Neutron Diffraction Studies of Tetrahedral Cluster ... - ACS Publications

5 Neutron Diffraction Studies of Tetrahedral Cluster Transition Metal Hydride Complexes:

HFeCo (CO) (P(OCH ) ) and H Ni (C H )

Downloaded by UNIV OF SYDNEY on January 8, 2016 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/ba-1978-0167.ch005

3

3

3

3

3

4

5

5

9

4

THOMAS F. KOETZLE and RICHARD K. McMULLAN-Department of Chemistry, Brookhaven National Laboratory, Upton, NY 11973 ROBERT BAU, DONALD W. HART, RAYMOND G. TELLER, DONALD L. TIPTON and ROBERT D. WILSON -Department of Chemistry, University of Southern California, Los Angeles, CA 90007 1

Structures of the tetrahedral cluster transition metal hydride complexes HFeCo (CO) (P(OCH ) ) and H Ni (C H ) have been investigated by low-temperature neutron diffraction techniques. Both complexes have approximateC symmetry. In HFeCo (CO) (P(OMe) ) , the hydride ligand is found outside the FeCo cluster, 0.978(3) Åfrom the Co face and essentially on the molecular threefold axis, triply bridging the cobalt atoms. In H Ni (Cp) , the three hydride ligands are face bridging, and their mean displacement from the faces of the cluster is 0.90(3)Å.The H N i core may be envisaged as a distorted cube with one vertex unoccupied. The observed geometries of the two clusters considered here suggest a plausible model for chemisorption of hydrogen on {111} or {001} surfaces of ccp or hcp metals, respectively, in which hydrogen atoms are located approximately 1 Åabove the centers of triangles of metal atoms. 3

9

3 3 3

3

4

5

5

4

3v

3

9

3 3

3

3

3

4

4

3

A

variety

of

factors

contribute

4

to

L polynuclear metal hydride complexes.

the

great

current

interest

in

These include the novel geometries

found i n these systems and their usefulness as models for the bonding of hydrogen to metals, such as m a y occur i n catalysis ( i ) or hydrogen-storage applications (2). A comprehensive review of metal h y d r i d e complexes, i n w h i c h polynuclear species are i n c l u d e d , has been published b y Kaesz a n d Saillant (3). 1

Present address:

Department of Chemistry, Northwestern University, Evanston, I L 60201.

0-8412-0390-3/78/33-167-061/$05.00/0 ©

American Chemical Society

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

62

TRANSITION M E T A L HYDRIDES

D i r e c t location of h y d r i d e ligands i n m e t a l complexes b y x-ray d i f f r a c t i o n m a y be difficult, especially i n the case of b r i d g i n g hydrides c o m m o n l y occurring i n polynuclear systems.

Several cases have been reported where the h y d r i d e

was f o u n d successfully despite this d i f f i c u l t y .

F o r example, C h u r c h i l l a n d D e -

Boer have located the b r i d g i n g h y d r i d e i n H O s ( C O ) i o ( C H C H P M e 2 P h ) (4) 3

2

w h i l e we have used a F o u r i e r - a v e r a g i n g technique to locate the f a c e - b r i d g i n g hydrogen atoms i n H U R e 4 C O ) 1 2 (5).

However, x-ray diffraction studies cannot,

i n a n y event, be expected to provide m e t a l - h y d r o g e n b o n d lengths of accuracy m u c h better than ± 0 . 1 Λ.

T h u s , precise i n f o r m a t i o n on geometries of m e t a l

h y d r i d e complexes has depended on neutron diffraction.

T h e sensitivity of

neutron diffraction to light atoms i n general and hydrogen i n particular is caused


b y the large relative cross sections of these atoms c o m p a r e d w i t h those of heavy atoms.

F o r example, the ratio a(H):a(Os), is 0.12 for neutrons a n d 1.7 Χ 1 0 "

for x-rays (20 = 0° ).

4

T h u s , the relative contribution of hydrogen i n a structure

containing o s m i u m w i l l be roughly three orders of magnitude greater i n neutron than i n x-ray diffraction. In this chapter w e b r i e f l y review some results of p r i o r neutron d i f f r a c t i o n studies a n d present new results for two tetrahedral cluster-complexes w i t h faceb r i d g i n g h y d r i d e ligands:

HFeCo3(CO) (P(OMe) )3 and H N i ( C p ) . 9

pentadienyl, and Ph-phenyl.)

