Synthesis, molecular structure and theoretical considerations on Mo(PhCCCOOEt)3(PMe3)

Article abstract of DOI:10.1016/S0022-328X(98)00770-0

The reaction of trans-Mo(C₂H₄)₂(PMe₃)₄ with PhCCCOOEt affords (PhCCCOOEt)₃(PMe₃) (1). The molecular structure of 1 has been determined by means of X-ray analysis. NMR studies have shown that 1 undergoes a dynamic process in solution due to the alkyne rotation. The related energy barrier is experimentally determined to be 15.1 kcal mol^-1. The well established EHMO theoretical arguments, which uniquely explain the chemical bonding in these complexes, are extended to interpret the nature of the barrier and its dependence on the π-donor or π-acceptor capability of the axial ligand. The conceptual continuity between the chemical bonding of d⁶ metal species coordinated by at least two alkyne molecules (e.g. M(alkyne)₂L₄, M(alkyne)₂L₂ and M(alkyne)₃L) is highlighted.

Full text of DOI:10.1016/S0022-328X(98)00770-0

Journal of Organometallic Chemistry 569 (1998) 21–27
Synthesis, molecular structure and theoretical considerations on
Mo(PhCꢀCCOOEt)3(PMe3)
Carlo Mealli a,*, Dante Masi a, Agust´ın Galindo b, Antonio Pastor b
a Istituto Studio Stereochimica ed En., dei Composti di Coordinazione, (ISSECC-CNR), Via J. Nardi 39, Firenze, Italy
b Departamento de Qu´ımica Inorga´nica, Uni6ersidad de Se6illa, Aptdo 553, 41071 Se6illa, Spain
Received 12 February 1998
Abstract
The reaction of trans-Mo(C2H4)2(PMe3)4 with PhCꢀCCOOEt affords (PhCꢀCCOOEt)3(PMe3) (1). The molecular structure of
1 has been determined by means of X-ray analysis. NMR studies have shown that 1 undergoes a dynamic process in solution due
to the alkyne rotation. The related energy barrier is experimentally determined to be 15.1 kcal mol−1. The well established EHMO
theoretical arguments, which uniquely explain the chemical bonding in these complexes, are extended to interpret the nature of
the barrier and its dependence on the y-donor or y-acceptor capability of the axial ligand. The conceptual continuity between the
chemical bonding of d6 metal species coordinated by at least two alkyne molecules (e.g. M(alkyne)2L4, M(alkyne)2L2 and
M(alkyne)3L) is highlighted. © 1998 Elsevier Science S.A. All rights reserved.
Keywords: Alkyne complex; Molybdenum complex; EHMO calculations
1. Introduction
and molybdenum [7], are scanty. We have now synthe-
sized the molybdenum complex Mo(PhCꢀCCOOEt)3
(PMe3) (1) and, in this paper, we present its molecular
structure and behavior in solution. EHMO arguments,
which uniquely describe the chemical bonding in this
type of molecule and which have been developed by
various authors [8], are adapted to interpret some as-
pects of the fluxionality process. Moreover we address
qualitatively the conceptual continuity between d6
metal complexes which contain at least two alkyne
molecules coordinated.
In the course of our studies of molybdenum and
tungsten bis(ethylene) complexes [1], we have investi-
gated systematically the reactivity of the M(C2H4)2
moiety toward a variety of reagents. For example, the
interaction with carbon dioxide is shown to produce a
unique example of ethylene carboxylation at the transi-
tion metal center [2]. As an extension of our research,
the reactivity of the trans-Mo(C2H4)2(PMe3)4 complex
toward the alkyne PhCꢀCCOOEt has been tested by us.
Although W(alkyne)3(CO) compounds were first re-
ported in 1963–64 [3], there is a continuous interest in
this chemistry, for example, because some of these
compounds promote alkyne–alkyne coupling reactions
as it has been recently described [4]. The tungsten
derivatives have attracted the greatest attention [5]
while those of the other Group 6 metals, chromium [6]
2. Experimental section
Microanalyses were by the Microanalytical Service of
the University of Sevilla. IR spectra were recorded on
1
Perkin-Elmer Model 883 spectrophotometer. H-, 13C-
and 31P-NMR spectra were run on Bruker AMX-300
and Bruker AMX-500 spectrometers. 31P shifts were
measured with respect to external 85% H3PO4. 13C-
* Corresponding author. Tel.: +39 55 2346653; fax: +39 55
2478366.
