Location of Terminal Hydride Ligands in Transition Metal Hydrides

3 Location of Terminal Hydride Ligands in Transition Metal Hydrides JAMES A. IBERS

Downloaded by UNIV OF BATH on March 13, 2017 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/ba-1978-0167.ch003

Department of Chemistry, Northwestern University, Evanston, IL 60201

The location of the positions of terminal hydride ligands in transition metal complexes using x-ray diffraction techniques is examined by reference to some recent structure determinations. These include studies of the complexes RuHX(CO)(PPh ) , where X = TolNNNTol and TolNCHNTol, RuH (N B H SMe )(PPh ) •3C H , OsH(CSSMe)(CO) (PPh)•½CH, [PtH(PhHNNCMe )(PPh ) ][BF ], and [Pt((tert-Bu)P(CH)P(tert-Bu))]. 3

2

32

2

2

10

6

8

2

3 3

6

6

6

2

2

2

2 3

2

3 2

4

2

lonfusion over the stereochemical role of the h y d r i d e l i g a n d was \ ^ largely cleared up some 12 years ago (see for example Ref. 1 ). T h e location f r o m x - r a y diffraction data of the h y d r i d e position i n R h H ( C O ) ( P P h ) (2, 3) was a c r u c i a l step i n this process since this was the first example of the location of a hydride ligand i n the presence of other, bulkier ligands on a transition metal. T h e R h - H bond length of 1.60(12)Â, a value that remains reasonable today, was d e t e r m i n e d b y F o u r i e r methods based on room-temperature diffraction data whose intensities had been estimated visually. T h e notion that the hydride ligand is " b u r i e d " i n the metal orbitals, though seriously u n d e r m i n e d b y this structure determination, was finally put to rest w i t h the determination of the M n - H distance of 1.602(16)À b y using neutron diffraction techniques (4). Since then the determination of the position of a t e r m i n a l h y d r i d e l i g a n d i n a transition metal h y d r i d e complex b y x-ray diffraction methods has become rather routine. I n favorable cases, the position of the h y d r i d e l i g a n d can be d e t e r m i n e d b y leastsquares procedures to an apparent accuracy of about ± 0.05Â from x-ray intensity data collected by diffractometer methods at room temperature. But not all cases are favorable. T h e purpose of this present chapter is to present some recent experiences f r o m our laboratory o n the determination of h y d r i d e positions i n various transition metal hydrides (5-10). In so doing, w e hope to point to some of the potential problems involved. Since this book is devoted to transition metal hydrides, the reasons for our studying the specific c o m p o u n d s discussed below w i l l be given w i t h utmost brevity. But w e emphasize that i n these days of the 3

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American Chemical Society

Bau; Transition Metal Hydrides Advances in Chemistry; American Chemical Society: Washington, DC, 1978.

3

3.

Location of Terminal Hydride

IBERS

27

Ligands

ubiquitous h y d r i d e l i g a n d , the fact that the c o m p o u n d s studied happened to be hydrides was i n c i d e n t a l to our c h e m i c a l interests. Experimental

Procedures

T h e experimental procedures followed i n collecting the d i f f r a c t i o n data were standard i n this laboratory (see for example Ref. 11). Diffraction data were obtained at room temperature on a P i c k e r F A C S - I diffractometer using either filtered copper radiation or monochromatized m o l y b d e n u m radiation. Efforts were m a d e to collect as large data sets as possible. A l l data sets were corrected for absorption effects. T h e compounds to be discussed have molecular weights i n the range 8 8 0 - 1 0 7 0 a m u , p r i m a r i l y because they contain P P h or related phosphine groups. A l l least-squares refinements of these structures were carried out by f u l l - m a t r i x methods. W h e r e there were no c o m p l i c a t i n g features, the h y d r i d e positions were i n c l u d e d as part of the refinement, w i t h their positional a n d isotropic t h e r m a l parameters b e i n g varied. P h e n y l groups were refined as r i g i d groups by using techniques first developed for R h H ( C O ) ( P P h ) (3). Abbreviations used i n this chapter are: P h = p h e n y l , T o i = p - t o l y l , M e = m e t h y l , E t = ethyl, tert-Bu = terf-butyl, and C y = cyclohexyl. Unless otherwise stated, these group atoms were constrained to vibrate isotropically. Table I lists selected i n f o r m a t i o n on the various compounds to be discussed, i n c l u d i n g some details on data collection a n d on M - H b o n d lengths.