3

3

breviations used i n this paper are as follows:

4

4

(Ab

Me-methyl, Et-ethyl, Cp-cyclo-

In a d d i t i o n to these tetrahedral complexes, sin

gle-crystal, neutron d i f f r a c t i o n data c u r r e n t l y are available for the triangular r u t h e n i u m a n d o s m i u m species H R u ( C O ) ( C = C - C ( M e ) ) (6), H O s ( C O ) R , 3

9

3

3

9

R = H , v i n y l (7), a n d H D O s ( C O ) i - ( C H D ) (8), as w e l l as for the d o d e c a n i c k e l 3

0

cluster anions [ H N i i ( C O ) 2 i ] ~ a n d [ H N i i ( C O ) 2 i ] 2

3

2

2

2 -

(9).

d i f f r a c t i o n study of H N b e l n also has been reported (10).

A neutron p o w d e r The hydride ligand

was located at the center of the octahedral N b cluster, s i m i l a r to the situation 6

i n the dodecanickel anions mentioned above where the h y d r i d e ligands occur i n octahedral sites i n the n i c k e l framework.

Prior Neutron Diffraction

Work on Transition Metal Hydride

Complexes

T h e first neutron d i f f r a c t i o n study of a transition m e t a l h y d r i d e c o m p l e x was that of K R e H ( 11 ), reported i n .1964, that showed that the [ H R e ] " dianion 2

9

9

2

forms a tricapped trigonal prism w i t h a mean r h e n i u m - h y d r o g e n b o n d distance of 1.68(1) Â.

This investigation, together with subsequent x-ray (12) a n d neutron

(13) diffraction studies of H M n ( C O ) s , demonstrated unequivocally that hydrogen is a stereochemically active l i g a n d i n the latter complex, a n d that t e r m i n a l m e t a l - h y d r o g e n distances correspond to those expected for n o r m a l covalent bonds.

M o r e recently, a substantial b o d y of accurate data on t e r m i n a l a n d

bridging metal-hydrogen bonds has emerged based on neutron diffraction studies of 23 complexes, listed i n Table I. W e have published a review covering this work u p to 1976 (26), a n d results for p o l y h y d r i d e complexes are discussed i n an acc o m p a n y i n g article (27).


5.

K O E T Z L E E T AL.

T a b l e I.

Neutron Diffraction

63

Studies

T r a n s i t i o n M e t a l H y d r i d e C o m p l e x e s S t u d i e d by N e u t r o n Diffraction" A.

Mononuclear

HMn(CO) (J3) HZnN(Me)C H N(Me) D M o ( C p ) (15) H M o ( C p ) (16) H Ta(Cp) (77) K +[H Re]2-(n) H 0 s ( P M e P h ) (18) 5

2

2

2

(14)

2

2

3

2

2

9

4

2

3

B.


4

2

Binuclear

[ E t N ] [ H C r ( C O ) ] - (19) ( P h P ) N ] [ H C r ( C O ) ] - (20) H M o ( C p ) ( C O ) ( P M e ) (27) H W ( C O ) ( N O ) (22) H W ( C O ) ( N O ) ( P ( O M e ) ) (23) H R e ( P E t P h ) (24) [ H I r ( C M e 5 ) ] B F - (25) +

4

2

3

1 0

+

2

2

2

2

2

1 0

4

2

9

2

8

8

2

3

3

2

2

4

5

2

C.

+

4

Polynuclear

HFeCo (P(OMe) ) H Ni (Cp) [ ( P h P ) N ] ] H N i ( C O ) ] - (9) [ ( P h P ) N ] + [ H N i ( C O ) ] - (9) H N b I „ (10) H R u ( C O ) ( C = C - C ( M e ) ) (6) H Os (CO) (7) H O s ( C O ) ( C H ) (7) H D O s ( C O ) ( C H D ) (8) 3

3

3

4

3

4

3

2

3

2

3

+

1 2

2

2

2 1

1 2

2 1

3

2

6

3

2

9

3

3

1 0

3

a

M e : methyl; Et: ethyl;

3

1 0

2

3

1 0

C p : cyclopentadienyl;

Ph: phenyl.