0022-328X/98/$ - see front matter © 1998 Elsevier Science S.A. All rights reserved.
PII S0022-328X(98)00770-0

22
C. Mealli et al. / Journal of Organometallic Chemistry 569 (1998) 21–27
NMR spectra were referenced using the 13C resonance
of the solvent as an internal standard but are reported
with respect to SiMe4. All preparations and other oper-
ations were carried out under oxygen-free nitrogen
following conventional Schlenk techniques. Solvents
were dried and degassed before use. The complex trans-
Mo(C2H4)2(PMe3)4 was prepared as previously de-
scribed ([1]a).
Table 1
Crystal data and structure refinement for 1
Empirical formula
Molecular weight
Crystal system
Space group
C36H39MoO6P
694.62
Trigonal
R3 (No 146)
10.3080(10)
10.308(5)
10.308(2)
107.53(2)
107.530(10)
107.53(2)
898.6(5)
1
1.283
0.450
293(2)
0.71069
360
˚
a (A)
˚
b (A)
˚
c (A)
h (°)
i (°)
k (°)
2.1. Synthesis of Mo(PhCꢀCCOOEt)3(PMe3) (1)
3
˚
V (A )
A solution of trans-Mo(C2H4)2(PMe3)4 (0.22 g, 0.5
mmol) in 30 ml of Et2O was reacted with
PhCꢀCCOOEt (2.1 ml of a solution 0.9 M in THF).
The mixture was stirred for 1 h at room temperature
(r.t.) and the solvent removed under vacuum. The
residue was extracted with Et2O (10 ml), centrifuged
and concentrated. Storage at −30°C gave complex 1 as
pale orange crystals. Subsequent recrystallization from
Et2O affords white crystals of 1 (40% yield) which are
suitable for the X-ray analysis. IR (Nujol): 1680 cm−1
Z
Dcalc. (g cm−3
)
v, (mm−1
)
Temperature (K)
Wavelength (A)
F(000)
Theta range for data collection (°) 3.36 to 24.97
Index ranges
˚
−125h511, −125k511,
05l512
3348
2030 [Rint=0.0286]
Full matrix least-squares on
F2
2030/1/137
1.026
Reflections collected
Independent reflections
Refinement method
1
(very br, CO). Major isomer: H-NMR (300 MHz, 250
3
K, CD3COCD3): l 1.25 (t, 9H, 3 CH2CH3; JHH=7.1
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I\2s(I)]
R indices (all data)
Absolute structure parameter
Largest difference peak and hole
(e A-3)
Hz), 2.07 (d, 9H, P(CH3)3, JP–H=9.2 Hz), 4.29 (q, 6H,
3
3 CH2CH3; JHH=7.1 Hz), 7.33 (m, 6 H, Ph), 7.39 (m,
R1=0.0182, wR2=0.0475
R1=0.0183, wR2=0.0476
−0.01(3)
3
3 H, Ph), 7.54 (d, 6H, Ph; JHH=6.9 Hz). 31P{1H}-
NMR (202 MHz, 250 K, CD3COCD3): l 9.41 (s).
0.167 and −0.108
13C{1H}-NMR (75.5 MHz, 298 K, C6D6): l 14.0 (s,
1
CH2CH3), 16.8 (d, P(CH3)3, JCP=24.7 Hz), 60.4 (s,
CH2CH3), 127.9 (s, Ph), 128.6 (s, Ph); 129.8 (s, Ph);
2
139.9 (s, Ph); 166.2 (d, ꢀCCOO, JCP=17.9 Hz), 174.5
thermal parameters to all atoms. In the last L.S. cycles,
anisotropic thermal parameters were used for all non-
hydrogen atoms. Fractional atomic coordinates for 1
are given in Table 2. Hydrogen atoms were included
2
(d, PhCꢀ, JCP=9.0 Hz), 196.2 (s, COO). Minor iso-
mer: 31P{1H}-NMR (202 MHz, 250 K, CD3COCD3): l
8.60 (s). Anal. Found: C, 62.1; H, 5.4. Calc. for
C36H39PO6Mo: C, 62.2; H, 5.6%.
