a prelim-
inary report we have shown that bis(phos-
phine)platinum complexes of the type [(dcpe)Pt(ECp*)2]
(E=Al (1(Al)), Ga (1(Ga)); dcpe=bis(dicyclohexyl-
phosphino)ethane), carrying no additional p-acceptor
ligands like CO, are readily accessible [3] by trapping
the reactive intermediate [(dcpe)Pt], produced via re-
ductive alkyl elimination from the platinum hydride
compound [(dcpe)Pt(H)(CH2t-Bu)], with the low valent
aluminum and gallium species Cp*Al and Cp*Ga. Here
we wish to describe two new compounds obtained by
the same strategy, namely [(dcpe)Pt(InCp*)2] (1(In))
and {(dcpe)Pt[GaC(SiMe3)3]2} (2(Ga)). This latter com-
pound is of particular interest in comparison with its
close congener [(dcpe)Pt(GaCp*)2] because of the lig-
and effects at the Ga center on the PtꢀGa bond.
The coordination chemistry of Group 13 elements in
a low oxidation state is presently a topic of intensive
research that has led to the successful synthesis of
appealing new types of transition metal compounds [1].
Common synthetic routes to transition metal Group 13
metal compounds with the Group 13 metal in the
formal oxidation state +I are the substitution of
weakly bonded ligands like CO or olefins by ligands of
the type ER (E=Al, Ga, In; R=e.g. Cp*, C(SiMe3)3,
* Corresponding authors. Tel.: +49-234-3224174; fax: +49-234-
3214174 (R.A.F.); tel.: +49-6421-2825563; fax: +49-6421-2825566
(G.F.).
E-mail addresses: rfischer@aci.ruhr-uni-bochum.de (R.A. Fischer),
frenking@chemie.uni-marburg.de (G. Frenking).
0277-5387/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0277-5387(01)01027-0

536
D. Weiß et al. / Polyhedron 21 (2002) 535–542
2. Experimental
(SiMe3)3]4 were suspended in 0.4 ml of methylcyclohex-
ane in an evacuated and sealed NMR tube. After the
reaction mixture was heated 4 h at 70 °C the reaction
was complete according to the 31P NMR spectrum. The
sample was treated following the procedure described
for compound 1(In) to give yellowish orange rhombic
crystals. Yield of the isolated crystalline product: 87 mg
2.1. Reagents, sol6ents, and general procedures
All manipulations were carried out under purified Ar
atmosphere using standard vacuum techniques. The
solvents were distilled over sodium and stored over
molecular sieves prior to use. Procedures described in
the literature were followed for the preparation of
1
(49%). The compound decomposes slowly at 80 °C. H
NMR: l 0.69 (s, 54H, 18CH3, 2C(SiMe3)3), 2.20–0.97
[(dcpe)Pt(H)(CH2t-Bu)]
[4],
(InCp*)6
[5]
and
(complex
and
broad
superpositions,
48H,
[GaC(SiMe3)3] [6]. Elemental analyses were performed
by the Microanalytical Laboratory of the Ruhr-Univer-
sita¨t Bochum. The NMR spectra were recorded in
benzene-d6 at 298 K using a Bruker Avance DPX-250
spectrometer (1H, 250.1 MHz; 13C, 62.9 MHz; 31P,
101.3 MHz). Chemical shifts were referenced to the
solvent resonances as internal standards and or-
thophosphoric acid as external standard, respectively.
Infrared spectra were recorded as KBr pellets using a
Perkin–Elmer 1720 X FT-IR spectrometer. Absorp-
tions are described as follows: strong (s), medium (m),
weak (w), and shoulder (sh).
Cy2P(CH2)2PCy2). 13C{1H} NMR: l 6.8 (18CH3,
C(SiMe3)3), 23.3–35.8 (complex superpositions). 31P
NMR: l 62.6 (s with 195Pt satellites, J(PꢀPt)=3285 Hz).
1
IR: 2923 (vs), 2845 (s), 1448 (m), 1409 (w), 1342 (w),
1251 (s), 1173 (w), 1103 (m, b), 1004 (m, b), 846 (vs, b),
741 (w), 673 (m), 652 (m), 585 (m), 522 (w) 463 (w).
Anal. Calc. for C46H102Ga2P2PtSi6 (1220.30): C 45.28,
H 8.42. Found: C 45.57, H 8.56%.
