New perspective on the formation and reactivity of metal-metal-bonded dinuclear rhodium and iridium complexes

Article abstract of DOI:10.1021/om970347j

Reactions of the binuclear complexes [{M(μ-Pz)(CNBu^t)₂}²] (M = Rh (1), Ir (2); Pz = pyrazolate) with diiodine give the metal-metal-bonded complexes [{M(μ-Pz)(I)(CNBu^t)₂}₂] (M = Rh (3), Ir (4)). A further reaction with diiodine afford the complexes [{M(μ-Pz)(I)-(CNBu^t)₂}₂(μ-I)]I, which react cleanly with CF₃SO₃Me replacing the ionic iodide by the triflate group. The M(I) complexes 1 and 2 are easily oxidized by mild oxidants such as [Cp₂Fe]⁺ to give the metal-metal-bonded dications [{M(μ-Pz)(CNBu^t)₂MMeCN)}₂]2+, which add halide ions yielding the neutral complexes [{M(μ-Pz)(X)(CNBu^t)₂}₂] (M = Rh, X = Cl (7); M = Rh, Ir, X = I). These reactions indicate that the metal-metal bond formation has an electron-transfer character. Evidence of the nucleophilic attack by the metal-metal bond in complexes 3 and 4 of small molecules such as diiodine is provided by the reaction of [I(Py)₂]BF₄ with 7, which adds the electrophilic iodine(I) to give the bridging iodide complex [{Rh(μ-Pz)(Cl)-(CNBu^t)₂}₂(μ-I)] ⁺. In addition, an electrophile such as MeCF₃SO₃ abstracts one of the iodide ligands trans to the metal-metal bond in 3 to give MeI and the cationic complex [Rh₂(μ-Pz)₂(I)(CNBU^t)₄]CF ₃SO₃. This cation adds complex 3, which behaves as a metalloligand through an iodide atom, to give the tetranuclear complex [{Rh₂(μ-Pz)₂(I)(CNBu^t)₄} ₂(μ-I)]CF₃-SO₃ (8). The X-ray structure of 8 contains two metal-metal-bonded binuclear 'Rh₂(μ-Pz)₂-(I)(CNBu^t)₄' units linked through a slightly asymmetric bridging iodine atom, showing an unusual Rh-I-Rh bond angle of 129.99(4)°. Reaction of 3 with IC≡C-Ph gives the acetylide complex [(CNBu^t)₂(I)Rh(μ-Pz)₂(μ-I)Rh(η ¹-C≡C-Ph)(CNBu^t)₂]I. EHMO calculations on com-plexes [Rh₂(μ-Pz)₂(CNBU^t)₄] ²⁺, [Rh₂(μ-Pz)₂(I)(CNBu^t)₄] ⁺ [A]⁺, and [{Rh(μ-Pz)(I)(CNBU^t)₂}₂] successfully explain the reactivity at the metal-metal bond, that of the iodide ligand in 3 with electrophiles, and those of [A]⁺ with nucleophiles. Complex 3 and the cation [Rh₂(μ-Pz)₂(I)(CNBu^t)₄] ⁺ react with MeI in the presence of direct sunlight to give the oxidative-addition product [(CNBu^t)₂(I)Rh(μ-Pz) ₂(μ-I)Rh(Me)(CNBu^t)₂]⁺ in high yield.

Full text of DOI:10.1021/om970347j

4718
Organometallics 1997, 16, 4718-4727
New P er sp ective on th e F or m a tion a n d Rea ctivity of
Meta l-Meta l-Bon d ed Din u clea r Rh od iu m a n d Ir id iu m
Com p lexes
Cristina Tejel, Miguel A. Ciriano,* J ose´ A. Lo´pez, Fernando J . Lahoz, and
Luis A. Oro*
Departamento de Qu´ımica Inorga´nica, Instituto de Ciencia de Materiales de Arago´n,
Universidad de Zaragoza-CSIC, E-50009 Zaragoza, Spain
Received April 23, 1997X
Reactions of the binuclear complexes [{M(µ-Pz)(CNBut)2}2] (M ) Rh (1), Ir (2); Pz )
pyrazolate) with diiodine give the metal-metal-bonded complexes [{M(µ-Pz)(I)(CNBut)2}2]
(M ) Rh (3), Ir (4)). A further reaction with diiodine afford the complexes [{M(µ-Pz)(I)-
(CNBut)2}2(µ-I)]I, which react cleanly with CF3SO3Me replacing the ionic iodide by the triflate
group. The M(I) complexes 1 and 2 are easily oxidized by mild oxidants such as [Cp2Fe]+ to
give the metal-metal-bonded dications [{M(µ-Pz)(CNBut)2(MeCN)}2]2+, which add halide
ions yielding the neutral complexes [{M(µ-Pz)(X)(CNBut)2}2] (M ) Rh, X ) Cl (7); M ) Rh,
Ir, X ) I). These reactions indicate that the metal-metal bond formation has an electron-
transfer character. Evidence of the nucleophilic attack by the metal-metal bond in complexes
3 and 4 of small molecules such as diiodine is provided by the reaction of [I(Py)2]BF4 with
7, which adds the electrophilic iodine(I) to give the bridging iodide complex [{Rh(µ-Pz)(Cl)-
(CNBut)2}2(µ-I)]+. In addition, an electrophile such as MeCF3SO3 abstracts one of the iodide
ligands trans to the metal-metal bond in 3 to give MeI and the cationic complex [Rh2(µ-
Pz)2(I)(CNBut)4]CF3SO3. This cation adds complex 3, which behaves as a metalloligand
through an iodide atom, to give the tetranuclear complex [{Rh2(µ-Pz)2(I)(CNBut)4}2(µ-I)]CF3-
SO3 (8). The X-ray structure of 8 contains two metal-metal-bonded binuclear ‘Rh2(µ-Pz)2-
(I)(CNBut)4’ units linked through a slightly asymmetric bridging iodine atom, showing an
unusual Rh-I-Rh bond angle of 129.99(4)°. Reaction of 3 with ICtC-Ph gives the acetylide
complex [(CNBut)2(I)Rh(µ-Pz)2(µ-I)Rh(η1-CtC-Ph)(CNBut)2]I. EHMO calculations on com-
plexes [Rh2(µ-Pz)2(CNBut)4]2+, [Rh2(µ-Pz)2(I)(CNBut)4]+ [A]+, and [{Rh(µ-Pz)(I)(CNBut)2}2]
successfully explain the reactivity at the metal-metal bond, that of the iodide ligand in 3
with electrophiles, and those of [A]+ with nucleophiles. Complex 3 and the cation [Rh2(µ-
Pz)2(I)(CNBut)4]+ react with MeI in the presence of direct sunlight to give the oxidative-
addition product [(CNBut)2(I)Rh(µ-Pz)2(µ-I)Rh(Me)(CNBut)2]+ in high yield.
In tr od u ction
In contrast, metal-metal-bonded species generally
result from oxidative-addition reactions to related bi-
nuclear iridium complexes. Thus, addition of MeI or I2
to the complexes [{Ir(µ-L)(CO)(L′)}2] (L ) SBut,6 pyra-
zolate,7 7-azaindolate,8 1,8-diamidonaphthalene;9 L′ )
monodentate phosphine or CO) and [{Ir(µ-Pz)(µ-SBut)-
(CO)(L′)}2]10 in all cases resulted in the formation of the
Ir(II)-Ir(II) species. Meanwhile, for rhodium, acetyl
Rh(I)-Rh(III) complexes result11 from the addition of
MeI to [{Rh(µ-SBut)(CO)(L′)}2], but Rh(II)-Rh(II) com-
plexes result from the reaction of [{Rh(µ-L)(CO)(L′)}2]
Recent studies on binuclear complexes of rhodium and
iridium containing alkyl isocyanide ligands have shown
a high electronic density at the metal centers1 and may
be considered to be “supernucleophiles”2 due to their
high reactivity. Thus, the complex [{Rh(µ-Pz)(CN-
But)2}2] adds a variety of chlorocarbons with C-Cl bond
cleavage to give dirhodium(III) complexes.3,4 In par-
ticular, we recently reported5 that the addition of methyl
iodide to the binuclear complexes [{M(µ-L)(CNBut)2}2]
and [(cod)M(µ-Pz)2M(CNBut)2] (M ) Rh, Ir; L ) pyra-
zolate (Pz), SBut) take place sequentially in two separate
steps, each one at a single metal center for rhodium.
These fast reactions lead to the M(III)-M(III) complexes
even for iridium.
(6) (a) El Amane, M.; Maisonnat, A.; Dahan, F.; Poilblanc, R. New
J . Chem. 1988, 12, 661. (b) Kalck, P.; Bonnet, J .-J . Organometallics
1982, 1, 1211.
(7) (a) Bushnell, G. W.; Fjeldsted, D. O. K.; Stobart, S. R.; Wang, J .
J . Organometallics 1996, 15, 3785. (b) Coleman, A. W. T.; Eadie, D.
T.; Stobart, S. R. J . Am. Chem. Soc. 1982, 104, 922. (c) Beveridge, K.
A.; Bushnell, G. W.; Dixon, K. R.; Eadie, D. T.; Stobart, S. R. J . Am.
Chem. Soc. 1982, 104, 920. (d) Powell, J .; Kuksis, A.; Nyburg, S. C.;
Ng, W. W. Inorg. Chim. Acta 1982, 64, L211.
X Abstract published in Advance ACS Abstracts, September 15, 1997.
(1) Tejel, C.; Villoro, J . M.; Ciriano, M. A.; Lo´pez, J . A.; Eguiza´bal,
E.; Lahoz, F. J .; Bakhmutov, V. I.; Oro, L. A. Organometallics 1996,
15, 2967.
(8) Ciriano, M. A.; Pe´rez-Torrente, J . J .; Oro, L. A. J . Organomet.
Chem. 1993, 445, 273.
(2) Collman, J . P.; Brauman, J . I.; Madonik, A. M. Organometallics
1986, 5, 310.
(3) Ciriano, M. A.; Tena, M. A.; Oro, L. A. J . Chem. Soc., Dalton
Trans. 1992, 2123.
(4) Tejel, C.; Ciriano, M. A.; Oro, L. A.; Tiripicchio, A.; Ugozzoli, F.