T h e studies of H M o ( C p ) 2 ( C O ) 4 ( P M e ) (21), H W ( C O ) ( N O ) (22), a n d 2

2

2

9

H W ( C O ) 8 ( N O ) ( P ( O M e ) 3 ) (23) are of p a r t i c u l a r significance a n d p r o v i d e de2

finitive evidence that m e t a l - h y d r o g e n - m e t a l bridges i n these binuclear species are best described as closed, three-center bonds w i t h significant m e t a l - m e t a l interaction (22).

It is not surprising that this is the case since m e t a l orbitals of

proper s y m m e t r y that interact w i t h H ( l s ) also can interact w i t h one another, as has been pointed out by H o f f m a n n (28).

In H W ( C O ) ( N O ) ( P ( O M e ) ) (Figure 2

8

3

1), the tungsten-hydrogen-tungsten bridge was f o u n d to be slightly asymmetric w i t h the h y d r i d e l i g a n d displaced t o w a r d the W ( C O ) s group, as c o u l d be p r e dicted on electron-counting considerations.

It is likely that such asymmetry also

exists i n H W ( C O ) g ( N O ) , but the effect c o u l d not be measured since both crys2

talline forms of this latter complex exhibit disorder w i t h rotation of the molecule a r o u n d the pseudo t w o f o l d axis passing through the h y d r o g e n atom.



64

TRANSITION M E T A L

Figure

1.

View of

HYDRIDES

HW (CO) (NOXP(OMeh) 2

s

Experimental A p o w d e r e d sample of H F e C o ( C O ) 9 ( P ( O M e ) 3 ) 3 was s u p p l i e d by H . D . Kaesz a n d Β. T. H u i e of the U n i v e r s i t y of C a l i f o r n i a , L o s Angeles, a n d recrystallized from a hexaneiCH2Cl2 (6:1) mixture. Single crystals of H N i 4 ( C p ) 4 were supplied by J. M u l l e r of T h e Technical University of Berlin. L a r g e single crystals of both compounds were affixed to a l u m i n u m pins a n d m o u n t e d i n cryostats on an automated four-circle diffractometer (29, SO) at the B r o o k h a v e n H i g h F l u x B e a m Reactor. H F e C o ( C O ) ( P ( O M e ) ) was studied at 90° K , a n d H N i ( C p ) at 81 °K. C r y s t a l data a n d e x p e r i m e n t a l parameters are s u m m a r i z e d i n T a b l e II. 3

3

3

T a b l e II.

9

3

3

3

3

Space group C e l l parameters α b c β C e l l vol N o . of molecules per unit cell (Z) M o l wt Calc. density Absorption coefficient ( μ ) Wavelength Sample vol D a t a collection temp D a t a collection l i m i t (sin0/X) N o . of reflections used i n structure analysis F i n a l agreement factors α

6

9

H Ni (Cp) 3

P2x/c 15.957(8) Å 10.611(5) 18.383(9) 98.70(2)° 3077(3) Å 4 858.0 1.85 g / c m 1.54 c m " 1.1598(1) Å 31.2 m m 90.0(4)°K 0.68 Â " 8229 R = 0.070 R = 0.035 3

3

1

3

1

F

wF

4

F

0

C

0

wF

0

c

2

2

0

4

C2/c 28.312(13) Å 9.234(5) 14.783(7) 103.35(2)° 3760(3) Å 8 498.2 1.76 g / c m 1.94 c m " 1.0183(1) Å 12.5 m m 81(1)°K 0.68 Â " 2656 R = 0.107 R F = 0.067 3

3

1

3

1

F

W

Calculated assuming an incoherent scattering cross section for hydrogen of 40 barn. R =2\F \F \\/XF R =&w\F \F \\ /ZwF V .

b

4

C r y s t a l D a t a a n d E x p e r i m e n t a l Parameters HFeCo (CO) (P(OMehh

α

4

/2


5.