Table 2
Fractional atomic coordinates (×104) and equivalent isotropic dis-
placement parameters (A ×10 ) of Mo(PhCꢀCCOOEt)3(PMe3) (1)
2.2. X-ray structure determination of
Mo(PhCꢀCCOOEt)3(PMe3) (1)
2
3
˚
x
y
z
U(eq)a
A summary of the crystal data, including the results
of the structural refinement are given in Table 1. A
white crystal was mounted on a Enraf-Nonius CAD4
diffractometer. The cell dimensions were refined by
least-squares fitting of 25 reflections in the 2q range of
10–12°. The intensity data were corrected for Lorentz
and polarization effects. Atomic scattering factors are
those reported by Cromer and Waber [9]. The structure
was solved by combined Patterson and Fourier meth-
ods. An empirical absorption correction was applied via
„-scan measurements and the transmission factors are
in the range 93.73–99.90. All the computational work
was carried out on a Digital DEC 2000 AXP worksta-
tion using the program SHELX96 [10]. The program
ZORTEP was also used for molecular drawings [11].
Refinement was carried out by means of full matrix
least-square calculations, initially attributing isotropic
Mo(1)
P(1)
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
O(1)
8829
8829
8829
6609(1)
31(1)
44(1)
39(1)
40(1)
48(1)
83(1)
109(2)
69(1)
75(1)
63(1)
44(1)
63(1)
89(1)
93(1)
85(1)
64(1)
6609(1)
10539(2)
9601(2)
9455(3)
8193(4)
6937(5)
5335(3)
10143(3)
8445(2)
11917(2)
12250(3)
13557(4)
14532(4)
14218(3)
12913(3)
6609(1)
8600(2)
7277(2)
5732(2)
3314(3)
2553(4)
7051(4)
5273(2)
4888(2)
9204(2)
10412(3)
10922(4)
10266(5)
9092(4)
8536(4)
10276(2)
9156(2)
8666(2)
8532(5)
8804(6)
5349(3)
8037(3)
8970(2)
11648(2)
12939(3)
14229(4)
14222(4)
12952(4)
11673(3)
O(2)
C(1,1)
C(2,1)
C(3,1)
C(4,1)
C(5,1)
C(6,1)
a
U(eq) is defined as one third of the trace of the orthogonalized Uij
tensor.

C. Mealli et al. / Journal of Organometallic Chemistry 569 (1998) 21–27
23
with fixed isotropic contributions at the calculated posi-
tions. No unusual trend in DF versus Fo or (sin q)/u
was observed. A final difference synthesis showed no
significant electron density residues. Listing of atomic
coordinates of the hydrogen atoms, anisotropic thermal
parameters of all and structure factors are provided as
supplementary material.
a small broad hump at 1.88 ppm close to the slightly
broad doublet due to PMe3 and, analogously, 31P{1H}-
NMR spectrum shows a small broad resonance at 7.6
ppm close to the singlet resonance for the PMe3 ligand.
Considering the results reported by Cooper and
coworkers ([5]a,b) where some W(alkyne)3(L) com-
plexes showed dynamic NMR behavior due to alkyne
rotation, we have performed a variable temperature
NMR study for 1.
2.3. Extended Hu¨ckel molecular orbital study
At 250 K, 31P{1H}-NMR spectrum (CD3COCD3)
shows two sharp singlets (9:1 ratio). After heating, both
signals get wider and at 323 K only one broad band
centered at 8.1 ppm was observed. This fact suggests
the minor presence of a second isomer of 1, probably
with a different orientation of the alkyne ligands. The
structural disposition of the alkyne ligands for the
major isomer was supposed to be that found in the
solid state (see ahead) and was schematically shown in
I.
All the MO calculations were of the extended Hu¨ckel
type [12] using
a weighted-modified Wolfsberg–
Helmholz formula [13]. The geometry of the starting
model Mo(HCꢀCH)3(PH3) is based on that of the
X-ray structure. The STO parameters for molybdenum
are taken from literature [14], while those of the main
group elements are the standard ones tabulated in the
program CACAO [15]. The input files are available
from the authors on request [16].
3. Results and discussion
The reaction of trans-Mo(C2H4)2(PMe3)4 complex
with three equivalents of PhCꢀCCOOEt affords crystals
of Mo(PhCꢀCCOOEt)3(PMe3) (1) after work up (Eq. 1)
in good yields. The use of a large excess of alkyne
produced intractable materials,
trans-Mo(C2H4)2(PMe3)4
+PhCꢀCCOOEt[Mo(PhCꢀCCOOEt)3(PMe3) (1)
In order to establish the structure of the minor
isomer the 1H-NMR spectrum was recorded at low
temperature. At 250 K, the minor isomer gives signals
for the PMe3 ligand at 1.85 ppm and for the Et groups
at 4.18 and 1.07 ppm. Unfortunately the present NMR
data are not clear enough to detect unequivocally the
three possible structures II, III and IV as previously
done by Cooper and coworkers for an analogous case
([5]a,b).
probably due to the presence of alkyne polymerization
products.