Organometallics 1994, 13, 4153.
(5) Tejel, C.; Ciriano, M. A.; Edwards, A. J .; Lahoz, F. J .; Oro, L. A.
Organometallics 1997, 16, 45.
(9) Ferna´ndez, M. J .; Modrego, J .; Lahoz, F. J .; Lo´pez, J . A.; Oro, L.
A. J . Chem. Soc., Dalton Trans. 1990, 2587.
(10) (a) Pinillos, M. T.; Elduque, A.; Lo´pez, J . A.; Lahoz, F. J .; Oro,
L. A. J . Chem. Soc., Dalton Trans. 1991, 1391. (b) Pinillos, M. T.;
Elduque, A.; Oro, L. A.; Lahoz, F. J .; Bonati, F.; Tiripicchio, A.;
Tiripicchio-Camellini, M. J . Chem. Soc., Dalton Trans. 1990, 989.
(11) Mayanza, A.; Bonnet, J .-J .; Galy, J .; Kalk, P.; Poilblanc, R. J .
Chem. Res., Synop. 1980, 146, J . Chem. Res., Miniprint 1980, 2101.
S0276-7333(97)00347-6 CCC: $14.00 © 1997 American Chemical Society

Metal-Metal-Bonded Dinuclear Rh and Ir Complexes
Organometallics, Vol. 16, No. 21, 1997 4719
(L ) 1,8-diamidonaphthalene) with diiodine.12 The
formation of a metal-metal bond in these reactions has
been associated to the nature of the metal (favored for
the transition metals in the third-row versus those in
the second-row) and to the flexibility of the framework
of metals and bridging ligands.13,14 Noticeably, further
breaking of the metal-metal bond is unusual, although
it has been reported for some iridium complexes with
SBut.6b,10b
We now present our reactivity studies of the binuclear
supernucleophiles [{M(µ-Pz)(CNBut)2}2] (M ) Rh, Ir)
with diiodine that, in contrast to our previous findings
with methyl iodide, show the formation of metal-metal-
bonded species [{M(µ-Pz)(I)(CNBut)2}2]. These com-
plexes also result from electron-transfer reactions. We
also report reactions of the new complexes with elec-
trophiles occurring at the metal-metal bond and at the
ligands trans to this bond. The former result in addi-
tions, and the latter result in an iodide abstraction to
render a metal-metal-bonded cationic complex, which
is able to add nucleophiles such as [{M(µ-Pz)(I)(CN-
But)2}2] to give an interesting coupled tetrarhodium
cation.
Sch em e 1
When the sample is cooled at 213 K, the 1H NMR
spectrum shows the presence of two complexes in a 6:4
ratio along with free acetonitrile. Comparison of the
intensities of the coordinated acetonitrile (δ 2.75) and
that of pyrazolate (δ 7.71) along with the C2v symmetry
of the major compound allows its identification as [{Rh-
(µ-Pz)(CNBut)2(MeCN)}2]2+, while the Cs symmetry for
the minor compound indicates the formula [{Rh(µ-Pz)-
(CNBut)2}2(MeCN)(Me2CO)]2+
.
Oxidation of the iridium complex 2 with [Cp2Fe]PF6
in acetonitrile also occurs readily to give the white
complex [{Ir(µ-Pz)(CNBut)2(CH3CN)}2](PF6)2 ([6](PF6)2)
in quantitative yield. Characterization of [6](PF6)2
relies on analytical and spectroscopic data, as com-
mented for [5](PF6)2. The acetonitrile ligands are more
tightly bound for the iridium complex than for the
rhodium counterpart, and thus, [6]2+ is the main
compound in solution along with small amounts of the
monosubstituted complex [{Ir(µ-Pz)(CNBut)2}2(CH3CN)-
(Me2CO)]2+. Complexes [5]2+ and [6]2+ are, thus, Rh-
(II)-Rh(II) and Ir(II)-Ir(II) species, respectively, con-
taining a metal-metal bond with labile ligands in a
trans arrangement (Scheme 1).
Cyclic voltammetry experiments on the more stable
complex 1 in acetonitrile, using NBu4PF6 as supporting
electrolyte, show the presence of one fully irreversible
oxidation peak at 201 mV (vs SCE). This is a two-
electron process since the oxidation of 1 and 2 requires
2 molar equiv of [Cp2Fe]PF6 for the reactions to go to
completion. The irreversibility in our case could be
attributed to the coordination of acetonitrile to the
oxidized species, [{Rh(µ-Pz)(CNBut)2}2]2+, which pre-
vents the regeneration of 1.
Treatment of acetone-d6 solutions of complexes 5 and
6 with a solution of KI results in the immediate
displacement of the labile ligands and formation of
complexes 3 and 4 quantitatively (1H NMR evidence).
Not surprisingly, addition of LiCl to a solution of 5 in
acetone-d6 gives the previously reported3 complex [{Rh-
(µ-Pz)(Cl)(CNBut)2}2] (7). The separation of the oxida-
tive addition of iodine to 1 and 2 in two fomal steps,
the chemical oxidation and the capture of the halide
ligands, clearly indicates that the formation of the
metal-metal bond has the nature of an electron-
transfer (ET) reaction.
In ter p r eta tion of th e Differ en ces in th e Oxid a -
tive-Ad d ition Rea ction s of MeI a n d Diiod in e. To
form the homovalent M(II)-M(II) complexes, the two
electrons should be removed from the HOMO, and in
this way the metals act in a cooperative way, while the
oxidation of a single metal center would lead to the
mixed-valence M(I)-M(III) complexes. At this point,
what produces the different type of behavior shown by
the reactions of the [{M(µ-Pz)(CNBut)2}2] (M ) Rh, Ir)
systems with MeI and with diiodine? The difference in
the type of products should be attributed to the distinc-
tive types of reaction, which is determined by the
characteristics of the substrates. Thus, MeI has one
Resu lts a n d Discu ssion
Oxid a tive-Ad d ition a n d Oxid a tion Rea ction s
Lea d in g to Meta l-Meta l Bon d F or m a tion . The
binuclear complexes [{M(µ-Pz)(CNBut)2}2] (M ) Rh (1),
Ir (2)) react with 1 molar equiv of diiodine to give the
complexes [{M(µ-Pz)(I)(CNBut)2}2] (M ) Rh (3), Ir (4)),
which are isolated as air-stable orange and yellow solids,
respectively. The elemental analysis indicates the
1
addition of only 1 mol of diiodine, and the H and 13C-
{1H} NMR spectra are consistent with the presence of
a single isomer of C2v symmetry. Therefore, complexes
3 and 4 are formulated as the M(II)-M(II) (d7-d7)
binuclear derivatives, where its diamagnetism indicates
the spin-pairing via metal-metal bond formation and
the iodo ligands are trans to the metal-metal bond.
According to the oxidation of both metal centers, the
two observed ν(CN) frequencies are ca. 87 cm-1 higher
than in the starting M(I) compounds.
The reactions leading to 3 and 4 could be considered
a two-electron transfer (ET) process giving the cation
[{M(µ-Pz)(CNBut)2}2]2+ followed by the incorporation of
the iodide ligands. As no kinetic measurements are
available, since the reactions are very fast, we have
studied the proposed redox process and the subsequent
incorporation of the iodide ligands separately.
Ch em ica l Oxid a t ion s Lea d in g t o Met a l-Met a l
Bon d F or m a tion . Addition of a mild oxidant such as
[Cp2Fe]PF6 to a solution of 1 in acetonitrile gives the
yellow complex [{Rh(µ-Pz)(CNBut)2(CH3CN)}2](PF6)2
([5](PF6)2) in high yield. Solutions of [5](PF6)2 in acetone
or acetonitrile show molar conductivity values typical
for a 2:1 electrolyte. The acetonitrile ligands are quite
labile in solution. Broad signals for the pyrazolate
1
ligands are observed in the H NMR spectrum at room
temperature due to the rapid exchange on the NMR
time scale of acetonitrile with the acetone-d6 solvent.
(12) Oro, L. A.; Ferna´ndez, M. J .; Modrego, J .; Foces-Foces, C.; Cano,
F. H. Angew. Chem., Int. Ed. Engl. 1984, 23, 913.
(13) Fackler, J . P., J r. Polyhedron 1997, 16, 1.
(14) He, X.; Maisonnat, A.; Dahan, F.; Poilblanc, R. Organometallics
1991, 10, 2443.

4720 Organometallics, Vol. 16, No. 21, 1997
Tejel et al.
Sch em e 2
F igu r e 1. Molecular representation of the cation [{Rh2(µ-
Pz)2(I)(CNBut)4}2(µ-I)]CF3SO3, [8]+, with the numbering
scheme used. Some carbon atoms are unlabeled for clarity.
normal van der Waals distances. The molecular struc-
ture of the tetranuclear cation [8]+ (Figure 1) contains
two binuclear “Rh2(µ-Pz)2(I)(CNBut)4” units linked
through a slightly asymmetric bridging iodine atom.
Within each binuclear moiety the two metals exhibit a
clear Rh-Rh bond doubly bridged by two bidentate
pyrazolate ligands. Each metal completes a distorted
octahedral environment with two terminal CNBut groups
and two iodide atoms (one terminal and one bridging)
located nearly opposite to the metal-metal bond (Rh-
Rh-Iterm av. 163.51(3)°; Rh-Rh-Ibridg av. 162.03(3)°).
Although chemically equivalent, the two binuclear
moieties Rh2(µ-Pz)2(I)(CNBut)4 exhibit statistically dif-
ferent geometrical parameters, with the most evident
distinctions affecting to the Rh-I and Rh-Rh bond
distances (see Table 1).
