Conformational isomerism in triosmium clusters: Structures of yellow and red Os3(CO)11[P(p-C6H4F)3] and Os3(CO)11(PBut3)

Article abstract of DOI:10.1021/om980365g

The clusters Os₃(CO)₁₁(PR₃) (R = p-C₆H₄F, 1; Bu^t, 2) have been prepared by the addition of PR₃ to Os₃(CO)₁₁(CH₃CN), and their structures determined. There are yellow and red forms of 1 (1y and 1r, respectively). The structure of 1y has the typical structure for Os₃(CO)₁₁(PR₃) (PR₃ = phosphine or phosphite) complexes with the axial carbonyls perpendicular to the Os₃ triangle, whereas in 1r the individual Os(CO)₃ (L) (L = CO, PR₃) units are twisted with respect to each other such that neighboring carbonyls on adjacent Os atoms are in a staggered configuration. The reason for the twisting in 1r is believed to be due to intermolecular edge-to-face interactions between fluorophenyl rings in the solid. In solution 1 is yellow with spectroscopic properties typical of Os₃(CO)₁₁(PR₃) compounds. Unlike 1y and its analogues, 2 is deep red both in the solid and in solution. The structure of 2 reveals that like 1r it also has the twisted structure. It is believed that the large size and strong donor properties of the PBu^t₃ ligand destabilize the edge-bridging molecular orbitals in 2 such that the twisted form, which is sterically favored, is adopted. Consistent with this view is that the Os-Os bond cis to the PBu^t₃ in 2 is long, at 2.9416(3) ?; in 1y and 1r the corresponding lengths are 2.872(1)and 2.888(1) (two independent molecules in the unit cell of 1y) and 2.9117(4) ?, respectively.

Full text of DOI:10.1021/om980365g

Organometallics 1998, 17, 5267-5274
5267
Con for m a tion a l Isom er ism in Tr iosm iu m Clu ster s:
Str u ctu r es of Yellow a n d Red Os3(CO)11[P (p-C6H4F )3] a n d
Os3(CO)11(P Bu t )
3
Valerie. M. Hansen,† Andrew K. Ma,† Kumar Biradha,‡
Roland K. Pomeroy,*,† and Michael J . Zaworotko‡
Department of Chemistry, Simon Fraser University, Burnaby, British Columbia,
Canada V5A 1S6, and Department of Chemistry, Saint Mary’s University, Halifax,
Nova Scotia, Canada B3H 3C3
Received May 11, 1998
The clusters Os3(CO)11(PR3) (R ) p-C6H4F, 1; But, 2) have been prepared by the addition
of PR3 to Os3(CO)11(CH3CN), and their structures determined. There are yellow and red
forms of 1 (1y and 1r , respectively). The structure of 1y has the typical structure for Os3-
(CO)11(PR3) (PR3 ) phosphine or phosphite) complexes with the axial carbonyls perpendicular
to the Os3 triangle, whereas in 1r the individual Os(CO)3(L) (L ) CO, PR3) units are twisted
with respect to each other such that neighboring carbonyls on adjacent Os atoms are in a
staggered configuration. The reason for the twisting in 1r is believed to be due to
intermolecular edge-to-face interactions between fluorophenyl rings in the solid. In solution
1 is yellow with spectroscopic properties typical of Os3(CO)11(PR3) compounds. Unlike 1y
and its analogues, 2 is deep red both in the solid and in solution. The structure of 2 reveals
that like 1r it also has the twisted structure. It is believed that the large size and strong
donor properties of the PBut ligand destabilize the edge-bridging molecular orbitals in 2
3
such that the twisted form, which is sterically favored, is adopted. Consistent with this
view is that the Os-Os bond cis to the PBut in 2 is long, at 2.9416(3) Å; in 1y and 1r the
3
corresponding lengths are 2.872(1)and 2.888(1) (two independent molecules in the unit cell
of 1y) and 2.9117(4) Å, respectively.
Ch a r t 1
In tr od u ction
The structures of M3(CO)12 (M ) Ru, Os) consist of a
triangular metal skeleton with six equatorial and six
axial carbonyl ligands.1,2 The axial carbonyls on neigh-
boring metal atoms are eclipsed to one another, giving
the molecules D3h symmetry (Chart 1). For reasons
discussed below we designate M3(CO)11(L) (L ) two-
electron donor ligand) complexes based on the D3h form
of the parent carbonyl as the “E” conformer. The
eclipsed arrangement is not the most favored conforma-
tion from steric considerations. Lauher has calculated
that the D3 conformer (Chart 1) of Ru3(CO)12 with
adjacent axial carbonyls in a staggered orientation
should be some 3.0 kcal mol-1 more stable than the D3h
conformer in the absence of electronic factors.3 We label
M3(CO)11(L) clusters based on the D3 version of M3-
(CO)12 as the “S” form. As Lauher pointed out, an
M(CO)4 unit is isolobal with CH2 that has two frontier
orbitals (a1 and b1), and three such groups should bind
with strict D3h symmetry. Twisting of the M(CO)4 units
with respect to each other to give the sterically preferred
D3 form would destroy this favorable overlap.3 Molec-
ular orbital calculations on D3h Fe3(CO)12 and Ru3(CO)12
support this view.4,5
Derivatives of the type M3(CO)11(PR3) (i.e., M ) Ru,
Os, not Fe) usually have close to E structures (i.e., based
on the D3h conformation).6-11 The distortion toward the
(4) Schilling, B. E. R.; Hoffmann, R. J . Am. Chem. Soc. 1979, 101,
3456.
(5) Delley, B.; Manning, M. C.; Ellis, D. E.; Berkowitz, J .; Trogler,
W. C. Inorg. Chem. 1982, 21, 2247.
(6) Deeming, A. J . In Comprehensive Organometallic Chemistry II;
Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon: Oxford,
U.K., 1995; Vol. 7, Chapter 12, p 683.
(7) Cullen, W. R.; Rettig, S. J .; Zhang, H. Can. J . Chem. 1996, 74,
2167. For structures of Ru3(CO)11(PR3) compounds published before
1995 see ref 6, Table 1 and references therein.
† Simon Fraser University. E-mail address: pomeroy@sfu.ca.
‡ Saint Mary’s University.
