4 Molecules with Bridging Hydride Ligands
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Direct Comparisons of Μ(μ-Η)Μ Bonds with M-M or M ( μ - C l ) M Bonds MELVYN ROWEN CHURCHILL Department of Chemistry, State University of New York, Buffalo, NY 14214
A series of trinuclear osmium carbonyl derivatives that contain single unsupported equatorial μ -hydride ligands, including (μ -H)(H)Os (CO) , (μ -H)(H)Os (CO) (PPh ), "Os (CO) (EtC≡CH) ", and (μ -Η) Os Re (CO) , have been investi gated. The Os(μ-H)Os distances range from 2.989(1) to 3.083(3)Å,as opposed to a normal Os-Os bond length of 2.877(3) Åin Os (CO) . In (μ -H) Os (CO) , the dihydrido -bridged Os-Os bond is 2.681(1) Åin length while in (μ -Η)Os (CO) (μ -CH•CH •PMe Ph), the dibridged Os-Os bond distance is 2.800(1)Å.Similar effects are noted for ruthenium complexes based upon studies on Ru (CO) , (μ -Η)Ru (CO) (µ -C=NMe ), and (μ -Η) Ru (CO) (diphos). In the dinuclear species [(η -C me )MCl] (μ-X)(μ-Cl) (M = Rh, Ir; X = Cl, H), the μ -hydrido-μ -chloro-bridged complexes have M-M separations of 2.906(1)Å(M = Rh) and 2.903(1)Å(M = Ir) whereas the di-μ -chloro-bridged species have increased nonbonding Μ···M distances of 3.719(1)Å(M = Rh) and 3.769(1)Å(M = Ir). 2
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H
y d r i d e complexes of the transition metals o c c u p y a central role in contemporary chemistry, both because of their importance as catalytic or stoichiometric reagents for f u n d a m e n t a l organic transformations (e.g., the catalytic hydrogénation of unsaturated systems) a n d because of their c h e m i c a l interest per se. T h e structural chemistry of these species has been the subject of a remarkable n u m b e r of misconceptions a n d only now is b e g i n n i n g to emerge as a coherent, ordered discipline. Thus, prevailing attitudes on the nature of even the simplest 0-8412-0390-3/78/33-167-036/$06.25/0 ©
American Chemical Society
Bau; Transition Metal Hydrides Advances in Chemistry; American Chemical Society: Washington, DC, 1978.
4.
Molecules with Bridging Hydride
CHURCHILL
37
Ligands
system, the t e r m i n a l h y d r i d e l i g a n d , have undergone almost an entire cycle of change, as illustrated by the f o l l o w i n g approximate chronology. (1) P r i o r to 1960. T h e m e t a l - h y d r o g e n b o n d was believed to be very short (I ). As such, the hydrogen atom was v i e w e d as being b u r i e d i n the m e t a l d-orbitals a n d h a v i n g , i n essence, a negative or close-to-zero covalent radius. As a result of electron d i f f r a c t i o n studies on H C o ( C O ) a n d H F e ( C O ) 4 (2), i n w h i c h the carbonyl ligands were shown to have a tetrahedral disposition around the central metal atom, the h y d r i d e l i g a n d also was believed to exert no stereochemical i n fluence (i.e., it was thought not to occupy a regular coordination site on the metal atom). (2) ca. 1960-1970. Because of a series of x-ray diffraction studies beginning w i t h H P t B r ( P E t ) (in w h i c h the h y d r o g e n atom was not located directly) (3) a n d H R h ( C O ) ( P P h ) (in w h i c h the hydrogen atom was located directly) (4,5) a n d a u n i q u e early neutron d i f f r a c t i o n study of K [ R e H ] (6), attitudes changed drastically. T h e revised credo was that trie m e t a l - h y d r o g e n b o n d was entirely n o r m a l (i.e., c o u l d be predicted safely to be close to ( r ( M ) + 0.3) Â i n length, where r ( M ) is the covalent radius appropriate to the m e t a l u n d e r consideration, and 0.3 A is the approximate covalent radius for hydrogen) a n d that the h y d r i d e l i g a n d occupied a regular stereochemical site i n the coordination sphere of the transition metal. (3) ca. 1970-present. T h e view c u r r e n t l y accepted is that the m e t a l - h y d r o g e n distance w i l l be normal unless otherwise affected by external factors (e.g., a ligand of extremely h i g h trans effect). T h e categorical statement as to the h y d r i d e l i gand o c c u p y i n g a regular stereochemical site should be v i e w e d circumspectly. T h i s is usually tne case, but a caveat must be issued that b u l k y ligands m a y e n croach u p o n the cone-angle of space f o r m a l l y allotted to the coordination site of the hydride ligand. Perhaps the most flagrant examples of this violation occur i n the H R h ( P P h ) (7) a n d H R h ( P P h ) ( A s P h ) (8) molecules; here, the t r i p h e n y l p n i c o g e n ligands o c c u p y essentially regular tetrahedral sites a r o u n d the central Rh(I) atom, but the h y d r i d e ligands can be detected spectroscopically a n d are believed to o c c u p y sites d i r e c t l y trans to P P h or A s P h ligands (two r h o d i u m - h y d r o g e n stretches are observed for the m i x e d l i g a n d complex). A similar, but far less severe, case occurs i n the H M n ( C O ) s molecule; here the equatorial ligands are displaced by only 6-8° toward the terminal hydride ligand (9).