2.4. Crystal structure determination
Diffraction data from a ocre colored needle-shaped
crystal (compound 1(In), crystal size 0.10×0.20×0.20
mm3) and a rhombic yellowish orange crystal (com-
pound 2(Ga), crystal sizes 0.25×0.20×0.15 mm3), re-
spectively, were collected on a Bruker-axs-SMART
1000 diffractometer (v-scans). The structures were
solved by direct methods using the SHELXS-97 program
[7] and were refined by the full-matrix least square
method on F2 with the SHELXL-97 program. [8] Crystal-
lographic data for 1(In): empirical formula
C46H78In2P2Pt, Fw=1117.759, monoclinic, space group
2.2. Preparation of [(dcpe)Pt(InCp*)2] (1(In))
A
sample of 100 mg (0.145 mmol) of
[(dcpe)Pt(H)(CH2t-Bu)] and 73 mg (0.048 mmol) of
(InCp*)6 were combined and suspended in 0.4 ml of
methylcyclohexane in an evacuated and sealed NMR
tube. After the reaction mixture was heated 2 h at
60 °C the reaction was complete according to the 31P
NMR spectrum. The NMR tube was opened in the
glove box, the content was transferred to a Schlenk
tube and all volatile components were removed in
vacuo, whereby the raw product could be obtained in
quantitative yield. The resulting solid was recrystallized
by slow evaporation of a solution of 1(In) in toluene in
the glove box to give ocre colored needle-shaped crys-
tals. Yield of the isolated crystalline product: 106 mg
(65%). The compound decomposes slowly at 108 °C.
1H NMR: l 2.40 (s, 30H, 10CH3, Cp*), 2.00–0.95
,
P21/c, a=12.366(2), b=20.27(4), c=20.211(4) A, i=
3
,
,
106.629(5)°, V=4796(2) A , Z=4, u=0.71073 A, T=
203 K, reflections collected 20 642, 8424 independent
reflections (Rint=0.0708), reflections observed 6188,
460 parameters, completeness to q=25.68°: 89.8%,
final R[I\2|(I)]=0.0396, Rw[I\2|(I)]=0.0821,
semi-empirical absorption correction, GOF=0.958;
crystallographic data for 2(Ga): empirical formula
C46H102Ga2P2PtSi6, Fw=1220.30, triclinic, space group
(complex
and
broad
superpositions,
48H,
Cy2P(CH2)2PCy2). 13C{1H} NMR: l 11.3 (10CH3,
Cp*), 26.8–38.0 (complex superpositions), 113.1 (10
ring C atoms, Cp*). 31P NMR: l 61.0 (s with 195Pt
(
,
P1, a=13.569(2), b=14.117(2), c=16.195 A, h=
103.872(3), i=94.823(3), k=94.689(3)°, V=2984.4(8)
1
satellites, J(PꢀPt)=5077 Hz). IR: 2911 (s), 2845(s), 1443
3
,
,
A , Z=2, u=0.71073 A, T=203 K, reflections col-
(s), 1417 (m), 1377 (m), 1334(w), 1257 (s), 1172 (w),
1096 (s), 1022 (s), 883 (w) 847 (m), 802 (s), 733 (m), 638
(w), 587 (w), 521 (m), 459 (w). Anal. Calc. for
C46H78In2P2Pt (1117.79): C, 49.43; H, 7.03. Found: C,
49.16; H, 7.41%.
lected 15 597, 10 398 independent reflections (Rint
=
0.1055), reflections observed 7182, 572 parameters,
completeness to q=25.13°: 97.2%, final R[I\2|(I)]=
0.0590, Rw[I\2|(I)]=0.1461, empirical absorption
correction, GOF=0.970, R=[ꢀ(ꢁFoꢁ−ꢁFcꢁ)/ꢀꢁFoꢁ]1/2
,
Rw=[ꢀw(Fo2−Fc2)2/ꢀw(Fo2)2]1/2
.
Selected geometric
2.3. Preparation of [(dcpe)Pt{GaC(SiMe3)3}2] (2(Ga))
parameters are listed in Tables 1 and 2. The trimethylsi-
lyl groups of compound 2(Ga) were disordered and
have been calculated for two positions.