(8) Bruce, M. I.; Liddell, M. J .; Hughes, C. A.; Skelton, B. W.; White,
A. H. J . Organomet. Chem. 1988, 347, 157.
(1) Churchill, M. R.; Hollander, F. J .; Huthchinson, J . P. Inorg.
Chem. 1977, 16, 2655.
(2) Churchill, M. R.; DeBoer, B. G. Inorg. Chem. 1977, 16, 878.
(3) Lauher, J . W. J . Am. Chem. Soc. 1986, 108, 1521.
(9) Ehrenreich, W.; Herberhold, M.; Su¨ss-Fink, G.; Klein, H.-P.;
Thewalt, U. J . Organomet. Chem. 1983, 248, 171.
(10) Benfield, R. E.; J ohnson, B. F. G.; Raithby, P. R.; Sheldrick, G.
M. Acta Crystallogr. 1978, B34, 666.
10.1021/om980365g CCC: $15.00 © 1998 American Chemical Society
Publication on Web 10/29/1998

5268 Organometallics, Vol. 17, No. 24, 1998
Hansen et al.
(25 mL) was stirred at room temperature for 15 min. The
solvent was removed on the vacuum line and the residue
chromatographed on a silica gel column (1 × 20 cm) with
hexane/CH2Cl2 (9:1) as the eluant. The desired product was
isolated as the second (yellow) band and recrystallized from
S configuration can be measured by the C(ax)-M-M-
C(ax) dihedral angles. Bruce and co-workers found
these angles ranged from close to 0° to 16.8° (in Os3-
(CO)11[PPh(OMe)2]) in the five Os3(CO)11(PR3) struc-
tures determined at the time; in similar Ru compounds
this dihedral angle was as high as 24.4° (in Ru3(CO)11-
[P(OCH2)3CEt]).8 In the more highly substituted clus-
ters, M3(CO)12-x(PR3)x (x ) 2, 3), the twisting is even
more significant, with a maximum value for the C(ax)-
M-M-C(ax) dihedral angle of 44.8° in Ru3(CO)10-
(PPh3)2.12 Twisting to give a D3-type conformation is
also found in Os3(CO)6[P(OMe)3]6.13
In unpublished work, we have prepared over 20
clusters of the type Os3(CO)11(PR3) in order to ascertain
the effect of the R group on the barriers to carbonyl
nonrigidity that these molecules exhibit in solution.14
With the exception of one form of Os3(CO)11[P(p-
C6H4F)3] and the PBut3 analogue, the clusters are yellow
or orange. Crystal structures of the yellow and orange
Os3(CO)11(PR3) compounds by ourselves and others
show a geometry based on Os3(CO)12 with one of the
equatorial carbonyls replaced by the phosphorus ligand;
that is, the syn axial carbonyls are in an approximate
eclipsed (E) orientation.6,8-10,14 Here we describe the
structures of the two exceptions (which are red) that
reveal that they have the twisted S conformation, that
is, with the syn axial carbonyls staggered with respect
to each other. We also report the structure of two yellow
forms of Os3(CO)11[P(p-C6H4F)3] that have the common
E configuration.
hot hexane to give 1 in ∼50% yield. The PBut analogue, 2,
3
was prepared in a similar manner except that a 5-fold excess
of the phosphine was employed and the reaction required 18
h to go to completion (as indicated by IR spectroscopy). The
first product eluted in the chromatographic separation was
identified as Os3(CO)12; a second yellow band that has not been
identified was then eluted; the third orange-red band was the
desired product (the yield was ∼30%); a fourth yellow band
was also not identified. Os3(CO)11[P(p-C6H4F)3]: mp (red form)
167-168 °C; (yellow form) 168.5-169.5 °C (turning orange
∼70-110 °C); UV-vis (hexane) 29,100 (ꢀM × 10-4 ) 9.81),
24 500 (5.29) cm-1; IR (hexane) ν(CO) 2110 (w), 2057 (m-s),
2036.5 (m), 2022 (s), 2004 (vw), 1994 (w), 1981 (w), 1969 (vw),
1
1959.5 (vw) cm-1; H NMR (CD2Cl2) δ 7.50-7.39 (m), 7.21-
7.16 (m); 31P{1H} NMR (CD2Cl2) δ -3.6; 13C{1H} NMR (CD2-
Cl2/CH2Cl2 1:4; -53 °C) δ 193.2 (2C, J PC ) 8.4 Hz), 186.3 (2C),
184.1 (2C), 177.3 (1C), 175.6 (1C), 172.4 (1C), 172.1 (1C), 170.1
(1C) (pattern unchanged at -95 °C); MS (EI; m/z) 1196 [M]+.
Anal. Calcd for C29H12F3O11Os3P: C, 29.15; H, 1.01. Found:
C, 29.35; H, 1.07. Os3(CO)11{P[C(CH3)3]3}: mp 144.5-145.5
°C; UV-vis (hexane) 31 400 (ꢀM × 10-4 ) 7.91), 26 700 (6.49),
21 400 (5.66) cm-1; IR (hexane) ν(CO) 2105.5 (m), 2055 (s),
2020 (vs), 2009 (m), 2002 (w), 1991.5 (vw), 1981 (w), 1944 (w),
1930 (w) cm-1 1H NMR (CD2Cl2) δ 1.56 (d, J PH ) 12.5 Hz)
;
(unchanged in CD2Cl2/CHFCl2 at -120 °C); 31P{1H} NMR (CD2-
Cl2) δ 80.6 (s, unchanged in CD2Cl2/CHFCl2 at -120 °C);
13C{1H} NMR (CD2Cl2/CH2Cl2 1:4; -60 °C) δ 197.4 (2C, J PC
)
6.1 Hz), 185.5 (2C), 182.5, (2C), 179.3 (1C), 175.9 (1C), 173.2
(1C), 171.7 (1C), 169.5 (1C); MS (EI; m/z) 1082 [M]+. Anal.
Calcd for C23H27O11Os3P: C, 25.56; H, 2.52. Found: C, 25.69;
H, 2.43.