The presence of one PMe3 ligand in 1 is clearly
evident from NMR spectra. The three alkyne ligands
give only one set of signals in the 1H-and 13C{1H}-
NMR spectra. For example, in the 13C{1H}-NMR
spectrum two doublets at 174.5 and 166.2 ppm were
assigned to PhCꢀ (2JCP=9 Hz) and ꢀCCOO (2JCP
=
Coalescence temperatures were determined from
1
17.9 Hz) carbon atoms, respectively. A close inspection
of the NMR data at r.t. reveals, in addition to the
resonances due to 1, small signals in an approximate
31P{1H}- and H-NMR spectra and were used to calcu-
late a DG" value of 15.1 kcal mol−1. This value is
superior to that reported for W(Me3SiCꢀCSiMe3)3(CO)
(B8 kcal mol−1) ([8]a) and only slightly higher than
1
ratio 1:9 with respect to 1. H-NMR spectrum displays

24
C. Mealli et al. / Journal of Organometallic Chemistry 569 (1998) 21–27
rhenium compounds with general formula Re(-
alkyne)3(X) (14–15 kcal mol−1) [17].
The molecular structure of 1 (Fig. 1 and Table 3 for
selected bond lengths and angles) is typical of
M(alkyne)3L complexes (M=d6 metal, Cr(0) [6], Mo(0)
([7]a,b), W(0) ([5]a,b,d,e) or Re(I) [17] and L=two
electron | donor). The coordination geometry can be
described as either a distorted tetrahedron or a tapered
trigonal prism. The complex has a crystallographic C3
symmetry axis along the Mo–P vector. The higher C3v
symmetry is not attainable mainly because steric factors
prevent each COOEt substituent from being coplanar
with the CꢀC linkage. The latter is almost symmetri-
cally coordinated to the molybdenum atom with Mo–C
˚
distances of 2.066(2) and 2.059(2) A, respectively. The
˚
CꢀC bonds are elongated to 1.309(3) A with respect to
˚
the 1.18 A average value for uncoordinated alkynes
[18]. Also the carbon atoms linked to the CꢀC bond are
not linearly arranged, the CꢀC–C angles being equal to
136.6(2) and 137.6(2)°, for the phenyl and carboxy
substituent, respectively. The angles P(1)–Mo(1)–C(1)
[122.13(6)] and P(1)–Mo(1)–C(2) [85.25(6)°] are in the
ranges found for the structurally characterized
M(alkyne)3(L) complexes (117–127 and 80–90°, respec-
tively) [19].
The interpretation of the chemical bonding for com-
plexes of the type M(alkyne)3(L) is one of the most
successful applications of MO theory in organometallic
chemistry. It is worth mentioning that Tate and
coworkers, after obtaining the first specimen
W(alkyne)3(CO), were able to predict its structure by
using qualitative MO arguments [3] and their hypothe-
sis was later confirmed by X-ray crystallography ([5]e).
Further studies are due to King ([8]b) and more re-
cently, Wink and coworkers ([8]a) have presented a
comprehensive analysis of the bonding with implica-
tions for the fluxional behavior of coordinated alkynes.
The basic MO arguments may be summarized as fol-
lows. In structures of type I, the coordinated d6 metal
atom receives one electron pair from the axial |-donor
Fig. 1. Molecular structure of Mo(PhCꢀCCOOEt)3(PMe3) (1).
those of W(PhCꢀCPh)3(SnMe3)− (12.7 kcal mol−1
)
([8]a), Cr(PhCꢀCPh)3(CO) (13.1 kcal mol−1) ([8]a) and
W(PhCꢀCPh)3(SnPh3)− (13.1 kcal mol−1) ([5]b). Simi-
lar values have been determined for W(PhCꢀCPh)3(CO)
([5]b) (17.2 kcal mol−1) and W(PhCꢀCPh)3(PMe2Ph)
(16.4 kcal mol−1) ([8]a) compounds and for related d6
Table 3
˚
Selected bond distances (A) and angles (°) of Mo(PhCꢀCCOOEt)3
(PMe3) (1)
˚
Bond distances (A)
Mo(1)–C(1)
Mo(1)–C(2)
Mo(1)–P(1)
C(1)–C(2)
C(2)–C(3)
C(3)–O(1)
C(3)–O(2)
C(4)–O(2)
C(4)–C(5)
2.066(2)
2.059(2)
2.500(2)
1.309(3)
1.463(3)
1.201(3)
1.324(3)
1.462(3)
1.453(5)
and three more from the y MOs of the alkynes.