3

3

3

Discussion RuHX(COXPPh )2. 3

T h e compounds X = T o l N N N T o l (triazenido) a n d

T o l N C H N T o l (amidinato) are prepared as shown i n Reactions 1 (12) a n d 2 (6, These compounds were studied because of our interest i n the diverse

13).

b o n d i n g patterns of the triazenido a n d isoelectronic a m i d i n a t o ligands. RuH (CO)(PPh ) + TolNNHNTol — 2

3

2

RuH(TolNNNTol)(CO)(PPh ) + H 3

2

2

(1)

2

(2)

RuH (CO)(PPh ) + T o l N = C = N T o l — 2

3

2

RuH(TolNCHNTol)(CO)(PPh ) 3

T h e inner coordination spheres of these two complexes are presented i n F i g u r e 1.

W h i l e the R u - H distance is w e l l defined i n the a m i d i n a t o , it is not i n

the triazenido complex.

Yet reference to T a b l e I indicates that the triazenido

structure is "better" if one uses the unreliable criterion that the lower the R index, the better the structure.

In this instance, the low R index i n the triazenido

complex results f r o m an elaboration of the usual group refinement m o d e l (3), allowing for anisotropic motion of the group atoms.

T h i s elaboration introduces

a large n u m b e r of a d d i t i o n a l variables a n d provides us w i t h an o p p o r t u n i t y to lower the R index!

M o r e i m p o r t a n t l y , we established that the other features of

the structure were v i r t u a l l y unaffected b y this elaboration.

W e conclude that

w i t h problems of this type, such an elaboration probably is not justified by the expense involved.

W h y can't the h y d r i d e position be located accurately i n the


28

TRANSITION M E T A L H Y D R I D E S


N(3)

Figure 1. (top) The inner coordination sphere of RuH(TolNNNTolXCOXPPhz) ; (bottom) the inner coordination sphere of the isoelectronic RuH(TolNCHNTolXCO)(PPhzh 2

triazenido w h i l e it was refined isotropically i n the a m i d i n a t o c o m p l e x ?

In this

instance the reason is clear: i n the triazenido structure there is a C O - Η disorder w i t h about 80% of the C O at the position shown i n F i g u r e 1 (top). T h e overlap of the 20% C O w i t h the 80% H is sufficient to obscure the position of the hydride ligand. T h i s disorder is not imposed crystallographically. O n the other h a n d , the amidinato structure, w h i c h crystallizes i n a different space group, is perfectly ordered.


3.

Location of Terminal Hydride

IBERS

29

Ligands

In the triazenido complex, there is a slight trans l a b i l i z i n g effect of the h y d r i d e over the c a r b o n y l , as reflected i n the trans R u - N distances.

T h i s effect

is m u c h more pronounced i n the a m i d i n a t o complex because of the absence of disorder and perhaps because the f our-membered ring has opened u p so that atom N(2) is more nearly trans to the h y d r i d e ligand. These two structures illustrate a f u n d a m e n t a l a n d d i s t u r b i n g point about diffraction experiments.

It is not u n t i l the late stages of refinement, after c o n -

siderable time and money has been spent on the experiment, that one sometimes discovers his i n a b i l i t y to define accurately a salient feature of the s t r u c t u r e — i n this instance the h y d r i d e position i n the triazenido complex.

T h e r e is no w a y

f r o m the formula, space group, or films to have predicted this; nor are there any


usefully consistent methods that enable one to predict, especially i n c o m m o n l o w - s y m m e t r y space groups, w h e n disorder w i l l occur. RuH (N2BioH SMe2)(PPh3)3*3C H . 2

8

e

T h e synthesis of this c o m p o u n d is:

e

THF

RuH (N )(PPh ) + l,10-N B H SMe 2

2

3

3

2

(j/

N N

1 0

8

2

(14)

= 2240 c m " )

1 hr under N

2

1

RuH (N B H SMe )(PPh ) 2

2

1 0

8

(I»NN = 2060

2

3

3

(IS)

cm" )

(3)

1

T h e c o m p o u n d is of interest to us because of our studies of the v a r y i n g b o n d i n g modes of the N R species. 2

F r o m the very h i g h value of the N N stretching fre-

quency, we believed (and confirmed) that the complex represents the first exa m p l e of the linear attachment of an R N group to a transition metal. 2

The

structural study (Table I) was straightforward, and the inner coordination sphere is shown i n F i g u r e 2.

T h e R index is somewhat higher than expected, probably

because of some residual electron density i n the region of the S M e Nevertheless, the two hydride positions refined successfully.

2

group.

T h e resultant R u - H

distances, insignificantly different f r o m one another, are as expected.

The

trans-labilizing effect of the h y d r i d e ligand relative to a triarylphosphine group is apparent.