Exp er im en ta l Section
X-r a y An a lyses of 1 a n d 2. The preparation of the crystals
of the red and yellow forms of 1 is intriguing and hence is
presented here in detail. To pure compound 1 (20 mg) was
added (under nitrogen) hexane (10 mL) and the mixture
refluxed until 1 had dissolved. The Schlenk flask with the
solution was stored at -40 °C overnight. By this procedure,
1 was obtained as feather-like needles from which 1y1 was
obtained. When a similar hot hexane solution of 1 was set
aside at room temperature, besides the yellow feather-like
crystals of 1 it yielded a few well-formed red blocks of 1r . On
standing overnight the red crystals apparently cannibalized
the yellow form such that only red crystals remained. The
yellow solution from one of these crystallizations was discarded
to a Schlenk tube left open to the air in the fumehood where,
upon evaporation of the solvent, yellow crystals of 1 were found
that were visibly of superior quality than those resulting from
previous (careful) crystallizations. From these crystals sample
1y was obtained. Crystals of 2 were obtained by cooling a hot
hexane solution of the compound at 6 °C. Two crystalline
forms of 1y were investigated designated as 1y and 1y1. All
X-ray diffraction data for 1y, 1y1, 1r , and 2 were collected on
a Siemens SMART/CCD diffractometer equipped with an LT-
II low-temperature device. The diffraction data were corrected
for absorption by using the SADABS program. The program
SHELXTL16 was used for the structure solution, and the
refinement was based on F2. The p-fluorophenyl groups of 1y1
were disordered, and the disorder could not be readily modeled.
The refinement of this structure was therefore not pursued
further. For the other structures, all the non-hydrogen atoms
were refined anisotropically with the exception of four carbon
atoms of 1y, namely, C41, C64, C400, and C502, which gave
negative temperature factors and were therefore refined
isotropically. Hydrogen atoms were fixed in calculated posi-
Unless otherwise stated, manipulations of starting materials
and products were carried out under a nitrogen atmosphere
with the use of standard Schlenk techniques. Hexane was
refluxed over potassium, distilled, and stored over molecular
sieves before use; dichloromethane was dried in a similar
manner except that CaH2 was employed as the drying agent.
The precursory compound, Os3(CO)11(CH3CN), was prepared
by a literature procedure.15 NMR spectra were recorded on a
Bruker AMX400 spectrometer at the appropriate operating
frequencies for 1H, 13C, and 31P nuclei.
P r ep a r a tion of Os3(CO)11[P (p-C6H4F )3] (1) a n d Os3-
(CO)11(P Bu t3) (2). A solution of Os3(CO)11(CH3CN) (50 mg,
0.054 mmol) and P(p-C6H4F)3 (26 mg, 0.081 mmol) in CH2Cl2
(11) (a) Ang, H. G.; Kwik, W. L.; Leong, W. K.; Potenza, J . A. Acta
Crystallogr. 1989, C45, 1713. (b) Ang, H. G.; Cai, Y. M.; Kwik, W. L.;
Leong, W. K.; Tocher, D. A. Polyhedron 1991, 10, 881. (c) Ang, H.-G.;
Cai, Y. M.; Kwik, W. L. J . Organomet. Chem. 1993, 448, 219. (d) Ang,
H. G.; Koh, C.-H.; Koh, L.-L.; Kwik, W.-L.; Leong, W.-K.; Leong, W.-Y.
J . Chem. Soc., Dalton Trans. 1993, 847. (e) Ang, H.-G.; Ang, S.-G.;
Kwik, W.-L.; Zhang, Q. J . Organomet. Chem. 1995, 485, C10. (f)
J ohnson, B. F. G.; Lewis, J .; Norlander, E.; Raithby, P. R. J . Chem.
Soc., Dalton Trans. 1996, 3825. (g) Bartsch, R.; Blake, A. J .; J ohnson,
B. F. G.; J ones, P. G.; Mueller, C.; Nixon, J . F.; Nowotyny, M.;
Schumutzler, R.; Shepherd, D. S. Phoshorus, Sulfur, Silicon Relat.
Elem. 1996, 115, 201. (f) Biradha, K.; Hansen, V. M.; Leong, W. K.;
Pomeroy, R. K.; Zaworotko, M. J . J . Clust. Sci., submitted for
publication.
(12) (a) Bruce, M. I.; Liddel, M. J .; Hughes, C. A.; Patrick, J . M.;
Skelton, B. W.; White, A. H. J . Organomet. Chem. 1988, 347, 181. (b)
Bruce, M. I.; Liddel, M. J .; bin Shawkataly, O.; Hughes, C. A.; Skelton,
B. W.; White, A. H. J . Organomet. Chem. 1988, 347, 207.
(13) Alex, R. F.; Einstein, F. W. B.; J ones, R. H.; Pomeroy, R. K.
Inorg. Chem. 1987, 26, 3175.
(14) Biradha, K.; Gilmour, B. S.; Hansen, V. M.; Ma, A. K.; Pomeroy,
R. K.; Wong, E.; Zaworotko, M. J . Unpublished results. See also: Ma,
A. K. Ph.D. Thesis, Simon Fraser University, 1992.
(15) (a) J ohnson, B. F. G.; Lewis, J .; Pippard, D. A. J . Chem. Soc.,
Dalton Trans. 1981, 407. (b) Nicholls, J . N.; Vargas, M. D. Inorg. Synth.
1990, 28, 232.
(16) Sheldrick, G. M. SHELXTL; Siemens: Madison, WI, 1995.

Conformational Isomerism in Triosmium Clusters
Organometallics, Vol. 17, No. 24, 1998 5269
Ta ble 1. Cr ysta l Da ta a n d Str u ctu r e Refin em en t for Yellow a n d Red Os3(CO)11[P (p-C6H4F )3] (1y1, 1y, 1r )
a n d Os3(CO)11(P Bu t3) (2)
1y1
1y
1r
2
formula
temp (K)
cryst syst
space group
a (Å)
C
29H12F3O11Os3P
C29H12F3O11Os3P
183(2)
triclinic
P1h
9.616(4)
12.873(6)
26.433(12)
87.295(14)
83.12(2)
69.26(2)
3038(2), 4
2.613
C29H12F3O11Os3P
193(2)
monoclinic
P21/n
13.9258(7)
12.2851(6)
18.3017(10)
90
91.4040(10)
90
3130.1(3), 4
2.536
12.276
4491 [R(int) ) 0.0336]
0.0271
C23H27O11Os3P
183(2)
monoclinic
P21/c
17.0703(9)
11.2941(6)
17.3047(10)
90
119.1080(10)
90
2914.9(3), 4
2.463
13.153
6822 [R(int) ) 0.0339]
0.0248
193(2)
orthorhombic
Pnma
29.671(2)
11.8805(6)
10.1114(5)
90
b (Å)
c (Å)
R (deg)
â (deg)
90
90
γ (deg)
V (Å-3), Z
D (calcd) (Mg/m3)
3564.3(3), 4
2.227
10.781
2708 [R(int) ) 0.0465]
0.0931
abs coeff (mm-1
indpdt reflcns
)
12.648
10 507 [R(int) ) 0.0536]
0.0407
a
RF
b
RwF
0.269
0.0891
0.0594
0.0533
RF ) ∑|(|Fo| - |Fc|)|/∑|Fo|; Io ) 2.0σ(Io). RwF ) [∑(w(|Fo| - |Fc|)2)/∑(wFo2)]1/2. w ) [σ2(Fo)2 + kFo2]-1
.