ꢀꢀ
Another four electrons are needed for the metal to
reach saturation which can be in principle taken from
the filled yÞ MOs of the alkynes. The latter are subdi-
vided by the C3v symmetry in a2 and e combinations.
Since the metal atom has no orbitals of a2 symmetry, it
accepts only the electrons of e symmetry, i.e. just those
needed for saturation.
Our present theoretical interest is to understand bet-
ter the origin of the alkyne rotation barrier in our
complex I. In fact, we wish not only to verify the
consistency of the calculated value with that derived
from NMR experiments but also to evaluate the effects
associated with both the bulkyness of the alkyl sub-
stituents and the electronic nature of the axial ligand.
Bond angles (°)
C(2)–Mo(1)–C(1)
C(2)–Mo(1)–P(1)
C(1)–Mo(1)–P(1)
C(2)–C(1)–C(1,1)
C(1)–C(2)–C(3)
C(2)–C(1)–Mo(1)
C(1)–C(2)–Mo(1)
C(1,1)–C(1)–Mo(1)
C(3)–C(2)–Mo(1)
O(1)–C(3)–O(2)
O(1)–C(3)–C(2)
O(2)–C(3)–C(2)
C(5)–C(4)–O(2)
C(3)–O(2)–C(4)
37.00(9)
85.25(6)
122.13(6)
136.6(2)
137.6(2)
71.20(12)
71.79(12)
152.2(2)
150.6(2)
124.0(2)
124.2(2)
111.7(2)
108.2(3)
117.0(2)

C. Mealli et al. / Journal of Organometallic Chemistry 569 (1998) 21–27
25
The models Mo(RCꢀCR%)3L, (R=H, Ph and R%=H,
COOMe, L=PH3, Cl−, CO) have been used in the
present calculations. As schematized in V, only one
alkyne molecule is allowed a ƒ rotation while the other
two alkyne ligands remain fixed in the planes contain-
ing the axis M–Lax. While the observed CꢀC–X angles
(k) are ca. 137° in 1, we have initially fixed them to the
same 150° value used in the calculations of Wink and
Creagan ([8]a). However, it may be anticipated that the
parameter k is to be critical even for XꢁH, i.e. when the
steric problem is minimized.
Fig. 2. Evolution of the d metal centered MOs upon the 180° alkyne
rotation in the models [Mo(HCꢀCH)3Cl]− and Mo(HCꢀCH)3(CO).
lighted before ([8]a). In VI, we compared the frontier
MOs (centered on the metal d orbitals) of the two
alternative C3v structures with one axial Cl and CO
ligand, respectively. For clarity, only the orbital contri-
bution of the alkyne ligand in the plane of the drawing
is shown.
The calculated barrier of ca. 14 kcal mol−1 is rela-
tively close to the experimental value of 15.1 kcal
mol−1. As already pointed out by Wink and Creagan
([8]a), the HOMO is closely correlated with the barrier
as it is most destabilized at ƒ=90° (structure V).
Interestingly, the barrier decreases to ca. 11 kcal mol−1
by closing k to the experimental 137° value. Such an
angular variation affects the y alkyne frontier orbitals
ꢀꢀ
(their energetics and hybridization) as it has been previ-
ously discussed by Hoffmann et al. [20]. In particular,
the y* combination is lowered and the backdonation
ꢀꢀ
from the appropriate metal dp orbital improves. For
smaller k angles, the HOMO (largely centered on the
metal) is relatively more stable over the whole range of
ƒ rotations. Then, we have tried to evaluate the barrier
for a model closer to 1, namely with Ph and COOMe
substituents. In particular, the carboxylate and the CꢀC
groupings are fixed in orthogonal planes. Only for the
CꢀC–C angles of 137° does the barrier match that
calculated for three simpler HCꢀCH ligands but, in this
case, even a small angular increase raises the total
energy dramatically (by several eVs). In other words,
the short contacts between the ligands determine a very
high steric hindrance which affects the barrier itself.