A sample of 100 mg (0.145 mmol) of [(dcpe)Pt-
(H)(CH2t-Bu)] and 88 mg (0.073 mmol) of [GaC-

D. Weiß et al. / Polyhedron 21 (2002) 535–542
537
Table 1
,
Selected interatomic distances (A) and angles (°) for the complexes 1(Al), 1(Ga) (values for the second conformational Isomer have been omitted
for reasons of a better overview; see also [3]) and 1(In) and the model complexes 1M(Al), 1M(Ga) and 1M(In)
[(dcpe)Pt(AlCp*)2] [(dhpe)Pt(AlCp)2] [(dcpe)Pt(GaCp*)2] [(dhpe)Pt(GaCp)2] [(dcpe)Pt(InCp*)2] [(dhpe)Pt(InCp)2]
(1(Al)) (E=Al)
(1M(Al)) (E=Al) (1(Ga)) (E=Ga)
(1M(Ga))
(E=Ga)
(1(In)) (E=In)
(1M(In)) (E=In)
Bond lengths
Pt(1)ꢀE(1)
Pt(1)ꢀE(2)
E(1)···E(2)
Pt(1)ꢀP(1)
2.327(2)
2.335(2)
3.502
2.358
2.358
3.659
2.353
2.353
2.367
2.356
2.343
2.345
2.360
2.367
2.356
2.343
2.345
2.360
2.028
2.028
2.355(2)
2.367(2)
3.538
2.439
2.439
3.680
2.326
2.326
2.445
2.446
2.442
2.442
2.444
2.445
2.446
2.442
2.442
2.444
2.122
2.122
2.569(1)
2.556(1)
3.666
2.600
2.600
3.916
2.317
2.317
2.608
2.613
2.620
2.620
2.613
2.608
2.613
2.620
2.620
2.613
2.314
2.314
2.291(2)
2.294(2)
2.313(6)
2.288(6)
2.284(6)
2.302(7)
2.322(6)
2.306(7)
2.291(6)
2.310(6)
2.332(6)
2.334(7)
1.958
2.251(4)
2.256(5)
2.39(2)
2.33(2)
2.29(2)
2.37(2)
2.38(2)
2.40(2)
2.36(2)
2.32(2)
2.37(2)
2.44(2)
2.017
2.247(2)
2.242(2)
2.582(6)
2.584(6)
2.589(6)
2.572(6)
2.567(6)
2.605(6)
2.581(6)
2.554(6)
2.557(6)
2.593(5)
2.282
Pt(1)ꢀP(2)
E(1)ꢀC(11)
E(1)ꢀC(12)
E(1)ꢀC(13)
E(1)ꢀC(14)
E(1)ꢀC(15)
E(2)ꢀC(21)
E(2)ꢀC(22)
E(2)ꢀC(23)
E(2)ꢀC(24)
E(2)ꢀC(25)
Cpc*entroidꢀE(1)
Cpc*entroidꢀE(2)
1.975
2.058
2.280
Bond angles
E(1)ꢀPt(1)ꢀE(2)
P(1)ꢀPt(1)ꢀP(2)
P(1)ꢀPt(1)ꢀE(1)
P(2)ꢀPt(1)ꢀE(2)
P(1)ꢀPt(1)ꢀE(2)
P(2)ꢀPt(1)ꢀE(1)
97.37(8)
90.38(7)
115.27(6)
117.50(7)
119.40(7)
118.76(6)
103.2
88.8
116.0
116.0
116.7
116.7
97.07(7)
92.2(2)
119.1(2)
118.4(2)
117.6(1)
114.3(1)
99.9
90.5
116.8
116.8
117.2
117.2
91.34(2)
93.22(6)
117.71(4)
116.76(4)
120.16(4)
120.15(4)
97.7
91.0
117.3
117.3
117.5
117.5
3. Results and discussion
discussed in detail here. The 31P NMR spectra in
benzene show characteristic singlets with 195Pt-satellites
at 61.0 ppm (1J(PꢀPt)=5077 Hz; compound 1(In)) and
62.6 ppm (1J(PꢀPt)=3285 Hz; compound 2(Ga)) indicat-
The previously reported compounds [(dcpe)Pt-
(ECp*)2] (E=Al (1(Al)), Ga (1(Ga))) and the related
title compounds [(dcpe)Pt(InCp*)2] 1(In) and
{(dcpe)Pt[GaC(SiMe3)3]2} 2(Ga) were obtained by the
same synthetic pathway according to Scheme 1. When a
suspension of [(dcpe)Pt(H)(CH2t-Bu)] [4] in methylcy-
clohexane was treated with 2 equiv. of the respective
Group 13-diyl compound [5,6] and heated to 60–70 °C
for several hours in an evacuated and sealed NMR tube
the quantitative conversion of the starting material was
observed by in situ 31P NMR spectroscopy. By slow
evaporation of a solution of 1(In) or 2(Ga) in toluene
the product complexes could be gathered in the form of
ocre colored needles (1(In)) and yellowish orange rhom-
bic crystals (2(Ga)), respectively. Complexes 1(In) and
Table 2
,