a
b
Ta ble 2. Selected Bon d Len gth s (Å) a n d An gles (d eg) for 1 a n d 2
1y1
1ya
1r
2
Os(1)-Os(2)
Os(1)-Os(3)
Os(2)-Os(3)
Os(1)-P
Os-Ceq (range)
Os-Cax (range)
2.886(2)
2.900(2)
2.878(2)
2.36(1)
2.859(1), 2.844(1)
2.872(1), 2.888(1)
2.859(1), 2.856(1)
2.341(3), 2.331(3)
1.85(2)-1.90(2)
1.91(1)-1.95(1)
2.8835(4)
2.9117(4)
2.8654(4)
2.353(2)
1.903(9)-1.926(9)
1.922(8)-1.95(1)
2.9214(3)
2.9416(3)
2.8629(3)
2.478(1)
1.857(5)b-1.918(5)
1.928(5)-1.958(5)
Os-Os-Os (range)
P-Os(1)-C(13)
Ceq-Os-Ceq
59.37(3)-60.87(2)
102.3(4), 99.3(3)
100.8(5)-103.6(5)
173.8(6)-178.4(6)
59.26(1)-60.86(1)
98.7(2)
58.455(6)-61.124(7)
92.45(13)
99.8(4), 103.5(4)
98.5(2), 101.3(2)
Cax-Os-Cax (range)
170.6(4)c-175.1(3)
165.8(2)c-175.7(2)
a
b
Two independent molecules in unit cell. Os(1)-C(13). c C(11)-Os(1)-C(12).
tions and refined isotropically on the basis of the corresponding
C atoms [U(H) ) 1.2Ueq(C) ]. Pertinent crystallographic data
are given in Table 1; selected bond length and angle data are
given in Table 2. Fractional coordinates and anisotropic
displacement parameters have been deposited as Supporting
Information.
In one preparation of 1 it was noticed that there were
both yellow (1y) and red (1r ) crystals present in the
sample. Closer investigation revealed that when a hot
hexane solution (with or without a small amount of CH2-
Cl2) of 1 was allowed to cool to room temperature, both
red and yellow crystals formed. When this solution was
allowed to stand, the red crystals became more numer-
ous at the expense of the yellow crystals until after 24
h only red blocklike crystals remained. When hot
hexane solutions of 1 were steadily cooled to -40 °C,
only yellow crystals resulted. As such, the crystals were
feather-like needles and mostly unsuitable for crystal-
lographic study. A crystal (1y1) from this preparation
was, however, found that diffracted, but the structure
obtained from the diffraction data was partly disordered
(see below). By chance, it was noticed that a sample of
1 in hexane that had been allowed to evaporate in air
had yielded well-formed yellow crystals of 1 (1y).
Crystallographic investigation of one of these crystals
revealed it was yet another polymorph of the compound
and, furthermore, was an ordered structure.
Resu lts a n d Discu ssion
The clusters Os3(CO)11(PR3) (R ) p-C6H4F, 1; R ) But,
2) were prepared by treatment of Os3(CO)11(CH3CN) (eq
1) with PR3, a standard protocol for the synthesis of Os3-
Os3(CO)11(CH3CN) + PR3 25 °C8 Os3(CO)11(PR3) (1)
R ) p-C6H4F, 1; R ) But, 2
(CO)11(PR3) compounds.6,15 As mentioned in the Intro-
duction, we have synthesized over 20 compounds of this
type by this method. Although no quantitative studies
were carried out, it was noted that the rate of the
reaction depended both on the size (cone angle) and
donor properties of the PR3 ligand employed. The
syntheses of 1 and 2 partially illustrate this observation.
The preparation of 1 was complete in 15 min, as
indicated by IR spectroscopy, whereas that of 2 took 18
When 1y was heated, it turned orange in the ap-
proximate range 70-110 °C and melted at essentially
the same temperature as the red form. DTA of 1y and
1r exhibited peak maxima at 165.5 and 165.0 °C,
respectively, with no evidence of any phase change up
to the melting point for either polymorph.
h; the cone angles of P(p-C6H4F)3 and PBut are 145°
3
and 182°, respectively.17 The yield of 2 (∼30%) was the
lowest of the derivatives prepared. The Os3(CO)11(PR3)
derivatives are air stable crystalline solids that with the
exceptions described here are yellow or orange.
Dissolution of 1r in hexane gave yellow solutions with
spectroscopic properties identical to those of hexane
solutions of 1y; when 1r was dissolved in CH2Cl2 at -78
°C, the resulting solution was yellow. These observa-
tions suggest that the cause of the color change in 1 is
(17) Tolman, C. A. Chem. Rev. 1977, 77, 313.

5270 Organometallics, Vol. 17, No. 24, 1998
Hansen et al.
F igu r e 1. Infrared spectrum (CO-stretching region) of Os3(CO)11[P(p-C6H4F)3], 1, (A) and Os3(CO)11(PBut ), 2, (B) in hexane.
3
IR spectrum of 2 in the CO-stretching region is some-
what different from that of other Os3(CO)11(PR3) com-
plexes and is illustrated in Figure 1.