As the next step we have substituted the axial PH3
group for either the CO or Cl− ligands in the model
Mo(HCꢀCH)3(L). In this manner, we expected to esti-
mate the changes in the barrier when either the y-donor
(halides) or the y-acceptor (carbon monoxide) capabili-
ties of the axial ligand are modified. The trends follow
in the direction that emerges from other experimental
works. Namely, a CO ligand increases the barrier (17
kcal mol−1) while a halide decreases it (9 kcal mol−1).
It is possible to point out the electronic underpin-
nings of the latter result in a manner not totally high-
In both cases, the lower and filled a1 and 1e levels (z2,
xz and yz) are stabilized by the combinations of the
empty y* levels of the alkynes. Also shown is the
ꢀꢀ
higher set (2e) of the empty xy and x2-y2 orbitals which
are antibonding with the yÞ orbitals of the alkyne.
Recall that two out of three combinations of the second
alkyne y bonding MOs act as donors. Importantly, the
order a1B1e is inverted in going from the y donor to y
acceptor axial ligand. Namely, the related DE passes
from −0.3 to +0.4 eV when the axial ligand stabilizes
the degenerate xz and yz metal orbitals. As shown by
the Walsh diagrams of Fig. 2, the rotation of one
alkyne ligand (the one featuring orbital contributions in

26
C. Mealli et al. / Journal of Organometallic Chemistry 569 (1998) 21–27
VI) causes one member of the degenerate 1e level (xz)
to rise in energy because the stabilizing interaction with
In conclusion, upto three types of d6 complexes
containing at least two alkynes, namely M(alkyne)2L4
[23], M(alkyne)2L2 ([8]a), [21] and M(alkyne)3L, can be
correlated. In the octahedral trans-bis(alkyne) com-
plexes VIII, both the second y-bonding alkyne pair
remain extraneous to metal bonding.
the alkyne y* orbital vanishes. At the same time, the
ꢀꢀ
higher xy orbital of 2e, stabilizes because of the loss of
the antibonding interaction with alkyne yÞ orbital.
At ƒ=90°, the inversion of the roles between xz and
xy must have occurred (with respect to the alkyne, xz is
now acceptor and xy is back-donor). Such an inversion
implies an avoided crossing mechanism that is evident
in both diagrams of Fig. 2. Even though there is no
symmetry along the pathways (pointgroup C1), the
rising of the xz orbital starts from deeper energy in the
CO ligand case due to the stabilization of y*. In other
words, the inversion of the roles between xz and xy
orbitals, at the transition state, has a higher energy
cost. Although presented in a very qualitative way, the
substantial difference between the two cases is clear.
Wink and Creagan ([8]a) pointed out a continuity in
the interpretation of the bonding by passing from
M(alkyne)2L2to M(alkyne)3L complexes (M is a d6
metal in both cases). In the former species (e.g. Cr(-
PhCꢀCPh)2(dmpe) [21]) the quasi-orthogonal orienta-
tions of the alkynes (a slight bending away from the
equatorial L ligands is also observed) allows the great-
est involvement of both the two second pairs [22] as
donors toward two empty d-p metal hybrids (see VII).
The two trans alkyne ligands are staggered to maxi-
mize metal backdonation from different metal orbitals
(xz and yz) and also the four electron repulsions be-
tween second pairs and the filled t2g metal orbitals are
minimized. The point has been previously discussed by
other authors [24]. The loss of two L ligands implies a
slight reorientation of the alkynes but not a major
deviation from the basic staggered disposition (for ex-
ample in the complex Cr(PhCꢀCPh)2(dmpe) [21] the
dihedral angle between the two planes CrCꢀC increases
only from 90 to 105°). Nonetheless this and other
minor geometric rearrangements are sufficient for a full
involvement of the second pair in bonding, as detailled
by Wink and Creagan ([8]a). Finally, the ultimate sub-
stitution of one L ligand for a third alkyne is not
limited to a simple replacement of the two electron
donor functionality, but induces an overall geometrical
rearrangement to the unique and highly symmetric
structure (C3v).