The two Rh2N4 metallacycles adopt strongly puckered
boat conformations (av. Q ) 1.536(5) Å; θ ) 89.9(3)°, φ
) 0.0(3)°),16 allowing short Rh-Rh separations. These
intermetallic distances, 2.6320 and 2.6057(10) Å, com-
pare well with those observed in other related Rh(II)-
Rh(II) pyrazolate-bridged complexes with postulated
Rh-Rh single bonds, such as [Rh(η5-C5H5)(µ-Pz)]2 (2.657-
(3) Å),17 [Rh2(µ-Pz)2(I)2(CO)2(µ-dppm)] (2.612(3) Å),18 and
[Rh2(µ-Pz)2(µ-pc)(Br)(CO)(pcBr)] (2.581(1) Å) (pcBr )
P(o-BrC6F4)Ph2).19 Besides these comparative values,
the formation of the intermetallic bond is clearly cor-
roborated by the large shortening of the intermetallic
distance (1.28 Å; from 3.8996(6) Å in [{Rh(µ-Pz)(CN-
But)2}2] to 2.6189(7) Å) upon the oxidative-addition
reaction, by no means attributable to ligand steric
requirements. Although other related binuclear pyra-
zolato-bridged rhodium complexes have also shown the
exceptional flexibility of the Rh(µ-Pz)Rh framework,10b
our example displays, as far as we know, the greatest
change in the intermetallic separation upon the forma-
tion of the metal-metal bond.
polarized I-C bond favorable for nucleophilic substitu-
tion by electron-rich metal centers, while diiodine is an
oxidizing agent. The addition to a single metal center,
as shown by the reactions with MeI, is favorable for
supernucleophilic metal centers because it is a Lewis
acid-base reaction in our case. On the contrary, the
reactions with diiodine, leading to the metal-metal
bond, involve a net electron transfer. This explanation
could also be appropriated for the [{Ir(µ-NHR)(CO)2}2]
complex, which also shows a different behavior toward
MeI and diiodine.15
Abstr a ction of a n Iod id e Liga n d tr a n s to th e
Meta l-Meta l Bon d . F or m a tion a n d Str u ctu r e of
a Tetr a n u clea r Com p lex. Complex 3 reacts quickly
with 1 molar equiv of MeCF3SO3 in acetone to give MeI
and a dark-red solution of the cationic complex [Rh2(µ-
Pz)2(I)(CNBut)4]CF3SO3 ([A]CF3SO3) (Scheme 2). Our
initial attempts to isolate this complex only resulted in
oils. However, the cation can be “trapped” as the adduct
of [A]+ with 3 linked through an iodide ligand. Thus,
reaction of 3 with 0.5 molar equiv of MeCF3SO3 gives
dichroic red-green monocrystals of [{Rh2(µ-Pz)2(I)-
(CNBut)4}2(µ-I)]CF3SO3 ([8]CF3SO3) in good yield. The
analytical data of [8]CF3SO3 are in accordance with the
proposed formula, and a definitive confirmation of its
structure is obtained from an X-ray diffraction study.
Abstraction of the iodide ligand trans to the metal-
metal bond by the cation Me+ (as MeCF3SO3) is a
method to create vacant coordination sites at this
position, which can be covered by nucleophiles at this
exo site of the complex. One of these nucleophiles is
complex 3. However, there are two sites in the bi-
nuclear complex 3 susceptible for an electrophilic attack,
i.e., the metal-metal bond and the iodide ligands. The
preference for the exo site by the Me+ cation (as MeCF3-
SO3) should be attributed to the polar Rh-I bond, which
facilitates the reaction, and to the difficulty of the
methyl group to enter the endo site bridging the metal
atoms.
As expected, the bridging Rh-I distances, 2.7900(10)
and 2.8124(10) Å, are significantly longer than those of
the terminal iodide ligands, 2.7545(10) and 2.7279(10)
Å. The latter ones are similar to those reported for the
complexes [Rh2(µ-Pz)(I)2(CNBut)2(µ-dppm)2]+ (2.738(3)
(16) Cremer, D.; Pople, J . A. J . Am. Chem. Soc. 1975, 97, 1354.
(17) Bailey, J . A.; Grundy, S. L.; Stobart, S. R. Organometallics 1990,
9, 536.
(18) Oro, L. A.; Pinillos, M. T.; Tiripicchio, A.; Tiripicchio-Camellini,
M. Inorg. Chim. Acta 1985, 99, L13.
The crystal structure of [8]CF3SO3 consists of tetra-
nuclear rhodium(II) cations and triflate anions at
(15) Kolel-Veetil, M. K.; Rheingold, A. L.; Ahmed, K. J . Organome-
tallics 1993, 12, 3439.
(19) Barcelo´, F.; Lahuerta, P.; Ubeda, M. A.; Foces-Foces, C.; Cano,
F. H.; Mart´ınez-Ripoll, M. J . Chem. Soc., Chem. Commun. 1985, 43.

Metal-Metal-Bonded Dinuclear Rh and Ir Complexes
Organometallics, Vol. 16, No. 21, 1997 4721
(109.5°),22 and the strong steric requirements of the
ligands bonded to bridged metals, Rh(2) and Rh(3).
Ta ble 1. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for th e Ca tion [8]+ a
Rh(1)-Rh(2)
Rh(1)-I(1)
2.6320(10)
2.7545(10)
2.7900(10)
2.042(8)
2.031(7)
2.038(8)
Rh(3)-Rh(4)
Rh(4)-I(2)
2.6057(10)
2.7279(10)
2.8124(10)
2.061(8)
2.038(8)
2.028(7)
The terminal tert-butyl isocyanide groups exhibit
Rh-C bond distances in the range 1.896(11)-1.958(12)
Å (mean 1.925(8) Å) and maintain linear dispositions
around the coordinated carbon atoms (mean Rh-C-N,
174.1(7)°). These bonding parameters compare well
with those reported for two closely related rhodium(II)
dimers [Rh2(µ-Pz)(Cl)2(CNBut)2(µ-dppm)2]+ (Rh-C,
1.933(5) and 1.940(5) Å; Rh-C-N, 176.3(4) and
172.8(5)°)24 and [Rh2(µ-Pz)(I)2(CNBut)2(µ-dppm)2]+ (Rh-
C, 1.92(2) and 1.90(2) Å; Rh-C-N, 176.3(16) and
175.6(16)°),20 where the isocyanide ligands are also trans
disposed to bridging pyrazolate groups. The values
reported in the latter two compounds for the Rh-
N(pyrazolate) bonds are 2.069(5) and 2.067(5) Å24 and
2.056(12) and 2.054(11) Å,20 also in a good agreement
with the bond distances determined in [8]+, in the range
2.028(7)_2.061(8) Å (mean 2.039(4) Å). However, if
compared with the rhodium(I) parent compound [Rh-
(µ-Pz)(CNBut)2]2 (av. Rh-C 1.889(4); Rh-N 2.084(2) Å)1
or the related complex [(cod)Rh(µ-Pz)2Rh(CNBut)2] (av.
Rh-C 1.883(8); Rh-N 2.069(4) Å),1 these distances
reflect the different (lower) electronic density of the
metals, which causes slightly longer Rh-C and shorter
Rh-N bond distances in [8]+.
Electr on ic Str u ctu r e of Folded Bin u clear Rh (II)-
Rh (II) Sp ecies. In an attempt to gain insight into the
intermetallic bonding system changes occurring on the
binuclear rhodium species along the oxidative addition
of diiodine to 1, we have examined the electronic
structures of several potential intermediates at a quali-
tative level.
The electronic structure of dirhodium(II) complexes
(formally Rh24+) has been extensively studied at differ-
ent sophistication levels from a theoretical point of view
using different approaches25-32 and have been usually
contrasted and complemented with photoelectronic or
ESR spectroscopic experiments.33-36 However, most of
these studies have been concentrated on complexes of
the face-to-face type where the two metals, bridged by
four bidentate ligands, maintain their coordination
planes practically parallel. For these rhodium lantern-
type complexes, most of them containing bridging O-
donor ligands and no axial groups (or with weak σ-donor
as axial ligands), it now seems to be accepted that there
Rh(2)-I(3)
Rh(3)-I(3)
Rh(1)-N(1)
Rh(2)-N(2)
Rh(1)-N(3)
Rh(2)-N(4)
Rh(1)-C(7)
Rh(1)-C(12)
Rh(2)-C(17)
Rh(2)-C(22)
Rh(4)-N(10)
Rh(3)-N(9)
Rh(4)-N(12)
Rh(3)-N(11)
Rh(4)-C(43)
Rh(4)-C(48)
Rh(3)-C(33)
Rh(3)-C(38)
2.051(8)
2.031(8)
1.958(12)
1.896(11)
1.929(11)
1.935(11)
1.927(10)
1.949(11)
1.910(12)
1.898(11)
Rh(2)-Rh(1)-I(1)
Rh(2)-Rh(1)-N(1)
Rh(2)-Rh(1)-N(3)
161.31(4) Rh(3)-Rh(4)-I(2)
71.6(2) Rh(3)-Rh(4)-N(10)
71.3(2) Rh(3)-Rh(4)-N(12)
165.71(4)
71.9(3)
72.1(2)
Rh(2)-Rh(1)-C(12) 105.1(3) Rh(3)-Rh(4)-C(43) 100.1(3)
Rh(2)-Rh(1)-C(7)
I(1)-Rh(1)-N(1)
I(1)-Rh(1)-N(3)
I(1)-Rh(1)-C(7)
I(1)-Rh(1)-C(12)
N(1)-Rh(1)-N(3)
N(1)-Rh(1)-C(7)
N(1)-Rh(1)-C(12)
104.6(3) Rh(3)-Rh(4)-C(48) 102.9(3)
95.4(2) I(2)-Rh(4)-N(10)
95.4(2) I(2)-Rh(4)-N(12)
89.1(3) I(2)-Rh(4)-C(43)
87.7(3) I(2)-Rh(4)-C(48)
100.5(3)
95.9(2)
87.0(3)
88.5(3)
87.5(3) N(10)-Rh(4)-N(12) 88.0(3)
174.9(4) N(10)-Rh(4)-C(43) 171.9(4)
89.8(4) N(10)-Rh(4)-C(48)
87.6(4)
176.1(4)
94.5(4)
95.7(4)
165.89(4)
71.7(2)
N(12)-Rh(4)-C(43) 88.1(3) N(3)-Rh(1)-C(12)
N(12)-Rh(4)-C(48) 174.2(4) N(3)-Rh(1)-C(7)
C(7)-Rh(1)-C(12)
Rh(1)-Rh(2)-I(3)
Rh(1)-Rh(2)-N(2)
Rh(1)-Rh(2)-N(4)
88.0(5) C(43)-Rh(4)-C(48)
158.16(4) Rh(4)-Rh(3)-I(3)
71.4(2) Rh(4)-Rh(3)-N(9)
71.7(2) Rh(4)-Rh(3)-N(11)
71.9(2)
Rh(1)-Rh(2)-C(17) 101.5(3) Rh(4)-Rh(3)-C(33) 104.2(3)
Rh(1)-Rh(2)-C(22) 100.7(3) Rh(4)-Rh(3)-C(38) 100.3(3)
I(3)-Rh(2)-N(2)
I(3)-Rh(2)-N(4)
I(3)-Rh(2)-C(17)
I(3)-Rh(2)-C(22)
N(2)-Rh(2)-N(4)
90.7(2) I(3)-Rh(3)-N(9)
95.6(2) I(3)-Rh(3)-N(11)
95.8(3) I(3)-Rh(3)-C(33)
91.2(3) I(3)-Rh(3)-C(38)
87.0(3) N(9)-Rh(3)-N(11)
98.9(2)
97.7(2)
85.3(3)
90.3(3)
87.6(3)
175.9(4)
91.9(4)
91.8(4)
N(2)-Rh(2)-C(17) 172.6(4) N(9)-Rh(3)-C(33)
N(2)-Rh(2)-C(22)
N(4)-Rh(2)-C(17)
89.2(3) N(9)-Rh(3)-C(38)
89.0(3) N(11)-Rh(3)-C(33)
N(4)-Rh(2)-C(22) 172.3(3) N(11)-Rh(3)-C(38) 172.0(4)
C(17)-Rh(2)-C(22)
Rh(2)-I(3)-Rh(3)
94.0(4) C(33)-Rh(3)-C(38)
129.99(4)
88.1(5)
a
For comparison, each column collects related geometrical
parameters for each analogous, but crystallographically indepen-
dent, dinuclear moiety “Rh2(µ-Pz)2(I)(CNBut)4”.