The 13C NMR spectrum of 2 (13CO enriched) in CH2-
Cl2/CD2Cl2 at -60 °C exhibits three resonances of
intensity 2 in the 182-198 ppm region assigned to the
(pseudo) axial carbonyls and five resonances in the 168-
180 ppm region of intensity 1 and assigned to the
(pseudo) equatorial carbonyls. This is typical of Os3-
(CO)11(PR3) complexes (at temperatures just above -50
°C selective carbonyl exchange occurs in these com-
plexes, which causes the resonances to broaden and
collapse into the baseline at different rates).14,19,20 Both
1
the H and 31P{1H} NMR spectra of 2 in CFHCl2/CD2-
F igu r e 2. UV-vis spectrum of 1 (dotted line) and 2 (solid
Cl2 at -120 °C consist of singlets (with phosphorus
line) in hexane.
coupling in the case of the 1H NMR resonance). Al-
though this does not completely rule out that both the
E and S (see below) forms of 2 are present in solution
in rapid equilibrium, the NMR and IR evidence indi-
due to intermolecular forces (see below). The IR and
13C NMR spectra of 1y (CO region) are typical of the
Os3(CO)11(PR3) compounds that we have investigated;
cates that this is most unlikely; that is, there is only
the IR spectrum of 1y (CO stretching region) in hexane
one form of 2 in solution responsible for the three bands
is shown in Figure 1.
in the UV-vis spectrum.
Unlike the other Os3(CO)11(PR3) compounds studied,
2 is deep red in solution and yields deep red, almost
black, crystals when such solutions are cooled. The
yellow or orange Os3(CO)11(PR3) clusters have two weak
absorptions in the UV-vis spectrum in the region
24 000-32 000 cm-1 that have been assigned in the
parent Os3(CO)12 cluster to σ to σ* transitions involving
molecular orbitals associated with the Os3 unit;18 this
is shown for 1y in Figure 2. Cluster 2, however, exhibits
three bands in this region, at 31 400, 26 700, and 21 400
cm-1 (Figure 2); the lowest energy bands absorb most
of the green of the visible spectrum so that 2 is red. The
maxima of the lowest energy band in both 1y and Os3-
(CO)11(PEt3)14 are around 24 500 cm-1, which suggests
that the band at 29 900 cm-1 of 2 is also not due to the
same electronic transition as in the other Os3(CO)11(PR3)
molecules. The UV-vis spectrum exhibited by 2 indi-
cates its electronic structure is unique of the Os3(CO)11-
(PR3) compounds so far investigated. We believe that
this is strong evidence that 2 has a genuinely different
conformation for an Os3(CO)11(PR3) cluster and not just
a more distorted version of the typical conformation. The
Str u ctu r es of Os3(CO)11[P (p-C6H4F )3]. Three X-
ray structures of 1 were carried out: two of the yellow
(1y1, 1y) and one of the red form (1r ). Selected bond
lengths and angles for the structures of 1 are given in
Table 2. The p-fluorophenyl rings of 1y1 were disor-
dered and could not be readily modeled, and therefore
only the OsP and OsOs distances for 1y1 are given in
the table. Further discussion on the structure of the
yellow polymorphs is restricted to 1y, the ordered form;
this version contains two independent molecules in the
unit cell. The structure of 1y is as expected, with the
vectors that pass through a given Os atom and the axial
carbonyls associated with it essentially orthogonal to
the Os3 plane; a view of molecule 1 of 1y is given in
Figure 3. This geometry based on the D3h structure of
Os3(CO)12 (Chart 1) we designate as “E” (eclipsed or
erect). The range of the Os-Os bond lengths in the two
independent molecules of 1y is 2.844(1)-2.888(1) Å. The
average bond length in Os3(CO)12 is 2.877(3) Å,2 whereas
the Os-Os distances in Os3(CO)11(PPh3) range from
2.886 to 2.918(1) Å.8 As is generally observed in Os3-
(CO)11(PR3) clusters,6,8-11 the longest Os-Os bond is
(18) (a) Tyler, D. R.; Levensen, R. A.; Gray, H. B. J . Am. Chem. Soc.
1978, 100, 7888. (b) Tyler, D. R.; Altobelli, M.; Gray, H. B. J . Am. Chem.
Soc. 1980, 102, 3022.
(19) J ohnson, B. F. G.; Lewis, J .; Reichert, B. E.; Schorpp, K. T. J .
Chem. Soc., Dalton Trans. 1976, 1403.
(20) Alex, R. F.; Pomeroy, R. K. Organometallics 1987, 6, 2437.

Conformational Isomerism in Triosmium Clusters
Organometallics, Vol. 17, No. 24, 1998 5271
F igu r e 4. Partial packing diagram for 1y showing the
edge-to-face interactions of the p-fluorophenyl rings.
in 1y (2.877, 2.888(1), and 2.859, 2.844(1) Å, respec-
tively). These lengths suggest that the metal-metal
bonds in 1r are somewhat weaker than those in 1y,
which is therefore consistent with the view that in the
S form of M3(CO)12 there is less metal-metal bonding
than in the E form.3
The question naturally arises as to the origin of the
twisting in the red isomer of 1. The steric and σ-donor/
π-acceptor properties of the P(p-C6H4F)3 ligand lie
approximately in midrange for common P ligands: the
cone angle of the ligand is 145°, and the Tolman
electronic parameter 2071.3 cm-1 17
It appears unlikely,
.
F igu r e 3. Molecular structure of the yellow (top) and red
forms of Os3(CO)11[P(p-C6H4F)3] (1y and 1r , respectively).
therefore, that the stability of the S form of 1 can be
attributed to any unusual steric or donor properties of
the P(p-C6H4F)3 ligand. It is known, however, that
phenyl groups of molecules or ions pack effectively, with
specific geometries, so as to lead to solids that have high
crystallinity and low solubility.21 For example, recent
studies have described structures with [Ph4P]+ cations
which contain one-dimensional supramolecular motifs
that involve concerted, multiple phenyl embraces based
on edge-to-face interactions between phenyl groups of
different [Ph4P]+ cations.22a In 1y each P(p-C6H4F)3
group forms a sextuple embrace with a second P(p-
C6H4F)3 group through such aromatic edge-to-face
interactions.22b These aggregates pack in the y-direction
also via edge-to-face binding (Figure 4). On the other
hand, in 1r the P(p-C6H4F)3 ligands do not form a
sextuple embrace, but rather the phenyl rings of one
P(p-C6H4F)3 substituent interacts with the phenyl rings
of its two nearest neighbors by four edge-to-face interac-
tions to give a one-dimensional chain (Figure 5).23 We
believe that these interactions are responsible for the
twisting of Os3 framework in 1r . As mentioned previ-
ously, that 1r reverts to the yellow form on dissolution
in CH2Cl2 or hexane suggests that it is an intermolecu-
lar interaction in the solid state that is the cause of the
distortion in 1r . The presence of the electronegative
that which is cis to the phosphorus-donor ligand (i.e.,
Os(1)-Os(3)), although this lengthening is chemically
not significant in 1y.