Selected interatomic distances (A) and angles (°) for complex 2(Ga)
and the model complex 2M(Ga)
{(dcpe)Pt[GaC(Si- {(dhpe)Pt[GaC(SiH3)3]2}
Me3)3]2} (2(Ga))
(2M(Ga))
Bond lengths
Pt(1)ꢀGa(1)
Pt(1)ꢀGa(2)
Ga(1)···Ga(2)
Pt(1)ꢀP(1)
Pt(1)ꢀP(2)
Ga(1)ꢀC(3)
Ga(2)ꢀC(4)
2.315(1)
2.318(1)
3.556
2.286(2)
2.285(3)
2.06(1)
2.060(9)
2.360
2.360
3.657
2.348
2.348
2.060
2.060
2(Ga) have been characterized by means of H, 13C and
1
31P NMR, as well as IR spectroscopy, elemental analy-
sis and single crystal X-ray diffraction.
Bond angles
Ga(1)ꢀPt(1)ꢀGa(2) 100.28(4)
101.5
87.6
P(1)ꢀPt(1)ꢀP(2)
C(3)ꢀGa(1)ꢀPt(1)
C(4)ꢀGa(2)ꢀPt(1)
P(1)ꢀPt(1)ꢀGa(1)
P(2)ꢀPt(1)ꢀGa(2)
P(1)ꢀPt(1)ꢀGa(2)
P(2)ꢀPt(1)ꢀGa(1)
89.08(9)
177.7(3)
178.4(3)
119.45(7)
114.99(7)
117.75(7)
116.41(7)
175.8
175.8
115.0
115.0
119.4
119.4
3.1. NMR spectra
The 1H and 13C NMR spectra are complex and
provide only little information relevant to the discus-
sion of the PtꢀE bonding. Therefor, they will not be

538
D. Weiß et al. / Polyhedron 21 (2002) 535–542
two phosphorus atoms of the chelating ligand dcpe and
the two h5-Cp*In units. The PtꢀIn distances of 2.569(1)
,
and 2.556(1) A are distinctly longer in comparison with
,
the PtꢀIn distances of 2.441(2) A of the homoleptic
compound {Pt[InC(SiMe3)3]4}, reported by Uhl et al.
[9],
but
slightly
shortened
compared
with
[(dcpe)Pt(InR2)(R)] (R=CH2SiMe3), the only further
example of a structurally characterized molecular com-
pound with a direct, unsupported PtꢀIn bond (2.601(0)
Scheme 1.
,
A) [10]. Expectedly the PtꢀIn bonds are somewhat
shorter than estimated from covalent single bond radii
,
(2.80–2.89 A). The shortest PtꢀIn distance described
,
for binary alloys amounts to 2.534 A [11]. For the
In(1)ꢀPt(1)ꢀIn(2) angle a value of 91.34(2)° has been
determined, distinctly narrowed in comparison to the
corresponding EꢀPtꢀE angles of the complexes
[(dcpe)Pt(ECp*)2] (E=Al, Ga) with values in a range
of 95.21(7)°–100.02(7)°. This feature of the solid state
structure of 1(In) is presumably caused by the smaller
cone angle of the Cp*In unit (98°) within compound
1(In) in comparison to the cone angles of the Cp*Al
(107°) and Cp*Ga units (102°) within the homologous
AlꢀPt and GaꢀPt complexes. The small angle also
explains the somewhat larger extent of the deviation of
the Cp*centroidꢀInꢀPt vectors from linearity (169.52° and
172.75°), in comparison to the Al and Ga complexes
with average values for the Cp*centroidꢀEꢀPt angles of
176.10° and 175.01°. The PtꢀP distances of 1(In) are
nearly identical and exhibit values of 2.247(2) and
Fig. 1. Molecular structure of [(dcpe)Pt(InCp*)2] (1(In)) (ORTEP plot).
ing an analogous chemical environment of the
phosphorus atoms in each compound which is in
accordance with the approximate C2 symmetry of the
complexes confirmed by X-ray analyses. The values for
the chemical shifts are in a range comparable to the
values of 52.5 and 53.6 ppm found for the signals of the
homologous complexes [(dcpe)Pt(ECp*)2] (E=Al, Ga).