and 2.766(6) Å)20 and [Rh2(µ-3,5-Me2Pz)(I)2(CO)2(µ-
dppm)2]+ (2.757(2) Å)21 where the iodine atoms are trans
disposed to the Rh-Rh bond. Nevertheless, both Rh-I
distances are significantly longer than the average
values observed for terminal or bridging iodide ligands
in rhodium(III) complexes (2.71(7) and 2.73(1) Å, re-
spectively),22 probably as a consequence of the structural
trans-effect associated to the metal-metal bonds.23 The
unusual coordination mode of the bridging iodide rep-
resents the first case in Rh or Ir chemistry where a
iodine atom acts as the only ligand connecting two
nonbonded metals. The unexpected value of the Rh-
I-Rh bond angle, 129.99(4)°, probably represents a
compromise between the general tendency observed
when a single iodine atom acts as the sole bridging
ligand between nonbonded metals in molecular crystals,
with reported values close to an ideal sp3 hybridization
(24) Tortorelli, L. J .; Woods, C.; McPhail, A. T. Inorg. Chem. 1990,
29, 2726.
(25) Bursten, B. E.; Cotton, F. A. Inorg. Chem. 1981, 20, 3042.
(26) Le, J . C.; Chavan, M. Y.; Chau, L. K.; Bear, J . L.; Kadish, K.
M. J . Am. Chem. Soc. 1985, 107, 7195.
(27) Poblet, J . M.; Benard, M. Inorg. Chem. 1988, 27, 2935.
(28) Bear, J . L.; Yao, C.-L.; Liu, L.-M.; Capdevielle, F. J .; Korp, J .
D.; Albright, T. A.; Kang, S.-K.; Kadish, K. M. Inorg. Chem. 1989, 28,
1254.
(29) Kawamura, T.; Katayama, H.; Nishikawa, H.; Yamabe, T. J .
Am. Chem. Soc. 1989, 111, 8156.
(30) Cotton, F. A.; Feng, X. Inorg. Chem. 1989, 28, 1180.
(31) Natkaniec, L.; Pruchnik, F. P. J . Chem. Soc., Dalton Trans.
1994, 3261.
(32) Boyd, D. C.; Connelly, N. G.; Herbosa, G. G.; Hill, M. G.; Mann,
K. R.; Mealli, C.; Orpen, A. G.; Richardson, K. E.; Rieger, P. H. Inorg.
Chem. 1994, 33, 960.
(33) Kawamura, T.; Fukamachi, K.; Sowa, T.; Hayashida, S.; Yon-
ezawa, T. J . Am. Chem. Soc. 1981, 103, 364.
(34) Rizzi, G. A.; Casarin, M.; Tondello, E.; Piraino, P.; Granozzi,
G. Inorg. Chem. 1987, 26, 3406.
(20) Carmona, D.; Oro, L. A.; Pe´rez, P. L.; Tiripicchio, A.; Tiripicchio-
Camellini, M. J . Chem. Soc., Dalton Trans. 1989, 1427.
(21) Oro, L. A.; Carmona, D.; Pe´rez, P. L.; Esteban, M.; Tiripicchio,
A.; Tiripicchio-Camellini, M. J . Chem. Soc., Dalton Trans. 1985, 973.
(22) Allen, F. H.; Davies, J . E.; Galloy, J . J .; J ohnson, O.; Kennard,
O.; Macrae, C.F.; Mitchell, E. M.; Mitchell, G. F.; Smith, J . M.; Watson,
D. G. J . Chem. Inf. Comput. Sci. 1991, 31, 187.
(35) Best, S. P.; Clark, R. J . H.; Nightingale, A. J . Inorg. Chem. 1990,
29, 1383.
(23) Laguna, A.; Laguna, M.; J ime´nez, J .; Lahoz, F. J .; Olmos, E. J .
Organomet. Chem. 1992, 435, 235.
(36) Stranger, R.; Medley, G. A.; McGrady, J . E.; Garret, J . M.;
Appleton, T. G. Inorg. Chem. 1996, 35, 2268.

4722 Organometallics, Vol. 16, No. 21, 1997
Tejel et al.
F igu r e 3. Schematic interaction diagram of the I-Rh-
Rh-I σ-axial bonding system calculated from the interac-
tion of [Rh2(µ-Pz)2(CNBut)4]2+ with two axial iodide anions.
Only level d is empty.
F igu r e 2. Molecular orbital diagram calculated for the
species (a) [Rh2(µ-Pz)2(CNBut)4]2+, (b) [Rh2(µ-Pz)2(I)-
(CNBut)4]+, and (c) [Rh2(µ-Pz)2(I)2(CNBut)4].
orbital mixes with the pσ orbitals of the iodides, giving
rise to a low-energy totally-bonding orbital and a second
high-energy orbital with metal-metal bonding and
metal-iodide antibonding character (Figures 3a and 3c,
respectively), the latter one becoming the HOMO of the
molecule. As these two new MOs are fully occupied,
they do not substantially contribute to the metal-
halogen bond order. On the other hand, the antisym-
metric combination of the iodide pσ orbitals with the
empty metallic σ* orbital forms an occupied low-energy
orbital with a metal-metal antibonding and metal-
halogen bonding character (Figure 3b) and a completely
empty antibonding orbital (Figure 3d).37 From these
qualitative considerations, we can assume in a Mulliken
sense a bonding order of 0.5 for the Rh-I bond. This
fact explains the lability of acetonitrile or iodide groups
when bonded as axial ligands in 5 or 3 and the
particularly long Rh-I distances determined in the
X-ray analysis of [8]+. It is quite remarkable to note
how the MO arrangement occurring when the binuclear
species [Rh2(µ-Pz)2(CNBut)4]2+ adds two axial iodides
does not substantially modify the Rh-Rh bonding
system (Mulliken population: 0.21-0.27 e-) but it
stabilizes the M-axial ligand interaction through the
formation of a relatively weak bond. That is, in other
words, the electronic basis of the commonly referred
strong structural trans-effect of the σ Rh-Rh bonds.
The most interesting results are obtained when the
monocationic asymmetric [Rh2(µ-Pz)2(I)(CNBut)4]+ moi-
ety is analyzed. From a qualitative point of view, the σ
and σ* metal orbitals interact with an axial ligand donor
function, giving up a totally-bonding, a totally-antibond-
ing, and a nonbonding orbital.28 Our EHMO calculation
shows that this intermediate level presents a slight
metal-metal bonding and metal-halogen antibonding
character and additionally a large polarization toward
the formally pentacoordinated rhodium atom (Figure
4): 68% of the electron density is concentrated on this
atom, 6% on the octahedral rhodium atom, and 14% on
the iodine ligand. A similar polarization of the Rh-Rh
bond has been observed in the analysis of [Rh2{(NH)2-
CH}4(CH3CN)]+ 28 and [Rh2(tcl)4(CO)]+ (tcl ) ω-thioca-
prolactamate).27 This electron transfer along the I-Rh-
Rh system leads to a dramatic difference in the calculated
Mulliken charges for the metal atoms: +0.67 e- for that
linked to the halogen and +0.28 e- for the pentacoor-
exists a single σ metal-metal bond, corresponding to
the usual electronic configuration σ2π4δ2δ*2π*4 for the
Rh24+ unit.27 With strong σ-donors as the axial ligands,
the σ level is strongly destabilized and it could become
the HOMO.25,31,35 In any case, the only unoccupied
metal-metal orbital is the antibonding σ*, leading to
an intermetallic bond order of one.28-30
The substitution of bridging O-donor ligands by
bidentate N-donor groups causes the δ* and σ levels to
rise in energy, while π and π* are partially stabilized,
4+
resulting in an electronic configuration for the Rh2
unit of σ2π4δ2π*4δ*2, with the σ and π levels quite close
in energy. These changes arise from the increased
donor ability of the nitrogen atoms and the absence of
the oxygen lone pairs which were responsible for the
destabilization of the π and π* orbitals.28-30,34
In our analysis, based on extended Hu¨ckel molecular
orbital (EHMO) calculations, we have studied the
electronic structure of the dication [Rh2(µ-Pz)2(CNBut)4]2+
first. Using a simplified and symmetric model based
on the solid state geometry observed for [8]+ (see
Experimental Section), we have carried out our calcula-
tions on a Rh(II)-Rh(II) binuclear species (formally
Rh24+) without any axial iodide ligand. The relative
energy order of the intermetallic MOs (Figure 2a) is
similar to those reported for the face-to-face type com-
plexes, except for the δ and δ* levels which become
nearly degenerate. This situation is probably due to the
folded geometry of our compounds, which exhibit the
two metal coordination planes not parallel (R ) 44°),1
and to the comparatively increased metal-metal dis-
tance. These geometrical constraints restrain the over-
lap among all orbitals of the two metal centers but affect
to a greater extent the δ orbitals. Additionally, these
two orbitals (δ, δ*) are also destabilized by combination
with the pyrazolate π orbitals. In spite of the com-
mented differences, the electronic configuration for
Rh24+, σ2π4π*4δ2δ*2, maintains an empty σ* orbital, and
hence, a single metal-metal bond should be also present
in the examined binuclear moiety. As a point of
reference, the calculated Mulliken population between
both metals, 0.21 e-, also reflects the intermetallic bond.