The structure of 1r is also depicted in Figure 3. It is
immediately apparent that the structure is based on the
D3 form of Os3(CO)12 (Chart 1) with the Os(CO)4 units
twisted with respect to each other. We designate this
conformer as “S” (staggered or slanted). The extent of
this twisting can be measured in a number of ways; we
have chosen to take the absolute value of the dihedral
angle between the Cax-Os(2)-Cax-Os(3) and Cax-Os-
(3)-Cax-Os(2) planes of the Os2(CO)8 fragment of the
Os3(CO)11(PR3) cluster. In this way the direct steric and
electron effects of the PR3 ligand are avoided (cf. the
structure of 2 discussed below). This angle can range
from 0°, as in the ideal form of the parent carbonyl Os3-
(CO)12 with D3h symmetry, to 45°. In 1r this dihedral
angle (i.e., that between the C(22)Os(2)C(23)Os(3) and
C(31)Os(3)C(33)Os(2) planes) is 24.5°. (The Cax-Os(2)-
Os(3)-Cax dihedral angles used by Bruce and co-workers
to describe this twisting are 23.1° and 26.1°.) The
corresponding angle for 1y is 2.0° in both of the
independent molecules. The distances between the
pseudoaxial carbonyl on one osmium atom to the next
closest osmium atom (e.g., Os(1)‚‚‚C(23)) are in the
range 3.07(1)-3.26(1) Å and cannot be considered as
semibridging interactions. In 1y these distances are in
the range 3.32(1)-3.50(1) Å.
(21) Dance, I. G.; In Perspectives in Supramolecular Chemistry: The
Crystal as a Supramolecular Entity; Desiraju, G. R., Ed., Wiley:
Chicester, 1996; Vol. 2.
(22) (a) Dance, I. G.; Scudder, M. J . Chem. Soc., Dalton Trans. 1996,
3755. (b) Dance, I. G.; Scudder, M. J . Chem. Soc., Chem. Commun.
1995, 1039.
(23) Note the conversion of 1y to 1r would involve a small displace-
ment in the y-direction of one row of molecules with respect to the
adjacent row.
The Os-Os bond cis to the P-donor ligand in 1r
(2.912(1) Å) and that trans to this ligand (2.884(1) Å)
are lengthened compared to the corresponding distances

5272 Organometallics, Vol. 17, No. 24, 1998
Hansen et al.
the two independent molecules in the unit cell;9 in
Os3(CO)11[P(o-C6H4Me)3] the Os-Os bond cis to the
P-donor ligand has a length of 2.9429(4) Å.11f The
lengthening of the metal-metal bond cis to the PBut
3
ligand in 2 can be attributed to both steric and electronic
factors. It has previously been found that as the cone
angle of PR3 increases, the cis Os-Os distance in Os3-
(CO)11(PR3) compounds increases.8 In unpublished
work we have found that for Os3(CO)11[P(OPh)3] the cis
Os-Os length is 2.8941(6) Å, whereas in the PEt3
analogue the corresponding length is 2.9242(4) Å.11f The
cone angles of the phosphite (128°) and of the phosphine
(132°) are approximately equal, whereas their donor
properties are significantly different (Tolman electronic
parameters of 2085.3 and 2061.7 cm-1, respectively).17
The increase in the Os-Os distance in the PEt3 com-
pound therefore indicates that the σ-donor/π-acceptor
properties of the P ligand also influence the cis Os-Os
length; that is, better donor ligands lengthen the cis
Os-Os bond. The PEt3 and PBut3 ligands have similar
donor properties (Tolman parameters of 2061.7 and
2056.1 cm-1, respectively). It could be argued, therefore,
that steric interactions in 2 are responsible for an
increase of approximately 0.2 Å in the cis Os-Os length
F igu r e 5. Partial packing diagram for 1r .
in 2. The Os-Os distance trans to the PBut ligand in
3
2 is 2.9214(3) Å, which is the longest Os-Os bond of
this type yet reported for Os3(CO)11(PR3) complexes, an
observation that can undoubtedly be attributed to the
F igu r e 6. Molecular structure of 2.
strong σ-donor properties of PBut (see below). The
fluorine atom would be expected to lower the energy of
the acceptor π* MO of the fluorophenyl ring and could
lead to these interactions being stronger than in say Os3-
(CO)11(PPh3). The perfluorophenyl derivative Os3(CO)11-
[P(C6F5)3] has the E configuration,11a and we found no
evidence for a red form in our spectroscopic investigation
of this cluster.
3
remaining Os-Os bond length in 2 is 2.8629(3) Å.
Structures of Os3(CO)11(PR3) compounds reveal, not
unexpectedly, that the length of this Os-Os bond is
insensitive to the nature of R.7-12
There are two additional features of interest in the
structure of 2. The Os-P length of 2.478(1) Å in 2 is
by far the longest Os-P bond length for an Os3(CO)11-
(PR3) cluster previously reported or determined in this
laboratory.7-12,14 For example, the Os-P lengths in 1y,
1r , and Os3(CO)11(PEt3) are 2.331(3) and 2.341(3), 2.353-
(2), and 2.345(2) Å, respectively.11f Given that the PEt3
In 1r an oxygen atom of one of the pseudoaxial CO
ligands (i.e., O(33)) makes close approach (3.13(1) Å) to
the plane of one of fluorophenyl rings (ring 2; Figure
3), which might indicate a weak bonding interaction
arising from electron donation from the π MO on the
CO ligand to the π* MO of the fluorophenyl ring.
(Electron donation from groups other than CO to phenyl
rings has been proposed to occur in biochemical mol-
ecules.)24 Molecular orbital calculations (ab initio with
a 3-12G** basis set), however, based on distances and
angles found in 1r did not indicate any significant
interaction between a carbonyl group and fluorobenzene.