Ml)

Figure

2.

The inner coordination sphere of R u H ^ i V ^ B i o H s S M e ^ (PPhk


30

TRANSITION M E T A L H Y D R I D E S

T a b l e I.

Some E x p e r i m e n t a l Details

Space Group

Formula Units/ Cell

C,-P1 C * - C2/c Q - PI

2 8 2

C,-PI C - P2i/c C / i - C2/c

2 4

Compound RuH(TolNNNTol)(CO)(PPh ) RuH(TolNCHNTol)(CO)(PPh ) 3

3

RuH2(N2B

1 0

1

2

2h

2

H SMe2)(PPh3)3-3C H 6

8

OsH(CSSMe)(CO) (PPh3)2-V2C H 6

2

1

6

1

6

[PtH(PhHNNCMe )(PPh ) ] [BF ] [Pt((ieri-Bu) P(CH )3P(ieri-Bu)2)]2 2

2


û

3

2

2 / l

4

2

2

5

6

46

R(F ) = 2||Fo|-|F ||/Z|Fo|. 0

c

OsHfCSSMeXCO) PPha)2CeHe. 2

T h i s c o m p o u n d was synthesized by

the f o l l o w i n g route (16): Os(C H xCO).,(PPh ), + CS, --> Os,(PPh ) 2

4

3

;i 2

J Mel

°C

Ρ I

o

i

c

SMe

•

0

Ρ

C

I

SMe" (4)

s

T h e c o m p o u n d is interesting to us because of the novelty of the C S S M e l i g a n d a n d the possibility of v a r y i n g modes of attachment of this l i g a n d to transition metals. T h e structural study was completely straightforward (see T a b l e I) a n d the inner coordination sphere is shown i n F i g u r e 3.

Note, i n particular, the successful

refinement of the h y d r i d e position (and isotropic t h e r m a l parameter) i n the presence of a t h i r d - r o w element.

As judged b y the agreement a m o n g what are

expected to be chemically equivalent distances w i t h i n the complex, the assigned standard deviations are reasonable; hence, the O s - Η distance has been determined to ±0.06 Â.

T h e complex itself contains 50 nonhydrogen atoms and 36 hydrogen

atoms i n a d d i t i o n to the h y d r o g e n atom of the h y d r i d e l i g a n d .

As a result, the

c o m p o u n d w o u l d present a formidable neutron diffraction study.

Such a study

m i g h t not lead to a m u c h better d e t e r m i n a t i o n of the h y d r i d e position.

In our

opinion, a neutron diffraction study is not justified on a transition metal h y d r i d e c o n t a i n i n g only a t e r m i n a l h y d r i d e if this h y d r i d e position has been reasonably w e l l defined f r o m an x-ray study.

It is wiser to a p p l y the l i m i t e d neutron d i f -

fraction resources to the study of structures i n v o l v i n g b r i d g i n g h y d r i d e ligands where small positional changes m a y affect our interpretation of the b o n d i n g . N o t e the t r a n s - l a b i l i z i n g effect of the h y d r i d e l i g a n d relative to the C S S M e group.


3.

31

Location of Terminal Hydride Ligands

IBERS

o n Selected T r a n s i t i o n M e t a l H y d r i d e s No,

No. of Obser vations

(2γ) \

δ

of Vari ables

/ max

M-H

(A)

R(F ) 0

a

5847 5805 6544

0.562 0.576 0.479

436 192 324

0.038 0.063 0.072

7005 5635 5350

0.627 0.559 0.700

182 214 204

0.036 0.046 0.030

7 1.58(7) 1.74(7) 1.53(7) 1.64(6) No Η

Ref. 5 6 7 8 9 10


* T h e dimer has crystallographically imposed C2 symmetry.

[PtH(PhHNNCMe XPPh )2PF4]. 2

3

T h e synthesis of this c o m p o u n d is

(9):