Further interaction of the two iodides as axial ligands
with the binuclear entity [Rh2(µ-Pz)2 (CNBut)4]2+ modi-
fies the electronic structure of the binuclear moiety to
a situation very similar to that described by Cotton et
al. for the adducts formed by tetrakis(carboxylato)-
dirhodium(II) complexes and strong σ-donors.25 As the
iodide ligands approach the two metals, the metallic σ
(37) The performed calculations also identify the interactions be-
tween the π and π* metal orbitals with those of the iodide ligands of
appropiate symmetry (pπ). However, all of the resulting molecular
orbitals are under the HOMO level, making their influence in the Rh-
Rh or Rh-I bonding system negligible (see ref 31).

Metal-Metal-Bonded Dinuclear Rh and Ir Complexes
Organometallics, Vol. 16, No. 21, 1997 4723
F igu r e 4. Cacao drawing of the “nonbonding” σ orbital of
the I-Rh-Rh system.
dinated one. As a reference, the calculated charge for
the symmetric compound with two axial iodo ligands is
around +0.38 e-. This polarization induces a partial
disproportionation of the dirhodium Rh(II)-Rh(II) core
toward an asymmetric structure. Hence, the metal-
metal bond could be described as a dative bond from
the pentacoordinated to the octahedral metal (formally
I-Rh(III)-Rh(I)). In this situation, the cationic species
[Rh2(µ-Pz)2(I)(CNBut)4]+ seems to be susceptible of
nucleophilic attack at the terminal pentacoordinated
metal center: the coordination vacancy present together
with the remaining positive charge (+0.28 e-) support
this proposal.
F igu r e 5. Mass spectrum from the reaction mixture of
complex 7 with I2, showing the peaks for the cations [9]+,
[11]+, and [12]+ and the losses of halide.39
(Cl)(CNBut)4]+ ([12]+; Cs), and [{Rh(µ-Pz)(I)(CNBut)2}2-
(µ-I)]+ ([9]+). Figure 5 shows the MS (FAB+) of this
mixture of complexes. Noticeably, the terminal halogen
ligands are ionized in the mass spectrometer, but the
bridging iodo ligand remains strongly attached in the
cations, which is an additional useful observation to
characterize the compounds.
The cations [12]+ and [9]+ result from a gradual
replacement of the chloro ligands in [11]+ by the iodide
present in the reaction medium. Thus, the formation
of [11]+ is confirmed by carrying out the reaction of 7
with diiodine followed by the addition of methyltriflate,
which removes the ionic iodide. Furthermore, the
replacement of the chloro ligands in [11]+ and [12]+ by
iodide, to give [9]+, is immediate on addition of KI to
the above solution in acetone-d6. The lability of the
halogen ligands in Rh(III) species has already been
observed by Collman.2 In conclusion, the reaction of 7
with diiodine initially leads to [11]I, a complex with an
iodide ligand bridging the metals formerly bonded in
the starting material.
Ten ta tive Mech a n ism for th e Ru p tu r e of th e
Meta l-Meta l Bon d by Electr op h iles. The addition
of a iodine atom to the metal-metal bond in the pocket
of complex of 7 is indicative of a selective reaction
pathway, since six isomers could result from this
reaction. On the other hand, the oxidation of 7 by
diiodine is chemically inaccessible, since the cyclic
voltammetry of 7 shows an irreversible oxidation peak
at 1334 mV (vs SCE) in CH2Cl2, a value much larger
than that for diiodine.38 Therefore, a reaction pathway
other than a direct electron transfer should be proposed.
The reaction of diiodine with 7, as well as with 3 and
4, can be envisaged as the addition of one halogen atom
at the metal-metal bond. A reasonable mechanism
requires the interaction of a diiodine molecule with the
electronic density in the M-M metal bond, as shown in
Scheme 3 for 7. This hypothetical transition state (B)
could lead to the heterolytic cleavage of the I-I bond,
resulting in an iodide ligand bridging two rhodium
atoms, i.e., the cation [11]+ and an iodide anion. This
proposal involves the actuation of both metals in a
cooperative way, such that the metal-metal bond is
acting as a nucleophile. The transition state resembles
This description allows an easy understanding of the
formation of the tetranuclear cation [8]+ through a
nucleophilic attack of the binuclear complex [Rh2(µ-Pz)2-
(I)2(CNBut)4] to the cationic species [Rh2(µ-Pz)2(I)-
(CNBut)4]+.
Ad d it ion of E lect r op h iles t o t h e Met a l-Met a l
Bon d . Reactions of 3 and 4 with 1 molar equiv of
diiodine give the corresponding M(III)-M(III) complexes
analyzing as [{M(µ-Pz)(I)2(CNBut)2}2], which are iso-
lated as air-stable red and orange crystalline solids,
respectively. A formal oxidation of the starting M(II)-
M(II) complexes is evidenced by the shift of ν(CN) by
ca. 50 cm-1 to higher frequencies. The C2v symmetry
of the complexes, deduced from the 1H and 13C{1H}
NMR spectra, agrees with a boat conformation of the
six-membered Rh2(N-N)2 ring, but the low solubility of
these compounds in polar solvents makes the conductiv-
ity measurements inconclusive. However, they should
be formulated as [{M(µ-Pz)(I)(CNBut)2}2(µ-I)]I (M ) Rh,
[9]I; Ir, [10]I), i.e., ionic and containing just three
bridging ligands because of their reactions with methyl
triflate, which indicate their ionic nature. Thus, the
formation of MeI is observed immediately after mixing
[9]I and [10]I with MeCF3SO3, while the resonances for
the cations remain unalterated. On a preparative scale,
the resulting rhodium complex [{Rh(µ-Pz)(I)(CNBut)2}2(µ-
I)]CF3SO3 ([9]CF3SO3) is isolated in good yield. Com-
plex [9]CF3SO3 shows identical spectroscopic features
to [9]I except for those of the anion and behaves as a
1:1 electrolyte in nitromethane.
To understand the pathway for the breaking of the
metal-metal bond in the M(II)-M(II) complexes, we
reacted the chloro derivative complex 7 with 1 molar
equiv of diiodine. This allows us to determine the fate
of the halide ligand (Cl) and the added halogen. The
reaction is completed in 5 min in dichloromethane, and
1
the H and 13C{1H} NMR spectra of the crude orange
solid, isolated by evaporation of the solvent to dryness,
show the presence of three complexes, namely [{Rh(µ-
Pz)(Cl)(CNBut)2}2(µ-I)]+ ([11]+; C2v), [Rh2(µ-Pz)2(µ-I)(I)-
(38) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877.
(39) The isotopic profile of the peak at 777 does not correlate to that
calculated for the cation [C26H42N8Cl3Rh2]+.

4724 Organometallics, Vol. 16, No. 21, 1997
Tejel et al.
Sch em e 3
Sch em e 4
that proposed for the electrophilic additions of iodine
to carbon-carbon double bonds.
P h otoch em ica l Rea ction s Lea d in g to th e Ru p -
tu r e of th e Meta l-Meta l Bon d . Although the di-
nuclear cation [A]+ is prone to add a donor from the
medium at the vacant coordination position, our at-
tempts to isolate M(η1-XR) halocarbon adducts42 of this
cation with the iodo derivatives MeI and PhI result in
irresolvable oils. However, we were surprised that [A]+
in MeI gave rise to the oxidative-addition product
[(CNBut )2(I)Rh(µ-Pz)2(µ-I)Rh(Me)(CNBut )2]CF3SO3
([14]CF3SO3) (Scheme 4) in 60% yield in 1 week.
Characterization of ([14]CF3SO3) relies on the analytical
and spectroscopic data, and NOE experiments are
conclusive to localize the methyl group outside the
pocket of the complex.
As the addition of MeI to the cation ([A]+) is unlikely,
we investigated this reaction further to find that it
actually requires the irradiation with the direct sunlight
or with an intense visible lamp. Examination of the
crude product revealed the presence of [14]CF3SO3 as
the main component, along with [9]CF3SO3 and
[(CNBut)2(Me)Rh(µ-Pz)2(µ-I)Rh(Me)(CNBut)2]CF3SO3
([15]CF3SO3) as impurities. One can speculate that the
adduct of MeI with the dinuclear cation [A]+ could be
the intermediate for an inner-sphere electron transfer,
although it is not actually a typical decomposition
pathway taken by halocarbon complexes.43 However,
this hypothesis was ruled out because irradiation of
complex 3 in MeI under identical conditions as those
above gives [14]I even more cleanly. Small amounts
(<10%) of [8]I and [15]I are also found in the crude
product.
Definitive support for the nucleophilic attack by the
metal-metal bond to the diiodine molecule leading to
heterolytic rupture of the I-I bond comes from the
reactions of 3, 4, and noticeably 7 with the cationic
complex [I(Py)2]BF4, which possesses a positive iodine
atom. They quantitatively give the complexes [9]BF4,
[10]BF4, and [11]BF4, respectively, and pyridine (Py) (1H
NMR evidence) in which the added iodine(I) bridges the
rhodium atoms.