Str u ctu r e of Os3(CO)11(P Bu t3). The structure of 2
is shown in Figure 6; as can be seen from the figure, it
too has the twisted S conformation. The dihedral angle
between the Cax-Os-Cax-Os planes of the Os2(CO)8
grouping in 2 is 32.9° (Cax-Os(2)-Os(3)-Cax dihedral
angles of 32.4° and 33.8°). The distances between the
pseudoaxial carbonyls and the nearest nonbonded os-
mium atom are in the range 3.05(1)-3.20(1) Å and as
in 1r do not indicate any significant bonding interaction.
The Os-Os bond cis to the phosphine ligand in 2 is, at
2.9416(3) Å, among the longest Os-Os bonds reported
and PBut ligands have similar donor properties, the
3
lengthening of the Os-P bond in 2 can be attributed to
steric effects of the large PBut ligand (cone angle )
3
182°).17 That the pseudoaxial carbonyl ligands bound
to the same Os atom as the PBut substituent are bent
3
away from this group by some 10° from the angle
expected for octahedral geometry (Table 2) is consistent
with this view. The second noteworthy aspect of 2 is
that the angle the equatorial ligands make at Os(1) (i.e.,
C(13)Os(1)P(1)) is 92.5(1)°, whereas the other angles of
this type in 2 and those in 1y and 1r are in the range
98.5(1)-103.6(5)°.
Unlike P(p-C6H4F)3, PBut is an exceptional ligand.
3
It is the strongest σ-donor of the common phosphorus
ligands (the Tolman electronic parameter for PBut is
3
2056.1 cm-1); it also has one of the largest cone angles
(θ ) 182°) of the PR3 class of ligands.17 We believe that
these exceptional properties are responsible for the S
conformation adopted in 2.
for Os3(CO)11(PR3) compounds. In Os3(CO)11[PBut -
(NH2)] the corresponding length is 2.953(1) Å in one of
2
The metal-metal bonding in M3(CO)12 is often de-
scribed in terms of localized 2c-2e bonds of octahedral
M(CO)4 fragments. What is believed to be a more
correct description of the bonding in the M3 core,
however, is one that involves a central MO resulting
(24) (a) Burley, S. K.; Petsko, G. A. Adv. Protein Chem. 1988, 39,
125. (b) Waksman, G.; Kominos, D.; Robertson, S. C.; Pant, N.;
Baltimore, D.; Birge, R. B.; Cowburn, D.; Hanafusa, H.; Mayer, B. J .;
Overduin, M.; Resh, M. D.; Rios, C. B.; Silverman, L.; Kuriyan, J .
Nature 1992, 358, 646.

Conformational Isomerism in Triosmium Clusters
Organometallics, Vol. 17, No. 24, 1998 5273
Ch a r t 2
Ch a r t 3
exchange to take place (Chart 3).20 If such an explana-
tion is correct, then expansion of the 5d orbitals on the
Os atom of the Os(CO)3(PR3) grouping would also cause
an increased interaction with the corresponding filled
5d orbitals on the other Os atoms in the cluster; this is
also shown in Chart 3. Such an interaction would be
repulsive and lead to a lengthening of the cis Os-Os
bond with increased donor ability of the PR3 ligand, as
observed. (Consistent with this proposal is that 2 has
one of the lowest barriers to axial-equatorial, merry-
go-round CO exchange, and that the lowest-energy
exchange takes place across the long cis Os-Os bond.)14
from the overlap of atomic orbitals on the individual M
atoms, plus a lower energy pair of molecular orbitals
that involve overlap of atomic orbitals along the edges
of the M3 triangle.4,5 (Recall that an M(CO)4 fragment
is isolobal with CH2 with a1 and b1 frontier orbitals.)25
This is shown in Chart 2 (adapted from that in ref 4).
Lengthening the M-M vectors, as in 2, would decrease
these overlaps especially along the edges of the metal
triangle. As mentioned in the Introduction, it is be-
lieved that these edge-bridging MOs are responsible for
the D3h conformation of M3(CO)12.3 We believe that in
2 the bonding contribution of these edge MOs has been
reduced such that the D3 form is preferred for steric
reasons. The presence of three transitions in the UV-
vis spectrum implies that in 2 the e′ set of molecular
orbitals has been split, with perhaps one of the resulting
molecular orbitals higher in energy than the a1′ MO.
In (OC)5M′[Os(CO)3(PMe3)]2 (M′ ) Cr, Mo, W) clusters
which contain an M′Os2 triangle, the radial carbonyls
on the M′(CO)5 unit are staggered with respect to the
axial carbonyl groups on the Os atoms. It is thought
that the main bonding interaction of the group 6 metal
atom to the Os2 fragment is via a single 3c-2e MO (the
M′(CO)5 fragment is isolobal with CH3 with a single
frontier MO). Interestingly, the (OC)5M′[Os(CO)3-
(PMe3)]2 clusters are, like 2, deep red.26
Twisting of the Os(CO)3(L) (L ) CO, PBut ) groups
3
might be expected to reduce this repulsive interaction
and hence would also favor the S conformation as
adopted by 2. The donor properties of P(o-C6H4Me)3 are
somewhat less than those of PBut3 (Tolman parameters
of 2066.6 and 2056.1 cm-1, respectively)17 and might
account for why the E conformation is maintained in
Os3(CO)11[P(o-C6H4Me)3]. (The structure of Os3(CO)11-
(PCy3) shows it has the common E conformation con-
sistent with its UV-vis spectrum.)11f
Con clu sion s
Deeming, in a review of substituted derivatives of M3-
(CO)12 (M ) Ru, Os) with ligands coordinated only
through heteroatoms, has pointed out that clusters of
the type M3(CO)11(L) are based on the parent carbonyl
with either equatorial L (for L ) P-, As-, S- donor
ligand, etc.) or axial L (L ) MeCN, ButNC, NHd
C(CH2)5).6 It is only in the more highly substituted
derivatives such as Ru3(CO)8(L)4 (L ) P(OMe)2Ph, PMe2-
Ph, P(OMe)3) that distorted structures with semibridg-
ing carbonyls, similar to the structure of Fe3(CO)12, are
observed.27 For the mixed metal derivative FeRu2-
(CO)10(PPh3)2 a third structural form is observed in
which two of the carbonyl ligands have semibridging
interactions across different Ru-Fe bonds.28 As men-
tioned in the Introduction, although distortions from E
toward the S geometry are observed in some solid-state
structures of M3(CO)11(L) clusters, the distortions are
generally small. In the more highly substituted clus-
ters, however, the S geometry is more common. In Os3-
(CO)6[P(OMe)3]6 (also prepared in this laboratory) there
is significant distortion toward the S conformer and
there are no semibridging CO contacts. The distortion
in this compound, however, can undoubtedly be at-
tributed to the minimization of the steric interactions
Simple lengthening of the Os-Os edges cannot be the
complete explanation as to why 2 has the twisted S
conformation since, as mentioned above, the cis Os-Os
length in Os3(CO)11[P(o-C6H4Me)3] is longer than that
in 2, but the P(o-C6H4Me)3 derivative has the E struc-
ture.11f The cone angle of P(o-C6H4Me)3 (194°) is larger
than that of PBut .17 It is found that the barrier to CO
3
exchange in Os3(CO)11(PR3) is significantly lowered to
that in the parent Os3(CO)12.14,19,20 To rationalize this
observation, we have proposed that the better donor PR3
ligand increases the electron density at the Os atom of
the Os(CO)3(PR3) unit in Os3(CO)11(PR3) compared to
the Os atoms of the Os(CO)4 groupings in Os3(CO)12.