irans-PtHCKPPh ), +

Η,ΝΝΗΡΗ +

:l

NaBF,

PtH(H,NNHPhXPPh,),

+

J a cet' THF irans-PtHCl(PPh ), +

AgBF

;i

4

+

PhHNNCMe,

>

PtH(PhHNNCMe,XPPh,),+

Because there are at least three ways i n w h i c h the hydrazone ligand could attach itself to a transition metal and because hydrazone complexes of transition metals barely have been investigated, this type of c o m p o u n d is interesting to us. As contrasted w i t h the studies discussed above, w e were unable to locate a n d refine the h y d r i d e position i n this c o m p o u n d (see T a b l e I). T h e inner c o o r d i nation sphere (Figure 4) shows the h y d r i d e ligand d r a w n at an assumed position. T h e reasons for our failure are not clear, but we are led to the sobering conclusion that success cannot always be guaranteed i n studies of this k i n d . O n e might ask if intrinsic differences i n the M - H bond m i g h t affect our ability to locate the h y d r i d e l i g a n d . O b v i o u s l y , as one goes i n a f o r m a l sense f r o m M - H ~ to M ~ - H , that is, if the h y d r i d e were to lose its electron, the h y d r i d e ligand w o u l d become transparent to x-rays. In principle, then, our inability to locate h y d r i d e positions i n certain complexes might correlate w i t h such a hypothetical electron transfer. B u t spectroscopic differences also should manifest themselves. T h e spectroscopic properties of the P t - H bond i n this hydrazone complex are normal. Thus, i>pt_H 2220 c m a n d r p _ H 27.36 p p m . ( C o m p a r e these values w i t h those of 2119 c m a n d 23.37 p p m i n the a m i d i n a t o complex above where the h y d r i d e position was located without difficulty.) W e believe it to be far more likely that failure to locate the h y d r i d e position i n this hydrazone complex results f r o m undetected errors i n our data or i n our m o d e l a n d not f r o m an intrinsic property of the P t - H bond. +

+

=

- 1

=

t

- 1

[Pt(( tert-Bu)2P(CH )3P(tert-Bu) )] . W e e n d this brief s u m m a r y of recent structural studies i n our laboratory b y discussing a c o m p o u n d that apparently 2

2

2


32

TRANSITION

M E T A L HYDRIDES


C(4)

Figure 3. The inner coordination sphere of OsH(CSSMeXCO) (PPh ) 2

is not a h y d r i d e .

3 2

W e attempted the preparation of the new complexes M ( P P ) ,

where P P represents a bidentate phosphorus l i g a n d constrained by geometry to present cis phosphorus atoms to the metal M . T h e resultant bent M ( P P ) complex should exhibit h i g h reactivity and unusual chemistry. the d i m e r i c c o m p o u n d was characterized (10).

In the course of this work, precipitated a n d

[Pt((tert-Bu) P(CH )3P(tert-Bu) )] 2

2

2

2

T h e preparative scheme is: Na/Hg

PtCl (diphos) 2

THF

• ds-PtH (diphos) 2

(6) J[Pt(diphos)], ^

|Pt(diphos)|

diphos = (ieri-Bu) P(CH ) ,P(terf-Bu> 2

2

:

2

T h e inner coordination sphere of the d i m e r is shown i n F i g u r e 5.

B y the n o r m a l

rules of electron counting, the rare-gas configuration is obeyed at p l a t i n u m if a P t = P t double bond is invoked. or slightly long, single bond.

Yet the P t - P t distance of 2.765(1)Â is a normal

O n e thus wonders if the c o m p o u n d m i g h t be a


3.

IBERS

Location of Terminal Hydride Ligands y [Pt(diphos)] + D 2

2

33

PtD (diphos)

2

2

(7)

P t H D (diphos) PtH

2

(diphos)

toluene

[Pt(diphos)] + 2

hydride.

CHCI3 — •

P t C l (diphos) + C H C l 40% 92% 2

2

2

(8)

Some chemistry of this d i m e r (Reactions 7 a n d 8) suggests that it could

be a h y d r i d e , presumably c o n t a i n i n g P t - H - P t bridges. As we noted above, there are instances where one fails to locate the h y d r i d e


position i n a k n o w n transition m e t a l h y d r i d e .

Consequently, f a i l u r e to locate

residual electron density i n positions thought to be reasonable for hydride ligands hardly can be taken as strong evidence against a given compound being a hydride. T h e classic diagnostic tool for detecting hydrides has been the h i g h - f i e l d shift of the h y d r i d e proton i n the N M R spectrum.

T h e present d i m e r shows no such

shift; nor is there i r evidence for the presence of h y d r i d e bridges.

However,

H a y m o r e (17) and Stone (see C h a p t e r 8) (18) have demonstrated that the proton signal i n the N M R spectrum is not always f o u n d at h i g h f i e l d but rather m a y be f o u n d at low field.

Stone's example, i n fact, is the c o m p o u n d [Pt(M-H)(SiEta)-

(PCy3)] , a c o m p o u n d closely related to the present one. 2

N o x-ray evidence for



34

TRANSITION M E T A L HYDRIDES

Figure

5.

The inner coordination

b r i d g i n g hydrides was f o u n d there.

sphere of Bu)2)h

[Pt((teTt-Bu) P(CH2)^P(teTt 2

B u t there was i r evidence for P t - H b o n d -

i n g — e v i d e n c e that was later corroborated b y the successful location of the h y d r i d e signal i n the N M R spectrum.