For compounds of the type [(dcpe)Pt(ECp*)2] a steady
,
2.242(2) A. In comparison to the compounds
[(dcpe)Pt(AlCp*)2] and [(dcpe)Pt(GaCp*)2] with aver-
,
age PtꢀP distances of 2.286 and 2.259 A a shortening of
the PtꢀP distances following the order E=Al, Ga, In
1
increase of the coupling constant J(PꢀPt), accompanied
and a corresponding increase of the 31P coupling con-
1
by a shortening of the interatomic PꢀPt distances
stants J(PꢀPt) from 3482 to 4625 and 5071 Hz, respec-
following the order E=Al (1J(PꢀPt)=3482 Hz,
tively, was found. For the P(1)ꢀPt(1)ꢀP(2) angle a value
of 93.22(6)° has been determined, similar to the respec-
tive angles in the Al and Ga compounds of around
90.50° and 92.25° (average values for the two coexisting
isomers, see [3]). Within the Cp*In fragments, the h5-
bonding mode of the Cp* ring is maintained, although
small deviations from ideal h5-bonding are observed.
The Cp*centroidꢀIn distances amount to 2.282 and 2.280
d(PꢀPt)average=2.286 A), Ga (1J(PꢀPt)=4625 Hz,
,
d(PꢀPt)average=2.259 A), In (1J(PꢀPt)=5077 Hz,
,
,
d(PꢀPt)average=2.245 A) has been observed. In none of
2
the cases discussed a coupling J(PꢀP) was observed,
possibly due to dynamic conformational effects in
solution. Neither 195Pt nor 71Ga NMR resonances have
been identified yet either, most likely because of
extensive quadrupolar line broadening effects caused by
the Ga and In nuclei.
,
A. These values are close to the one found for the free
,
ligand in the gas phase (2.288(4) A) [5]. The Cp*centroidꢀE
distance is regarded as an indirect measure for the
polarity of the PtꢀE bond (E=Group 13 metal), a
subject that has already been discussed extensively in
the literature [2a,2c,3,12], so that the polarity of the
PtꢀIn bond of compound 1(In) seems to be low com-
pared with [(CO)5CrꢀInCp*], the only known metal
carbonyl complex of an Cp*In ligand and Cp*centroidꢀIn
3.2. Molecular structures of 1(In) and 2(Ga)
A single crystal X-ray diffraction study was per-
formed, the results of which confirm the structural
predictions based on analytical and spectroscopic data.
The molecular structure of the title compound 1(In) is
outlined in Fig. 1.
Compound 1(In) crystallizes in the monoclinic space
group P21/c (Z=4). As is has been shown for the
above mentioned congeners, the platinum center of
1(In) is coordinated in a nearly tetrahedral mode by the
,
bond distance of 2.168(4) A [13].
The complex {(dcpe)Pt[GaC(SiMe3)3]2} (2(Ga)) crys-
(
tallizes in the triclinic space group P1 (Z=2). The
molecular structure in the crystal is depicted in Fig. 2.

D. Weiß et al. / Polyhedron 21 (2002) 535–542
539
[16]. They differ clearly from the average GaꢀC dis-
The platinum center is coordinated by the phosphine
ligand dcpe and two (Me3Si)3Ga units in a distorted
tetrahedral mode.
,
tances of 1.993 and 1.982 A of the only (Me3Si)3Ga-
substituted metal carbonyl complexes known so far,
{Mn2(CO)8[m-GaC(SiMe3)3]2} and {Fe3(CO)9(m-CO)[m-
GaC(SiMe3)3]2} [17], which should provide a higher
polar character of the MꢀGa bond. The GaꢀC bond
length of the only further complex with terminal coor-
The Pt(1)ꢀGa(1) and Pt(1)ꢀGa(2) distances of
,
2.315(1) and 2.318(1) A represent the shortest PtꢀGa
bonds known so far. The PtꢀGa distances of the related
,
complexes [(dcpe)Pt(GaCp*)2] (2.355(2) to 2.382(2) A
dinating
(Me3Si)3Ga
units,
{Ni[GaC(SiMe3)3]4}
for the two coexisting isomers) and {(dcpe)Pt[Ga(CH2t-
,
amounts to 2.014(4) A [18]. The quantum-chemically
calculated GaꢀC distances of the homoleptic platinum
model complex [Pt(GaMe)4] are 2.040 (BP86) and 2.028
,
Bu)2](CH2t-Bu)} [14] (2.438(1) A) are distinctly longer.