4. Addendum
Conversely, it is easly seen that dp-y four electron
repulsions would be triggered for different orientations
of the alkynes. If the species M(alkyne)3L is thought of
as derived from M(alkyne)2L2 upon the replacement of
one |-donor ligand with an alkyne, hardly the second y
electron pair of the newly introduced ligand is used for
metal bonding. If the orthogonal disposition of the first
two alkynes is maintained as in the ideal precursor
M(alkyne)2L2, the structure V should be adopted by the
complexes M(alkyne)3L. By contrast, the conformation
V corresponds to a destabilized transition state and, we
know that the uninvolvement of one of the second pairs
in the C3v geometry is to be ascribed to the unique
unmatched symmetry of the combination a2. For this
reason, the chemical bonding in the molecule is highly
delocalized and can be uniquely rationalized in terms of
MO theory.
When this manuscript was ready to be submitted, a
mechanistic study of Yeh and coworkers [25] appeared
on the substitution reactions of W(PhCꢀCPh)3(CO). In
this work
a
semi-empirical MO calculation of
W(PhCꢀCPh)2(CO)(PMe3)2, with a bipyramid trigonal
geometry, is also briefly discussed. Such a compound
would fall inside the conceptual limits of the chemical
bonding in d6 metal complexes coordinated by at least
two alkyne molecules as discussed by us.
Acknowledgements
Financial support from the DIGICYT and Junta de
Andaluc´ıa and a CSIC-CNR joint project are gratefully

C. Mealli et al. / Journal of Organometallic Chemistry 569 (1998) 21–27
27
Kgonovich, M.I. Rybinskaya, J. Organomet. Chem. 102 (1975)
185. (d) H. Kolsorn, H. Meier, E. Muller, Tetrahedron Lett. 16
(1972) 1589. (e) R.B. King, J. Organomet. Chem. 8 (1967) 139.
(f) W. Strohmeier, D. Hobe, Z. Naturforsch. 19b (1964) 959.
[8] (a) D.J. Wink, B.T. Creagan, Organometallics 9 (1990) 328. (b)
R.B. King, Inorg. Chem. 7 (1968) 1044.
[9] D.T. Cromer, J.T. Waber, Acta Crystallogr. 18 (1965) 104.
[10] G.M. Sheldrick, SHELX93, Structure Solving Program, Univer-
sity of Go¨ttingen, Germany, 1993.
[11] L. Zsolnai, H. Pritzkow, ZORTEP, University of Heidelberg,
Germany, 1994.
[12] (a) R. Hoffmann, J. Chem. Phys. 39 (1963) 1397. (b) R. Hoff-
mann, W.N. Lipscomb, J. Chem. Phys. 37 (1962) 3489. (c) R.
Hoffmann, W.N. Lipscomb, J. Chem. Phys. 36 (1962) 2179,
2872.
acknowledged. We thank the Spanish Ministry of Edu-
cation for a postdoctoral grant (A. Pastor).
References
[1] See for example: (a) E. Carmona, J.M. Mar´ın, M.L. Poveda, J.L.
Atwood, R.D. Rogers, J. Am. Chem. Soc. 105 (1983) 3014. (b)
E. Carmona, A. Galindo, M.L. Poveda, R.D. Rogers, Inorg.
Chem. 24 (1985) 4033. (c) E. Carmona, A. Galindo, J.M. Mar´ın,
E. Gutie´rrez, A. Monge, C. Ruiz, Polyhedron 7 (1988) 1831.
[2] (a) A. Alvarez, E. Carmona, A. Galindo, et al., Organometallics
8 (1989) 2430. (b) A. Galindo, A. Pastor, P.J. Pe´rez, E. Car-
mona. Organometallics 12 (1993) 4443. (c) C. Collazo, M.M.
Conejo, A. Pastor, A. Galindo. Inorg. Chim. Acta (1997) (in
press).
[13] J.H. Ammeter, H.-B. Bu¨rgi, J.C. Thibeault, R. Hoffmann, J.
Am. Chem. Soc. 100 (1978) 3686.
[14] V.I. Baranovskii, A.B. Nikolskii, Teor. Eksp. Khim 3 (1967) 527.
[15] C. Mealli, D.M. Proserpio, J. Chem. Educ. 67 (1990) 399.
[3] (a) D.P. Tate, J.M. Augl, W.M. Ritchey, B.L. Ross, J.G. Gras-
selli, J. Am. Chem. Soc. 86 (1964) 3261. (b) D.P. Tate, J.M.
Augl, J. Am. Chem. Soc. 85 (1963) 2174.