According to the proposed mechanism, complex 3
should react with organoiodo compounds in which the
iodine atom possesses a small positive charge, such as
I-CtC-Ph.40 This reaction gives an orange-brown
solid analyzing as [(CNBut)2(I)Rh(µ-Pz)2(µ-I)Rh(η1-CtC-
Ph)(CNBut)2]I ([13]I). The 1H and 13C{1H} NMR spectra
of the solid indicate that [13]I is contaminated by small
amounts of two other products possessing C2v symmetry.
One of them is [{Rh(µ-Pz)(I)(CNBut)2}2(µ-I)]I ([7]I),
identified by comparison of the spectra with those of the
characterized compound, and the other probably corre-
sponds to the dialkynyl cation [{Rh(µ-Pz)(η1-CtCPh)-
(CNBut)2}2(µ-I)]+. The terminal η1-coordination of the
alkynyl group in [13]+ is deduced from the Cs symmetry
of the complex, along with the observation of two
acetylenic carbons in the 13C{1H} NMR spectrum dis-
tinctively coupled with a single 103Rh nucleus.41
The reaction between 3 and I-CtC-Ph should occur
via the nucleophilic attack of the metal-metal bond to
the positive iodine atom, following the halogen transfer,
to give [9]+ and Ph-CtC- as the counteranion. Indeed,
complex [9]+ is the initial product observed by monitor-
The UV-vis spectrum of 3 shows two absorption
bands in the visible region at λ ) 450 and 395 nm.
Absorption bands in this region have been assigned to
a dσ-dσ* transition from the dσ2 ground state of the
1
ing the reaction by H NMR spectroscopy. Phenylac-
etelyde reacts further with [9]+ replacing one or two
terminal iodide ligands to give the observed mixture,
essentially containing [13]+ and small equimolecular
amounts of [9]+ and [{Rh(µ-Pz)(η1-CtCPh)(CNBut)2}2-
(µ-I)]+.
d7 configuration systems.44 Therefore, the triplet ex-
2
cited state dσ1dσ*1 is easily reached on irradiation, and
this excited species should react with MeI. Surprisingly,
a high selectivity is observed for this photochemical
reaction.
As expected from this mechanism, complex 3 does not
react with MeI in the dark, since the iodine atom
supports a partial negative charge in the I-Csp bond
3
and the methyl group is not electrophilic enough to
interact with 3. In summary, complex 3 reacts at the
exo site with a strongly electrophilic methyl group such
as MeCF3SO3, while the endo site, i.e., the metal-metal
bond in 3, is the preferred position for iodine and
electrophilic iodine compounds.
Con clu sion s
The complexes [{M(µ-Pz)(CNBut)2}2] (M ) Rh (1), Ir
(2); Pz ) pyrazolate) are capable of recognizing the
different nature of their reactions with MeI and diiodine,
(42) (a) Butts, M. D.; Scott, B. L.; Kubas, G. J . J . Am. Chem. Soc.
1996, 118, 11831. (b) Kulawiec, R. J .; Crabtree, R. H. Coord. Chem.
Rev. 1990, 99, 89.
(43) Peng, T.-S.; Winter, C. H.; Gladysz, J . A. Inorg. Chem. 1994,
33, 2534.
(40) (a) Ghassemzadeh, M.; Harms, K.; Dehnicke, K. Chem. Ber.
1996, 129, 115. (b) Davies, J . A.; El-Ghanam, M.; Pinkerton, A. A.;
Smith, D. A. J . Organomet. Chem. 1991, 409, 367.
(41) Werner, H.; Wiedemann, R.; Mahr, N.; Steinert, P.; Wolf, J .
Chem. Eur. J . 1996, 2, 561.
(44) Bailey, J . A.; Grundy, S. L.; Stobart, S. R. Inorg. Chim. Acta
1996, 243, 47.

Metal-Metal-Bonded Dinuclear Rh and Ir Complexes
Organometallics, Vol. 16, No. 21, 1997 4725
for C26H42N8I2Rh2: C, 33.71; H, 4.57; N, 12.10. Found: C,
33.67; H, 4.28; N, 12.35. IR (CH2Cl2, cm-1): ν(CN) 2199 (s),
2173 (s). 1H NMR (room temperature, CDCl3): δ 7.59 (d, J )
1.9 Hz, 4H, H3,5Pz), 5.96 (t, J ) 1.9 Hz, 2H, H4Pz), 1.47 (s,
36H, CNBut). 13C{1H} NMR (room temperature, CDCl3): δ
since they give the M(III)-M(III) complexes either
through the mixed-valence M(I)-M(III) intermediates
or through the metal-metal-bonded complexes, respec-
tively. Our findings could be explained because of two
different properties, the nucleophilic and the reductor
characteristics of 1 and 2. It should be mentioned that,
in some instances, both substrates have been considered
alike for oxidative-addition reactions because they give
apparently analogous results.
1
138.7 (C3,5Pz), 138.0 (d, J RhC ) 56 Hz, CNBut), 104.5 (C4Pz),
58.4 (C-(CH3)3), 30.8 (C-(CH3)3). MS (FAB+, CH2Cl2, m/ z): 742
(10, M+), 707 (100, M - I+), 672 (40, M - 2I+), 1449 (10, 2M
- I+).
[{Ir (µ-P z)(I)(CNBu t)2}2] (4) was prepared as described for
3, starting from 2 (75 mg, 0.09 mmol) and I2 (22.4 mg, 0.09
mmol) to give a yellow-orange solid, which was recrystallized
twice from CH2Cl2/diethyl ether to render yellow neddles of
4, which were isolated by filtration. Yield: 70 mg (72%). Anal.
Calcd for C26H42N8I2Ir2: C, 28.26; H, 3.83; N, 10.14. Found:
C, 28.35; H, 3.88; N, 10.18. IR (CH2Cl2, cm-1): ν(CN) 2189
(s), 2151 (s). 1H NMR (room temperature, CDCl3): δ 7.45 (d,
J ) 1.9 Hz, 4H, H3,5Pz), 5.87 (t, J ) 1.9 Hz, 2H, H4Pz), 1.45 (s,
36H, CNBut). 13C{1H} NMR (room temperature, CDCl3): δ
138.5 (C3,5Pz), 104.2 (C4Pz), 58.0 (C-(CH3)3), 31.4 (C-(CH3)3).
MS (FAB+, CH2Cl2, m/ z): 1104 (56, M+), 977 (100, M - I+),
1998 (6, 2M - I - CNBut+).
The separation of an oxidative-addition reaction
across two metal atoms into two formal steps, the
chemical oxidation and the capture of the additional
ligands, indicates that this type of reaction is an
electron-transfer reaction. Once the oxidative-addition
reaction has been carried out, two reactive sites avail-
able for reactions with electrophiles remain in the
molecules [{M(µ-Pz)(X)(CNBut)2}2] (X ) Cl, I). These
are the metal-metal bond and the halogen ligands at
the trans positions. The reactions of these complexes
with diiodine leading to the breaking of the metal-
metal bond involve the electron density at this site,
which should act as a nucleophile. Probably the metal-
metal bond polarizes the diiodine molecule producing
its heterolytic cleavage. This proposal is supported by
the reaction of the complex [{Rh(µ-Pz)(I)(CNBut)2}2] (3)
with the iodine(I) complex [I(Py)2]+ at the metal-metal
bond leaving the iodine atom bridging the metal atoms.
The terminal halogen ligands in [{Rh(µ-Pz)(I)(CN-
But)2}2] are abstracted by strong electrophiles, such as
MeCF3SO3, to give the binuclear cationic complex [Rh2-
(µ-Pz)2(I)(CNBut)4]+ (A+). Notice that the latter reaction
leads to an important change in the nucleophilicity of
the metallic centers compared with complexes 1 and 3.
EHMO calculations for this cation reveal an uneven
distribution of the electron density on the metals,
resulting in a polarized Rh-Rh bond. This polarization
induces a partial disproportionation of the dirhodium
core toward an asymmetric structure, and the metal-
metal bond is best described as a dative bond from the
pentacoordinated metal to the octahedral metal (for-
mally I-Rh(III)-Rh(I)).
[{Rh (µ-P z)(CNBu t)2(CH3CN)}2](P F 6)2 ([5](P F 6)2). Solid
[Cp2Fe]PF6 (99.5 mg, 0.30 mmol) was added to a solution of 2
(100 mg, 0.15 mmol) in dry acetonitrile (10 mL) to give a pale-
orange solution almost immediately. After concentration to
1 mL, the addition of diethyl ether produces the precipitation
of [5](PF6)2. The solid was filtered, washed 3 × 10 mL of
diethyl ether to remove the ferrocene, and vacuum dried.
Yield: 140 mg (90%). Anal. Calcd for C30H48N10P2F12Rh2: C,
34.50; H, 4.63; N, 13.41. Found: C, 34.16; H, 4.27; N, 13.27.
IR (CH3CN, cm-1): ν(CN) 2224 (s), 2203 (s). 1H NMR (213 K,
acetone-d6) for [{Rh(µ-Pz)(CNBut)2(MeCN)}2]2+: δ 7.71 (d, J
) 1.7 Hz, 4H, H3,5Pz), 6.22 (t, J ) 1.7 Hz, 2H, H4Pz), 2.75 (s,
9H, CH3CN), 1.58 (s, 36H, CNBut); for [{Rh(µ-Pz)-
(CNBut)2}2(MeCN)(Me2CO)]2+: δ 7.77 (d, J ) 1.7 Hz, 2H, H3-
Pz), 7.65 (d, J ) 1.7 Hz, 2H, H5Pz), 6.23 (t, J ) 1.7 Hz, 2H,
H4Pz), 2.79 (s, 3H, CH3CN), 1.61 and 1.56 (s, 18H + 18H,
CNBut). 19F NMR (213 K, acetone-d6): δ 70.60 (d, J F-P ) 709
Hz, PF6). MS (FAB+, acetone, m/ z): 817 (4, M - 2CH3CN +
PF6+), 672 (100, M - 2CH3CN+). ΛM (4.9 × 10-4 M in acetone)
) 232 S cm2 mol-1
.