[Pt2(GaCp*)2(m2-GaCp*)3] [15], the only homoleptic
PtꢀGa complex, shows terminal PtꢀGa distances that
,
A (B3LYP), respectively. The Ga(1)ꢀPt(1)ꢀGa(2) angle
,
are with an average value of 2.329 A close to the
of 100.28(4)° of compound 2(Ga) is widened in com-
parison to the analogous angle of the complex
[(dcpe)Pt(GaCp*)2] with an average value of 96.14° for
the two conformational isomers. The nonbonding
respective values for 2(Ga). The Ga(1)ꢀC(3) and
,
Ga(2)ꢀC(4) bond lengths of 2.06(1) and 2.060(9) A
(Cp*centroidꢀGa distances of [(dcpe)Pt(GaCp*)2]: 2.017 to
,
2.067 A) correspond to the value found for the free
,
Ga(1)···Ga(2) distance of 3.556 A is comparable with
,
ligand (Me3Si)3Ga (monomer, gas phase) of 2.06(2) A
the corresponding value in [(dcpe)Pt(GaCp*)2] (3.510
,
and 3.538 A for the two isomers). In contrast the solid
state structure of tetrameric cluster compound
[(Me3Si)3Ga]4 shows GaꢀGa distances of 2.678(4) to
,
2.702(3) A. The cone angle of (Me3Si)3CGa within
compound 2(Ga) amounts to 73° (cone angle of Cp*Ga
within [(dcpe)Pt(GaCp*)2]: 102°), so that steric reasons
do not seem to predominate in the widening of the
Ga(1)ꢀPt(1)ꢀGa(2) angle. The average PtꢀP distances of
,
compound 2(Ga) (2.286 A) are significantly elongated
in comparison to the situation for [(dcpe)Pt(GaCp*)2]
,
(2.259 A). This is presumably a consequence of a
stronger Pt“Ga back-donation as predicted by quan-
tum chemical calculations for MꢀER bonds (M=tran-
sition metal, E=Group 13 element) with alkyl
substituents R at the Group 13 element in contrast to
the situation where R represents a p-donor ligand. This
is discussed below.
Fig. 2. Molecular structure of {(dcpe)Pt[GaC(SiMe3)3]2} (2(Ga))
(
ORTEP plot).
4. Theoretical studies on model complexes
The structures of the model complexes [(dhpe)Pt-
(InCp)2]
(1M(In))
and
{(dhpe)Pt[GaC(SiH3)3]2}
(2M(Ga)) (dhpe=1,2-diphosphinoethane) have been
optimized with the DFT functional BP86 and SVP
basis sets and a relativistic pseudopotential for Pt (Figs.
3 and 4) [19–27]. The comparison of the calculated and
experimental bond lengths and angles shows that the
structural data of 1M(In) and 2M(Ga) are in good
agreement with the observed values of 1(In) and 2(Ga)
(Tables 1 and 2). The use of Cp instead of Cp* explains
the somewhat longer EꢀC distances of the model com-
pounds.
The calculated structures of the aluminum and gal-
lium analogues 1M(Al) and 1M(Ga) of the model com-
pound 1M(In) have been reported by us in a previous
investigation [3]. The data of 1M(Ga) can now be
compared with those of 2M(Ga). In order to facilitate
Fig. 3. Calculated structure of [(dhpe)Pt(InCp)2] (1M(In)) (SCHAKAL
plot).

540
D. Weiß et al. / Polyhedron 21 (2002) 535–542
distance of 1M(Ga) is longer than in 2M(Ga) (Table 2)
and that the latter complex has a longer PtꢀP bond
than the former. We analyzed the bonding situation in
the model complexes with the NBO partitioning scheme
[26] in order to see whether the shorter PtꢀGa bond of
2M(Ga) correlates with stronger Pt“Ga back-dona-
tion. Tables 3 and 4 give the results of the NBO
analysis of 1M(E) (E=Al, Ga, In) and 2M(Ga).