This in turn expands the filled 5d orbitals on the unique
Os atom and allows better overlap with the π* MO of
the axial carbonyls on the neighboring Os atoms,
thereby lowering the activation energy needed to form
the CO-bridged intermediate necessary for carbonyl
(27) (a) Bruce, M. I.; Matisons, J . G.; Patrick, J . M.; White, A. H.;
Willis, A. C. J . Chem. Soc., Dalton Trans. 1985, 1223. (b) Bruce, M. I.;
Liddel, M. J .; bin Shawkataly, O.; Bytheway, I. Skelton, B. W.; White,
A. H. J . Organomet. Chem. 1989, 369, 217.
(28) Vena¨la¨inen, T.; Pakkanen, T. J . Organomet. Chem. 1984, 266,
269.
(25) Hoffmann, R. Angew. Chem., Int. Ed. Engl. 1982, 21, 711.
(26) Batchelor, R. J .; Davis, H. B.; Einstein, F. W. B. E.; J ohnston,
V. J .; J ones, R. H.; Pomeroy, R. K.; Ramos, A. F. Organometallics 1992,
11, 3555.

5274 Organometallics, Vol. 17, No. 24, 1998
Hansen et al.
between the bulky phosphite ligands that occupy all the
equatorial sites in the molecule. Interestingly, the Os3-
(CO)6[P(OMe)3]6 cluster is orange-red with three ab-
sorptions in the UV-vis spectrum. It is, however, not
as deeply colored as 2.13
[P(OCH2)3)CEt] (22.5°, 24.4°),8 which is not expected
from the arguments presented here but might be due
to packing effects. On the other hand, the same workers
found that the Cax-Os(2)-Os(3)-Cax angles in
M3(CO)12-x(PR3)x (x ) 2, 3) were much larger than in
the monosubstituted analogues,12 which may be ratio-
nalized with arguments presented above.
In this study, we have described two simple Os3(CO)11-
(L) derivatives that have the twisted (S) rather than
the previously observed E conformation. We believe
that in 1r (L ) P(p-C6H4F)3) the twisting is the result
of weak intermolecular bonding between the fluorophe-
nyl rings in the solid state, that is, due to a bonding
interaction that does not involve the metal skeleton.
That the yellow form of 1 has the common E geometry
with eclipsed adjacent carbonyl ligands indicates the
extrinsic bonding interaction in 1r is weak; the solution
properties of 1 are consistent with only the presence of
the E conformation in solution. The S configuration is
It is known that large basic phosphine ligands can
stabilize unusual bonding modes or electron configura-
tions, for example, the η2-H2 complex W(CO)3(PPri ) -
3 2
(η2-H2) and the 17e compound Re(CO)3(PCy3)2.29,30
Recently, the first Ru3 cluster with a 44e configuration,
Ru3(µ3-H)2(µ-CO)3(CO)3(PCy3)3, has been described in
which the tricyclohexylphosphine ligands are undoubt-
edly crucial to its stability.31 The synthesis and struc-
ture of 2 demonstrate that a large basic phosphine
ligand can also distort the geometry of a simple metal
carbonyl cluster compound.
also found for 2 (L ) PBut ) both in the solid state and
3
in solution. We believe that the conformation of 2 is
the result of both the exceptional σ-donor and steric
Ack n ow led gm en t. We thank the Natural Sciences
and Engineering Research Council of Canada and St.
Mary’s University for financial support. We also thank
Dr. Noham Weinberg for the MO calculations.
properties of the PBut ligand which destabilize the
3
peripheral Os-Os bonding in the Os3 skeleton so that
the sterically preferred S conformer is favored. These
results provide support of the theoretical study of
Lauher, who found that the S conformation of Ru3(CO)12
is only some 3 kcal mol-1 higher in energy than the E
form.3
It is interesting that Bruce and co-workers found that
distortion toward the S conformer was more prevalent
in the structures of Ru3(CO)11(PR3) compounds than
their Os analogues, consistent with the weaker metal-
metal bonding in the second-row derivative. (We have
been unsuccessful in attempts to prepare the Ru con-
gener of 2.) It should, however, be mentioned that the
Cax-Os(2)-Os(3)-Cax angles for Ru3(CO)11(PCy3) (3.7°,
4.7°) are much smaller than those in Ru3(CO)11-
Su p p or tin g In for m a tion Ava ila ble: Figure of the mo-
lecular structure of 1y1; listing of atomic coordinates, bond
lengths and angles, anisotropic thermal parameters, and
hydrogen atom coordinates for compounds 1y, 1y1, 1r , and 2
(31 pages). Ordering information and Internet access instruc-
tions are given on any current masthead page.
OM980365G
(29) Kubas, G. J .; Ryan, R. R.; Swanson, B. I.; Vergamini, P. J .;
Wasserman, H. J . J . Am. Chem. Soc. 1984, 106, 451.
(30) Walker, H. W.; Rattinger, G. B.; Belford, R. L.; Brown, T. L.
Organometallics 1983, 2, 775.
(31) Su¨ss-Fink, G.; Godefroy, I.; Ferrand, V.; Neels, A.; Stoeckli-
Evans, H. J . Chem. Soc., Dalton Trans. 1998, 515.