[16] e-mail
addresses:
mealli@cacao.issecc.fi.cnr.it
and
galindo@cica.es
[4] (a) W.-Y. Yeh, C.-L. Ho, M.Y. Chiang, I.-T. Chen,
Organometallics 16 (1997) 2698. (b) W.-Y. Yeh, S.-M. Peng,
G.-H. Lee, J. Chem. Soc. Chem. Commun. (1993) 1056. (c)
W.-Y. Yeh, L.-K. Liu, J. Am. Chem. Soc. 114 (1992) 2267. (d)
W.-Y. Yeh, S.-M. Peng, L.-K. Liu, Inorg. Chem. 32 (1993) 2965.
[5] (a) D.J. Wink, N.J. Cooper, Organometallics 10 (1991) 494. (b)
J.M. Maher, J.R. Fox, B.M. Foxman, N.J. Cooper, J. Am.
Chem. Soc. 106 (1984) 2347. (c) W.-Y. Yeh, C.-S. Ting, S.-M.
Peng, G.-H. Lee, Organometallics 14 (1995) 1417. (d) K.W.
Chiu, D. Lyons, G. Wilkinson, M. Thornton-Pett, M.B. Hurst-
house, Polyhedron 2 (1983) 803. (e) R.M. Laine, R.E. Moriarty,
R. Bau, J. Am. Chem. Soc. 94 (1972) 1402. (f) W.-Y. Yeh, C.-S.
Ting, C.-F. Chih, J. Organomet. Chem. 427 (1991) 257. (g) K.J.
Odell, E.M. Hyde, B.L. Shaw, I. Shepherd, J. Organomet. Chem.
168 (1979) 103. (h) J.A. Connor, G.A. Hudson, J. Organomet.
Chem. 185 (1980) 385. (i) J. Okuda, K.H. Zimmermann, J.
Organomet. Chem. 384 (1990) C21.
[17] (a) S.K. Tahmassebi, W.S. McNeil, J.M. Mayer, Organometal-
lics 16 (1997) 5342. (b) R.R. Conry, J.M. Mayer, Organometal-
lics 12 (1993) 3179. (c) S.K. Tahmassebi, R.R. Conry, J.M.
Mayer, J. Am. Chem. Soc. 115 (1993) 7553. (d) A.B. Mannion,
T.K.G. Erikson, E. Spaltenstein, J.M. Mayer, Organometallics 8
(1989) 1871.
[18] F.H. Allen, O. Kennard, D.G. Watson, L. Brammer, A.G.
Orpen, R. Taylor, J. Chem. Soc. Perkin Trans. 2 (1987) 1.
[19] Systematic search in the Cambridge Structural Database.
[20] D.M. Hofmman, R. Hofmmann, C.R. Fisel, J. Am. Chem. Soc.
104 (1982) 3858.
[21] J.E. Salt, G.S. Girolami, G. Wilkinson, M. Montevalli, M.
Thornton-Pett, M.B. Hursthouse, J. Chem. Soc. Dalton Trans.
(1985) 685.
[22] J.L. Templeton, Adv. Organomet. Chem. 27 (1989) 1 and refer-
ences therein.
[23] (a) T.-Y. Hsiao, P.-L. Kuo, C.-H. Lai, C.-H. Cheng, C.-Y.
Cheng, S.-L. Wang, Organometallics 12 (1993) 1094. (b) C.-H.
Lai, C.-H. Cheng, C.-Y. Cheng, S.-L. Wang, J. Organomet.
Chem. 458 (1993) 147.
[24] C. Bachman, J. Demynck, A. Veillard, J. Am. Chem. Soc. 100
(1978) 2366.
[6] (a) P.W. Jolly, U. Zakrzewski, Polyhedron 10 (1991) 1427. (b)
N.E. Kolobova, O.S. Zhvanko, V.G. Andrianov, H.A. Kara-
petyan, Yu. T. Struchkov, Koord. Khim. 6 (1980) 1407.
[7] (a) T. Szymanska-Buzar, T. Glowiak, J. Organomet. Chem. 467
(1994) 223. (b) R. Bowerbank, M. Green, H.P. Kirsch, A.
Mortreux, L.E. Smart, F.G. Stone, J. Chem. Soc. Chem. Com-
mun. (1977) 245. (c) A.N. Nesmayanov, V.V. Krivyich, V.S.
[25] W.-Y. Yeh, S.-M. Chien, J. Organomet. Chem. 548 (1997) 191.
.