[{Ir (µ-P z)(CNBu t)2(CH3CN)}2](P F 6)2 ([6](P F 6)2) was pre-
pared as described for 5, starting from 2 (75 mg, 0.09 mmol)
and [Cp2Fe]PF6 (58.9 mg, 0.18 mmol) to give white microc-
rystals of [6](PF6)2. Yield: 100 mg (92%). Anal. Calcd for
A second pathway to break the metal-metal bond is
a photochemically assisted addition. The visible light
should promote an electron from the dσ2 ground state
of the d72 configuration to an excited state dσ1dσ*1. The
metal-metal bond is broken in the excited state, and
hence, a structure with a large metal-metal separation
should be the reactive species
C
30H48N10P2F12Ir2: C, 29.46; H, 3.96; N, 11.45. Found: C,
29.38; H, 3.53; N, 11.31. IR (CH2Cl2, cm-1): ν(CN) 2218 (s),
2181 (s). 1H NMR (213 K, acetone-d6): δ 7.45 (d, J ) 2.1 Hz,
4H, H3,5Pz), 6.17 (t, J ) 2.1 Hz, 2H, H4Pz), 2.86 (s, 9H, CH3-
CN), 1.58 (s, 36H, CNBut). MS (FAB+, acetone, m/ z): 850
(100, M - 2CH3CN+). ΛM (5.0 × 10-4 M in acetone) ) 224 S
cm2 mol-1
.
[{Rh 2(µ-P z)2(I)(CNBu t)4}2(µ-I)]CF 3SO3 ([8]CF 3SO3). To
a solution of 3 (100 mg, 0.108 mmol) in acetone (10 mL),
MeCF3SO3 (6 µL, 0.054 mmol) was added. The initial orange
solution turns dark red in 10 min. The solution was concen-
trated to ca. 3 mL and carefully layered with 20 mL of diethyl
ether to give dark red monocrystals with a green metallic
luster, suitable for X-ray diffraction studies. The solid was
filtered, washed with 2 × 10 mL of diethyl ether, and vaccum
Exp er im en ta l Section
All reactions were carried out under argon using standard
Schlenk techniques. The complexes [{M(µ-Pz)(CNBut)2}2] (M
) Rh, 1; Ir, 2) were prepared according to literature methods.1
Solvents were dried and distilled under argon before use by
standard methods. For additional general information, includ-
ing a list of the spectrophotometers and equipment used for
the physical characterization, see ref 1.
P r ep a r a t ion of t h e Com p lexes. [{R h (µ-P z)(I)(CN-
Bu t)2}2] (3). Addition of solid I2 (37.7 mg, 0.15 mmol) to a
yellow solution of [{Rh(µ-Pz)(CNBut)2}2] (1) (100 mg, 0.15
mmol) in acetone (20 mL) immediately produces an orange
solution. The solution was stirred for 30 min in the dark and
concentrated to ca. 2 mL. Slow addition of diethyl ether (15
mL) to the evaporated solution rendered compound 3 as orange
neddles, which were filtered, washed with cold diethyl ether,
and dried under vacuum. Yield: 127 mg (92%). Anal. Calcd
dried. Yield: 70 mg (69%). Anal. Calcd for C53H84N16
-
I3F3SO3Rh4: C, 33.95; H, 4.52; N, 11.95; S, 1.71. Found: C,
34.03; H, 4.46; N, 11.85; S, 2.06. MS (FAB+, CH2Cl2, m/ z):
1449 (10, M+), 707 (100, M - [3]+).
[{Rh (µ-P z)(I)(CNBu t)2}2(µ-I)]I ([9]I). The addition of solid
I2 (38.1 mg, 0.15 mmol) to an acetone (15 mL) solution of [{Rh-
(µ-Pz)(I)(CNBut)2}2] (3) (139 mg, 0.15 mmol) leads to the
immediate precipitation of a red microcrystalline solid. After
the mixture was stirred for 1 h, the suspension was concen-
trated to dryness. The residue was dissolved in 3 mL of
dichloromethane and layered with pentane (20 mL) overnight

4726 Organometallics, Vol. 16, No. 21, 1997
Tejel et al.
to render red prismatic crystals. The solution was decanted,
and the crystals were washed with cold diethyl ether and
vacuum dried. Yield: 174 mg (98%). Anal. Calcd for
([13]I). I-CtC-Ph (40 mg, 0.17 mmol) was added to a
solution of 3 (100 mg, 0.11 mmol) in dichloromethane (5 mL).
The reaction was completed in 4 h at room temperature.
Evaporation of the resulting solution to 1 mL and addition of
diethyl ether (10 mL) rendered dark-orange crystals, which
were separated by filtration, washed with diethyl ether, and
vacuum dried. Yield: 91 mg (73%). Anal. Calcd for
C
26H42N8I4Rh2: C, 26.46; H, 3.58; N, 9.49. Found: C, 26.39;
H, 3.61; N, 9.32. IR (CH2Cl2, cm-1): ν(CN) 2240 (s), 2219 (s).
1H NMR (room temperature, CDCl3): δ 8.05 (d, J ) 2.2 Hz,
4H, H3,5Pz), 6.13 (t, J ) 2.2 Hz, 2H, H4Pz), 1.61 (s, 36H,
CNBut). 13C{1H} NMR (room temperature, CDCl3): δ 145.5
C
34H47N8I3Rh2: C, 35.38; H, 4.10; N, 9.71. Found: C, 35.38;
(C3,5Pz), 120.2 (d, J RhC ) 45 Hz, CNBut), 107.0 (C4Pz), 60.3
H, 4.15; N, 9.88. IR (CH2Cl2, cm-1): ν(CN) 2235 (s), 2220 (s).
1H NMR (room temperature, CDCl3): δ 8.08 (d, J ) 2.1 Hz,
2H, H3Pz), 7.76 (d, J ) 2.1 Hz, 2H, H5Pz), 7.30 (m, 5H, Ph),
6.17 (t, J ) 2.1 Hz, 2H, H4Pz), 1.66 and 1.61 (s, 18H + 18H,
CNBut). 13C{1H} NMR (room temperature, CDCl3): δ 145.2
(C3Pz), 142.7 (C5Pz), 131.6, 128.1, 126.7, and 126.0 (Ph), 106.9
1
(C-(CH3)3), 30.2 (C-(CH3)3). MS (FAB+, CH2Cl2, m/ z): 1053
(100, M+), 926 (55, M - I+), 799 (12, M - 2I+).
[{Rh (µ-P z)(I)(CNBu t)2}2(µ-I)]CF 3SO3 ([9]CF 3SO3). To a
suspension of [8]I (100 mg, 0.085 mmol) in acetone (10 mL),
MeCF3SO3 (10 µL, 0.085 mmol) was added to give an orange
solution in 5 min. Concentration of this solution and layering
with diethyl ether overnight gave orange microcrystals, which
were isolated by filtration and washed with diethyl ether.
Yield: 82 mg (79%). Anal. Calcd for C27H42N8I3F3SO3Rh2: C,
26.97; H, 3.52; N, 9.32; S, 2.67. Found: C, 27.23; H, 3.58; N,
8.95; S, 2.45. 19F NMR (room temperature, CDCl3): δ -78.30
(s, CF3SO3). ΛM (2.8 × 10-4 M in nitromethane) ) 75 S cm2
(C4Pz), 97.1 (d, J C-Rh ) 8 Hz, Rh-CtC-Ph), 81.0 (d, J C-Rh
) 48 Hz, Rh-CtC-Ph), 60.6 and 60.4 (C-(CH3)3), 30.3 (C-
(CH3)3). MS (FAB+, CH2Cl2, m/ z): 1027 (100, M+), 900 (52,
M - I+), 799 (31, M - I - C2Ph+). ΛM (5.1 × 10-4 M in acetone)
2
1
) 80 S cm2 mol-1
.
[(CNBu t)2(I)Rh (µ-P z)2(µ-I)Rh (Me)(CNBu t)2]A ([14]A, A
) CF 3SO3, I). A solution of complex 3 (100 mg, 0.11 mmol)
in MeI (2 mL) was treated with MeCF3SO3 (12 µL, 0.11 mmol)
to give a purple solution. Irradiation for 2 h with a 400 W
visible lamp or under direct sunlight gave an orange solution.
Evaporation to dryness gave a crude solid containing 76% of
[14]CF3SO3, which was extracted with acetone (3 mL). Addi-
tion of diethyl ether (10 mL) to the extract gave orange
microcrystals, which were separated by decantation and
washed with diethyl ether. Yield: 71 mg (60%). Anal. Calcd
for C28H45N8I2F3SO3Rh2: C, 30.84; H, 4.16; N, 10.27; S, 2.94.
Found: C, 30.61; H, 3.88; N, 9.67; S, 2.81.
mol-1
.
The 1H and 13C{1H} NMR spectra, ν(CN), and mass
spectrum are identical as those described for [9]I.
[{Ir (µ-P z)(I)(CNBu t)2}2(µ-I)]I ([10]I). The addition of solid
I2 (13.8 mg, 0.055 mmol) to an acetone (15 mL) solution of [{Ir-
(µ-Pz)(I)(CNBut)2}2] (4) (60 mg, 0.055 mmol) leads to an orange
solution. Evaporation of this solution and layering with
diethyl ether gave [10]I as orange cubic crystals. The solution
was decanted, and the crystals were washed with cold diethyl
ether and vacuum dried. Yield: 59 mg (80%). Anal. Calcd
for C26H42N8I4Ir2: C, 22.98; H, 3.12; N, 8.24. Found: C, 22.54;
H, 2.50; N, 7.84. IR (CH2Cl2, cm-1): ν(CN) 2232 (s), 2210 (s).
1H NMR (room temperature, CDCl3): δ 8.08 (d, J ) 2.3 Hz,
4H, H3,5Pz), 6.11 (t, J ) 2.3 Hz, 2H, H4Pz), 1.61 (s, 36H,
CNBut). 13C{1H} NMR (room temperature, CDCl3): δ 144.5
(C3,5Pz), 107.3 (C4Pz), 59.8 (C-(CH3)3), 30.6 (C-(CH3)3). MS
(FAB+, CH2Cl2, m/ z): 1231 (100, M+), 1104 (65, M - I+), 977
(40, M - 2I+).