The calculated atomic partial charges show that the
platinum atom carries in all model compounds a nega-
tive charge between −0.2 and −0.3 e while the Group
13 elements have a large positive charge between +0.6
and +0.8. We want to point out that the atomic
partial charges may be a deceptive indicator for the
electrostatic forces between the atoms because they do
not give information about the topographical distribu-
tion of the electronic charge. The calculated partial
charges suggest that there is electrostatic attraction
between the positively charged element E and the nega-
tively charged Pt. However, the elements E in the
ligands ER behave like a Lewis base because they have
an electron lone pair as HOMO which donates elec-
tronic charge into the LUMO of the metal fragment.
The latter species is a Lewis acid where the Pt atom is
Fig. 4. Calculated structure of {(dhpe)Pt[GaC(SiH3)3]2} (2M(Ga))
(SCHAKAL plot).
the comparison we give the most important bond
lengths and angles in Table 1. Note that the PtꢀGa
Table 3
NBO analysis of [(dhpe)Pt(ECp)2] (1M(E)) (E=Al, Ga, In) at the BP86/SVP level
a
E
q[(dhpe)Pt]
q(Pt)
q(E)
q(Cp)
px(E)
py(E) a
pz(E)
Dq(E) c
Dqp(E) c Dqs(E) c Dq(Cp) c P(PtꢀE)
Al
Ga
In
−0.14
−0.03
−0.14
−0.28
−0.21
−0.32
0.68
0.59
0.77
−0.61
−0.58
−0.68
0.28
0.25
0.22
0.27
0.27
0.22
0.29
0.23
0.16
+0.04
−0.03
+0.10
−0.29
−0.22
−0.18
+0.33
+0.19
+0.29
+0.03
+0.04
+0.01
0.52
0.42
0.42
b
Free ligand ECp
Al
Ga
In
0.65
0.62
0.67
−0.65
−0.62
−0.67
0.13
0.15
0.13
0.13
0.15
0.13
0.21
0.15
0.13
Partial charges q, p-orbital populations, charge difference Dq of the ECp moieties between the complex and the free ligand, Wiberg bond index
P.
a p(p) AO of E.
b Values for the free ligand ECp, calculated in the geometry of the complexes.
c Negative values represent a higher electronic charge, and positive values represent a lower electronic charge in the complex relative to the free
ligand. De{[(dhpe)Pt(ECp)2]}=29.6 (E=Al), 18.3 (E=Ga), 19.1 kcal mol−1 (E=In). The data for 1M(Al) and 1M(Ga) have been taken from
reference [3].
Table 4
NBO analysis of {(dhpe)Pt[GaC(SiH3)3]2} 2M(Ga) at the BP86/SVP level
a
q[(dhpe)Pt]
q(Pt)
q(Ga)
q(R)
px(Ga)
py(Ga) a pz(Ga)
Dq(Ga) c Dqp(Ga) c Dqs(Ga) c Dq(R) c P(PtꢀE)
−0.14 −0.44 +0.30 +0.12 0.57
0.04
−0.23
0.66
−0.68
0.25
0.23
0.02
0.34
0.27
b
Free ligand GaCSiH3
0.80
−0.80
0.02
Partial charges q, p-orbital populations, charge difference Dq of the EC(SiH3)3 moieties between the complex and the free ligand, Wiberg bond
index P, R=C(SiH3)3.
a p(p) AO of E.
b Values for the free ligand GaC(SiH3)3, calculated in the geometry of the complexes.
c Negative values represent a higher electronic charge, and positive values represent a lower electronic charge in the complex relative to the free
ligand. De({(dhpe)Pt[GaC(SiH3)3]2})=33.0 kcal mol−1
.

D. Weiß et al. / Polyhedron 21 (2002) 535–542
541
the acceptor atom. We have recently shown in a de-
tailed analysis of the metalꢀER bonding in Group
13-diyl complexes that the electrostatic attraction of the
MꢀER bond arises from the local negati6e charge con-
centration of the o6erall positi6ely charged atom E with
the locally charged depleted metal atom M which has an
o6erall negati6e charge [28]. The partial atomic charges
give important information about the total charge flow
between the atoms in a molecule but they do not tell
where the charge is concentrated and where it is de-
pleted within an atomic basin.