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T h e structural chemistry of species containing b r i d g i n g h y d r i d e ligands has lagged b e h i n d that of the terminal h y d r i d e complexes, partly because of the later realization of their synthesis and partly because of the greater complexity of the molecules involved.
[A consistent p r o b l e m for x-ray studies is that the electron
density for a hydrogen atom (Z = 1) is far smaller than that for a transition metal (Z = 2 1 - 2 9 for a first-row transition m e t a l , 3 9 - 4 7 for a second-row m e t a l , a n d 7 1 - 7 9 for a t h i r d - r o w metal).
T h e h y d r i d e l i g a n d thus m a y be obscured b y
background noise on an electron density map. ] T h e first x-ray structural analysis on one of these species was that b y Doedens a n d D a h l (10) on the molecule [(7/ -C H )Mo(CO) ] (M -H)(M -PMe ). 5
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A l t h o u g h the b r i d g i n g h y d r i d e l i g a n d
was not located i n this study, its location was deduced by comparison w i t h related structures.
T h e structure was described at the t i m e as c o n t a i n i n g a localized,
bent, three-center m e t a l - h y d r o g e n - m e t a l b o n d without a m e t a l - m e t a l bond. Thus, despite a m o l y b d e n u m - m o l y b d e n u m distance of o n l y 3.262(7) Â [cf. the
Bau; Transition Metal Hydrides Advances in Chemistry; American Chemical Society: Washington, DC, 1978.
38
TRANSITION M E T A L HYDRIDES
molybdenum-molybdenum
single b o n d distance of 3.235(1)Å i n [(n5-CsHs)-
M o ( C O ) 3 ] 2 (12)], the core of this M - h y d r i d o species was d r a w n originally as that 2
i n Structure 1; i.e., without any direct m e t a l - m e t a l interaction.
A s i m i l a r sit-
uation arose i n the duo of complexes F e ( C O ) i 2 U 3 , 1 4 ) and [ H F e 3 ( C O ) n ~ ] (15). 3
W h i l e these were formally illustrated as Structures 2 and 3, the i r o n - i r o n distances of note are 2.558(1)Å for the F e ( M - C O ) F e system i n F e ( C O ) i a n d 2.577(3) Å 2
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this f o r m a l i s m , a change f r o m a f o r m a l m e t a l - m e t a l single b o n d to a f o r m a l no-bond situation is a c c o m p a n i e d b y a change i n m e t a l - m e t a l distance of only 0.019 Å. C l e a r l y , the original description of these bent m e t a l - h y d r o g e n - m e t a l
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bonds needed some m o d i f i c a t i o n .
/
H
\
(CO)
Mo
\
(OC) Fe—
3
H
.Fe
pFe(CO).
4
\
Me
(CO)
\
/
/
m
3
,Fe^j
\__ Mo
X
}
(OC) Fe-
\—Fe(CO)
4
\ / CO 2
Me
1
3
1/ CO 3
A f i n a l important structure i n the early days was [HCr2(CO)io~].
The
C r — C r distance here was 3.406(9) Å, i.e., some 0.44 Å longer than i n the conjugate base, [Cr2(CO)io ~], where the c h r o m i u m - c h r o m i u m b o n d length is 2.97(1)Å 2
ment of equatorial carbonyl ligands on the two c h r o m i u m atoms.
It is interesting
to note that the [ H C r ( C O ) i o ~ ] ion lies on a crystallographic center of symmetry. 2
F o r some ten years this molecule was believed to be the a r c h e t y p a l example of a molecule w i t h a truly linear metal-hydrogen-metal system. shown not to be the case.
This now has been
A neutron diffraction study shows the bridging hydride
l i g a n d to be disordered (18) a n d displaced f r o m co-linearity w i t h the C r — C r vector.
Early Studies on Rhenium-Carbonyl-Hydride
Clusters and Related
Species O u r i n i t i a t i o n into the f i e l d of transition metal h y d r i d e chemistry resulted f r o m collaborative research w i t h H . D . Kaesz o n the structures of a series of r h e n i u m - c a r b o n y l - h y d r i d e complexes. of
Single-crystal x-ray diffraction studies
H R e M n ( C O ) i 4 (19, 20) a n d derivatives of the [ H R e ( C O ) i " ] 2
2
[HRe (CO) 3
1 2
2
-]
(22, 23), [ H R e ( C O ) 6
4
1 2
2
-]
3
(24), a n d [ R e ( C O ) 4
2
1 6
2
(21),
- ] (25, 26)
anions led to our proposing some rules whereby the positions of b r i d g i n g h y d r i d e ligands could be determined indirectly.
( W e note parenthetically here that our
e q u i p m e n t a n d data were not, at this t i m e , good enough to enable us to locate h y d r i d e ligands directly.)
These simple, e m p i r i c a l rules were as follows:
(1) A n o r m a l n o n b r i d g e d r h e n i u m - r h e n i u m linkage was about 3.00 Å i n length i n a r h e n i u m c a r b o n y l cluster. T h i s distance expanded b y about 0.15 Å
Bau; Transition Metal Hydrides Advances in Chemistry; American Chemical Society: Washington, DC, 1978.
Molecules with Bridging Hydride Ligands
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CHURCHILL
Figure 1. Interatomic distances and angles within the equatorial planes of (top) [H Re (CO)i -] (see Ref. 21) and (bottom) [HRez(CO)i