Irradiation of a solution of 3 (100 mg, 0.11 mmol) in MeI (2
mL) for 2 h with a 400 W visible lamp or under direct sunlight
and evaporation to dryness gave a crude solid containing 81%
of [14]I, which was recrystallized as described above. Yield:
75 mg (65%). Anal. Calcd for C27H45N8I3Rh2: C, 30.35; H,
4.24; N, 10.49. Found: C, 30.58; H, 4.04; N, 10.34. IR (CH2-
Cl2, cm-1): ν(CN) 2224 (s), 2205 (s). 1H NMR (room temper-
ature, CDCl3): δ 8.07 (d, J ) 2.1 Hz, 2H, H3Pz), 7.30 (d, J )
2.1 Hz, 2H, H5Pz), 6.17 (t, J ) 2.1 Hz, 2H, H4Pz), 1.83 (d, J H-Rh
) 2.1 Hz, 3H, Me-Rh), 1.58 and 1.57 (s, 18H + 18H, CNBut).
13C{1H} NMR (room temperature, CDCl3): δ 145.2 (C3Pz),
140.2 (C5Pz), 106.8 (C4Pz), 60.3 and 60.0 (C-(CH3)3), 30.3 and
30.1 (C-(CH3)3), 8.60 (d, J C-Rh ) 21 Hz, Me-Rh). MS (FAB+,
CH2Cl2, m/ z): 941 (100, M+), 926 (10, M - Me+), 814 (45, M
- I+), 799 (8, M - I - Me+). ΛM (5.1 × 10-4 M in acetone) )
[{Rh (µ-P z)(Cl)(CNBu t)2}2(µ-I)]BF 4 ([11]BF 4). The addi-
tion of solid [I(Py)2]BF4 (24.9 mg, 0.067 mmol) to an acetone
(10 mL) solution of [{Rh(µ-Pz)(Cl)(CNBut)2}2] (7) (50 mg, 0.067
mmol) leads to an orange solution. Evaporation of this solution
to 2 mL and layering with diethyl ether (15 mL) gave [11]BF4
as orange microcrystals. The solution was decanted, and the
crystals were washed with cold diethyl ether and vacuum
dried. Yield: 50 mg (78%). Anal. Calcd for C26H42N8Cl2IRh2-
BF4: C, 32.63; H, 4.42; N, 11.70. Found: C, 32.57; H, 4.45;
N, 11.44. IR (CH2Cl2, cm-1): ν(CN) 2239 (s), 2231 (s). 1H NMR
102 S cm2 mol-1
.
Cr ysta l Str u ctu r e Deter m in a tion of [{Rh 2(µ-P z)2(I)-
(CNBu t)4}2(µ-I)]CF 3SO3 ([8]CF 3SO3). A summary of crystal
(room temperature, CDCl3): δ 7.75 (d, J ) 2.3 Hz, 4H, H3,5
-
Pz), 6.23 (t, J ) 2.3 Hz, 2H, H4Pz), 1.60 (s, 36H, CNBut).
13C{1H} NMR (room temperature, CDCl3): δ 141.5 (C3,5Pz),
106.6 (C4Pz), 60.6 (C-(CH3)3), 29.9 (C-(CH3)3). MS (FAB+,
CH2Cl2, m/ z for 35Cl): 869 (100, M+), 834 (52, M - Cl+), 799
data and refinement parameters is reported in Table 2.
A
dark-red prismatic crystal (0.50 × 0.44 × 0.32 mm) was used
to collect data on a Siemens-P4 diffractometer with graphite-
monochromated Mo KR radiation (λ ) 0.710 73 Å). Cell
constants were obtained from the least-squares fit on the
setting angles of 36 reflections in the range 16° e 2θ e 25°.
Reflections with 2θ in the range 3-45° were measured using
the ω/2θ scan technique and corrected for Lorentz and
polarization effects. Reflections were also corrected for ab-
sorption by a semiempirical method (Ψ-scans, 12 reflections).45
Three standard reflections were monitored every 97 measure-
ments throughout data collection as a check of crystal and
instrument stability; no important variation was observed.
The structure was solved by direct methods (SIR92)46 and
difference Fourier techniques and refined by full-matrix least-
squares on F2 (SHELXL93).47 Anisotropic thermal parameters
were used in the last cycles of refinement for all non-hydrogen
(7, M - 2Cl+). ΛM (4.9 × 10-4 M in acetone) ) 135 S cm2 mol-1
.
Rea ction of [{Rh (µ-P z)(Cl)(CNBu t)2}2] (7) w ith Iod in e
a n d Meth yl Tr ifla te. To a solution of 7 (70 mg, 0.095 mmol)
in acetone were successively added solid I2 (24 mg, 0.095 mmol)
and MeCF3SO3 (11 µL, 0.095 mmol) after 5 min. The resulting
orange solution was concentrated to 2 mL, and the extract was
layered with 20 mL of diethyl ether overnight to give orange
microcrystals. The solution was decanted, and the solid was
washed with diethyl ether and vacuum dried. Yield: 80 mg.
The crude solid contains 45% of [11]CF3SO3, 19% of [9]CF3-
SO3, and 36% of the new complex [Rh2(µ-Pz)2(I)(Cl)(CNBut)4(µ-
I)]CF3SO3 ([12]CF3SO3). 1H NMR for [12]+ (room temperature,
CDCl3): δ 8.06 (d, J ) 2.2 Hz, 2H, H3Pz), 7.74 (d, J ) 2.2 Hz,
2H, H5Pz), 6.19 (t, J ) 2.2 Hz, 2H, H4Pz), 1.60 and 1.59 (s,
18H + 18H, CNBut). 13C{1H} NMR for [12]+ (room temper-
ature, CDCl3): δ 141.8 (C3), 145.4 (C5), 107.3 (C4Pz), 60.7 (C-
(CH3)3), 30.1 (C-(CH3)3). Complex [9]+ pure results upon
addition of KI to this mixture in CDCl3 solution.
(45) North, A. C. T.; Phillips, D. C.; Mathews, F. S. Acta Crystallogr.,
Sect. A 1968, 24, 351.
(46) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A. J .
Appl. Crystallogr. 1994, 27, 435.
(47) Sheldrick, G. M. SHELXL-93; University of Go¨ttingen: Go¨t-
tingen, Germany, 1993.
[(CNBu t )2(I)R h (µ-P z)2(µ-I)R h (η1-CtC-P h )(CNBu t )2]I

Metal-Metal-Bonded Dinuclear Rh and Ir Complexes
Organometallics, Vol. 16, No. 21, 1997 4727
Ta ble 2. Cr ysta llogr a p h ic Da ta a n d Str u ctu r e
Refin em en t for [8]CF 3SO3
calculated and refined as riding on carbon atoms with a
common isotropic displacement parameter. The function
2
minimized was ∑[w(Fo - Fc2)2]. The refinement converged
chem formula
fw
C53H84F3I3N16O3Rh4S
1807.08
to R(F) ) 0.0476 (F2 > 2σ(F2), 6407 reflections) and wR(F2) )
0.1421 (all data) for 671 parameters, 170 restraints, and 10 940
unique reflections. The calculated weighting scheme was 1/[σ2-
cryst syst
space group
a, Å
monoclinic
P21/c (No. 14)
21.082(3)
2
(Fo2) + (0.0717P)2], where P ) (Fo2 + 2Fc )/3. Scattering factors
were used as implemented in the refinement program.47
b, Å
18.452(2)
c, Å
21.717(2)
â, deg
92.165(7)
8442(2)
4
Ack n ow led gm en t. We thank Direccio´n General de
Investigacio´n Cient´ıfica y Te´cnica (DGICYT) for finan-
cial support (Project Nos. PB95-221-C1 and PB94-1186).
V, Å3
Z
µ, mm-1
1.938
θ range of data collected, deg
index ranges
1.5_22.5
-1 e h e 22, -1 e k e 19,
-23 e l e 23
14 509
10944 (Rint ) 0.0263)
ψ-scan
0.291, 0.347
10940/170/671
0.0476 (6407 reflns)
0.1421
Ap p en d ix
Calculations of the extended Hu¨ckel type48 were
carried out using a modified version of the Wolfsberg-
Helmholz formula.49 The investigated compounds were
modeled with internal symmetry using the geometrical
parameters obtained from the reported crystal struc-
ture. Calculations and drawings were made with the
program CACAO,50 and the atomic parameters used are
those implemented in this program.
no. of measd reflns
no. of unique reflns
abs corr method
min, max transmission factors
no. of data/restraints/params
R(F) [F2> 2σ(F2)]a
wR(F2) [all data]b
S [all data]c
0.921
a
R(F) ) ∑||Fo| - |Fc||/∑|Fo|, for 6407 observed reflections.
wR(F2) ) (∑[w(Fo - Fc2)2]/∑[w(Fo2)2])1/2 c S ) [∑[w(Fo - Fc2)2]/
.
b
2
2
(n - p)]1/2; n ) number of reflections, p ) number of parameters.
Su p p or tin g In for m a tion Ava ila ble: Tables of crystal-
lographic data, atomic coordinates, isotropic and anisotropic
thermal parameters, bond distances and angles, and least-
squares planes for [8]CF3SO3 and an ORTEP diagram of [8]+
(17 pages). Ordering information is given on any current
masthead page.
atoms, except those atoms of the CNBut ligands and the triflate
anion involved in disorder. All of the tert-butyl groups were
found disordered in two positions, but five of them with a
unique tertiary carbon, and were refined with geometrical
restraints and a variable common carbon-carbon distance.
The occupancy factors were refined but fixed complementary
to 1.0 (C(9)-C(11) 0.63(3); C(13)-C(16) 0.57(1); C(19)-C(21)
0.55(4); C(23)-C(26) 0.71(2); C(35)-C(37) 0.54(2); C(39)-C(42)
0.62(2); C(45)-C(47) 0.52(2); C(50)-C(52) 0.61(1)). Hydrogen
atoms of the nondisordered ligands (pyrazolate anions) were
OM970347J
(48) Hoffmann, R. J . Chem. Phys. 1963, 39, 1397. Hoffmann, R.;
Lipscomb, W. N. Ibid. 1962, 36, 2179; 1962, 37, 2872.
(49) Ammeter, J . H.; Bu¨rgi, H. B.; Thibeault, J . C.; Hoffmann, R. J .
Am. Chem. Soc. 1978, 100, 3686.
(50) Mealli, C.; Proserpio, D. M. J . Chem. Educ. 1990, 67, 399.