The ligands ECp become slightly positively charged
in 1M(E) while the ligand GaR (R=C(SiH3)3) receives
a small negative charge in 2M(Ga) (Tables 3 and 4).
Note that the total charge of atom E changes only little
upon formation of the PtꢀECp and PtꢀGaR bonds (see
the Dq(E) values in Tables 3 and 4) but the differences
of the s and p charges Dqs(E) and Dqp(E) are much
larger. The change in the p charges Dqp(E) give directly
the Pt“E p-back-donation. The data in Table 3 show
that the Pt“E p-back-donation in 1M(E) has the trend
Al\Ga\In. The comparison of 1M(Ga) with 2M(Ga)
shows clearly that the Pt“GaCp p-back-donation is
significantly less than the Pt“GaR back-donation.
This is in agreement with the results, which have been
obtained for complexes (CO)4FeꢀER [29]. The stronger
Pt“GaR p-back-donation is in agreement with the
higher bond dissociation energy (BDE) of 2M(Ga). The
theoretically predicted BDE for dissociation of one
GaR ligand is De=33.0 kcal mol−1, while the BDE of
the PtꢀGaCp bond of 1M(Ga) is only De=18.3 kcal
mol−1. The bond energies of the other ECp complexes
are De=29.6 kcal mol−1 for 1M(Al) and De=19.1
kcal mol−1 for 1M(In).
metal fragment play also an important role concerning
the polarity and characteristics of the PtꢀE bond [3].
All complexes of the type [(dcpe)Pt(ER)2] show a dis-
torted tetrahedral coordination sphere at the platinum
center. It has to be emphasized that 2(Ga) possesses
extraordinarily short PtꢀGa bonds of 2.315(1) and
,
2.318(1) A. The calculated geometries of the model
complexes [(dhpe)Pt(InCp)2] 1M(In) and {(dhpe)Pt-
[GaC(SiH3)3]2} 2M(Ga) are in good agreement with the
experimental values of 1(In) and 2(Ga), respectively. In
these bis(phosphine)platinum complexes the ER frag-
ment functions as a two electron donor/acceptor ligand
substitute in a terminal position. The NBO analysis
shows that the Pt“GaC(SiH3)3 p-back-donation in
2M(Ga) is significantly larger than the Pt“GaCp p-
back-donation in [(dcpe)Pt(GaCp)2] (1M(Ga)) as a re-
sult of the weaker p-donor properties of the organic
ligand C(SiH3)3 in comparison with Cp. These accents
are also reflected in the bond dissociation energies of
1M(Ga) (De=18.3 kcal mol−1) and 2M(Ga) (De=33.0
kcal mol−1). For 1M(In) a value of De=19.1 kcal
mol−1, comparable with the bond strength of the
model compound 1M(Ga) has been calculated.
6. Supplementary data
Supplementary data are available on request, quoting
the deposition numbers CCDC 172598 and CCDC
172599 from The Director, CCDC, 12 Union Road,
Cambridge, CB2 1EZ, UK (fax: +4-1223-336033; e-
mail: deposit@ccdc.cam.ac.uk or www: http://www.
ccdc.cam.ac.uk).
Acknowledgements
5. Conclusions
The support of this work by the Degussa AG and the
W.C. Heraeus GmbH is gratefully acknowledged. We
also thank the Deutsche Forschungsgemeinschaft and
the Fonds der Chemischen Industrie for financial
support.
Recently the first examples of phosphine transition
metal complexes with low valent Group 13 metal sub-
stituents, [(dcpe)Pt(ECp*)2] (E=Al, Ga), have been
reported by our group [3]. The row of this type of
compounds has now been extended to the complexes
[(dcpe)Pt(InCp*)2] 1(In) and {(dcpe)Pt[GaC(SiMe3)3]}
2(Ga), which have been fully characterized and ana-
lyzed quantum-chemically. Thus we have shown that
bimetallic PtꢀE (E=Al, Ga, In) compounds can be
synthesized by trapping the reactive, low coordinated
fragment [(dcpe)Pt] with Group 13-diyl species like
ECp* (E=Al, Ga, In) or EC(SiMe3)3 (E=Ga). The
current data suggest that the concept depicted in
Scheme 1 is extendable to a variety of compounds of
the type [(dcpe)Pt(ER)2] which are now available with
different homologues of the Group 13 metal E and
organic groups R influencing the properties of the PtꢀE
bond. Besides, the spectator ligands at the transition
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