T h e r e is x-ray, N M R , a n d i r evidence to

support bridging hydrides i n the similar compound [ N i ( M - H ) ( C y P ( C H ) 3 P C y ) ] 2

(19).

2

2

2

In both of these v e r i f i e d b r i d g i n g h y d r i d e structures, the L M L / L M L i n -

terplanar angles are near 23°.

I n our d i m e r , this angle is 82°.

In essence then,

there is negative d i f f r a c t i o n a n d spectroscopic evidence for the presence of h y d r i d e bridges i n the present complex, but negative evidence is never f u l l y satisfactory.

H e r e we must appeal to stereochemical arguments that if the compound

were a h y d r i d e , the P P t P / P P t P interplanar angle w o u l d be close to 0 ° .

This

example illustrates, p a r t i c u l a r l y for transition metal complexes c o n t a i n i n g or possibly c o n t a i n i n g b r i d g i n g h y d r i d e ligands, that x-ray d i f f r a c t i o n a n d spectroscopic results m a y not always lead to a totally satisfying conclusion.


3. IBERS

Location of Terminal Hydride Ligands

35

Conclusions In this short chapter, I have attempted to provide an overview of the types of results that one can expect to obtain from x-ray diffraction studies of transition metal hydride complexes. The salient points seem to me to be: ( 1 ) one cannot predict at the outset whether one will be successful in locating the hydride position or, in fact, a given feature of a structure owing to the possibility of disorder and, at times, to unknown causes; (2) in favorable cases, the hydride position can be located to an accuracy commensurate with or exceeding our ability to use such information in theoretical models; (3) our expectations concerning the stereo chemistry of transition metal hydrides that contain terminal hydride ligands have changed little in the 12 years since the determination of the structure of RhH(CO)(PPh )3; hence it is unlikely that the study of such structures in them selves is a fertile field of structural chemistry.


3

Acknowledgment Most of the work discussed here was kindly supported by the National Science Foundation.

Literature Cited 1. 2. 3. 4.

Ibers, J. Α., Ann. Rev. Phys. Chem. (1965) 16, 375. La Placa, S. J., Ibers, J. Α., J. Am. Chem. Soc. (1963) 85, 3501. La Placa, S. J., Ibers, J. Α., Acta Crystallogr. (1965) 18, 511. La Placa, S. J., Hamilton, W. C., Ibers, J. Α., Davison, Α., Inorg. Chem. (1969) 8, 1928. 5. Brown, L. D., Ibers, J. Α., Inorg. Chem. (1976) 15, 2794. 6. Brown, L. D., Robinson, S. D., Sahajpal, Α., Ibers, J. Α., Inorg. Chem. (1977) 16, 2728 7. Schramm, K. D., Ibers, J. Α., Inorg. Chem. (1977) 16, 3287. 8. Waters, J. M., Ibers, J. Α., Inorg. Chem., (1977) 16, 3273. 9. Krogsrud, S., Toniolo, L., Croatto, U., Ibers, J. Α., J. Am. Chem. Soc. (1977) 99, 5277. 10. Yoshida, T., Yamagata, T., Tulip, T. H., Ibers, J. Α., Otsuka, S., J. Am. Chem. Soc., in press. 11. Nakamura, Α., Yoshida, T., Cowie, M., Otsuka, S., Ibers, J. Α., J. Am. Chem. Soc. (1977) 99, 2108. 12. Laing, K. R., Robinson, S. D., Uttley, M. F., J. Chem. Soc., Dalton Trans. (1974) 1205. 13. Robinson, S. D., Sahajpal, Α., J. Organomet. Chem. (1976) 117, C111. 14. Knoth, W. H., Hertler, W. R., Muetterties, E. L., Inorg. Chem. (1965) 4, 280. 15. Knoth, W. H., J. Am. Chem. Soc. (1972) 94, 104. 16. Collins, T. J., Roper, W. R., Town, K. G., J. Organomet. Chem. (1976) 121, C41. 17. Haymore, B. L., "Abstracts of Papers," Joint Conf. CIC/ACS, 2nd, Montreal, May 29-June 2, 1977, INDOR 126. 18. Stone, F. G. Α., ADV. CHEM. SER. (1978) 167, 000. 19. Jolly, P. W., Wilke, G., "The Organic Chemistry of Nickel," Vol. 1, p. 145, Academic, London, 1974. RECEIVED July 19, 1977.


Location of Terminal Hydride Ligands in Transition Metal Hydrides

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