Neutron Diffraction

K O E T Z L E E T AL.

65

Studies

F o r HFeCo3(CO)9(P(OMe)3) , starting phases were calculated based on the positions of nonhydrogen atoms determined f r o m a prior x-ray analysis (31 ), a n d all h y d r o g e n atoms were then located i n a d i f f e r e n c e - F o u r i e r synthesis. Initial refinement was c a r r i e d out w i t h an automated procedure using d i f f e r e n t i a l F o u r i e r syntheses (32), followed b y f u l l - m a t r i x least-squares based u p o n F o , i n c l u d i n g reflections w i t h F o < 0. Parameters were b l o c k e d into groups of ca. 250, a n d anisotropic t h e r m a l factors were used for a l l atoms. Satisfactory c o n vergence was achieved, a n d a l l b o n d distances were d e t e r m i n e d w i t h precision better than 0.004 Å. F o r H 3 N i ( C p ) , the initial phasing model consisted of the nickel and carbon atoms at positions d e t e r m i n e d i n an earlier x-ray study (33, 34), w i t h c y c l o p e n t a d i e n y l h y d r o g e n atoms i n calculated positions. T h e h y d r i d e ligands were lo cated i n a d i f f e r e n c e - F o u r i e r synthesis, a n d the structure was refined by leastsquares procedures, i n c l u d i n g only reflections w i t h F > l.5a(Fo ). The rela tively h i g h discrepancy between calculated a n d observed structure factors (see T a b l e II) results f r o m the fact that a large fraction of the reflections were m e a sured to have very low intensity, i.e. 3478 of a total of 5633 u n i q u e reflections were observed with F o < 3a(F(f). However, chemically equivalent bond lengths i n the H3NL1 core agree to w i t h i n 0.04 Λ. Anisotropic t h e r m a l factors r e f i n e d to q u i t e large values for certain atoms i n the C p rings, as m i g h t be expected if the barrier to rotation of the rings i n the solid state is assumed to be low. 3

2

2

4

4


0

2

2

2

Results HFeCo3(CO)g(P(OMe)3)3. T h i s complex is found to possess essentially Cs s y m m e t r y , w i t h the geometry shown schematically i n F i g u r e 2. F i g u r e 3 illus trates the molecular structure w i t h t h e r m a l ellipsoids a n d gives the a t o m i c n u m b e r i n g scheme. T h e h y d r i d e l i g a n d is located outside the F e C o 3 cluster, 0.978(3)Å f r o m the C o face, t r i p l y b r i d g i n g the cobalt atoms. E x c l u d i n g the cobalt-cobalt bonds, the e n v i r o n m e n t of each cobalt atom is a p p r o x i m a t e l y oc tahedral, and the position of the hydride ligand might be inferred by the presence of one vacancy c o m m o n to a l l three octanedra, trans to the t e r m i n a l c a r b o n y l on each cobalt atom. These results c o n f i r m the findings of H u i e et al. (31 ) based on their x-ray d i f f r a c t i o n study at 134° K . T h e b r i d g i n g h y d r i d e is f o u n d esv

3

0



TRANSITION M E T A L

Figure S. Molecular structure of HFeCo^CO)^(P(OMe)s)a with thermal ellipsoids drawn to enclose 50 % probability (Ref. 35). Methoxy groups have been removed for clarity. (Top) View normal to the threefold molecular axis; (bottom) view approximately along the three fold axis.


HYDRIDES

5.

KOETZLE

ET

Neutron Diffraction

AL.

T a b l e III.

67

Studies

Selected B o n d Distances a n d Angles i n H F e C o 3 (CO)9(P(OMe)3)3a

Distances

Angles

(°)

Mean

1.742(3) 1.731(3) 1.728(3) 1.734(4)

Co(l)-H-Co(2) Co(l)-H-Co(3) Co(2)-H-Co(3) Mean

92.1(1) 91.7(1) 91.5(1) 91.8(2)

Co(l)-Fe Co(2)-Fe Co(3)-Fe Mean

2.563(2) 2.556(2) 2.558(2) 2.559(2)

Fe-Co(l)-H Fe-Co(2)-H Fe-Co(3)-H Mean

89.5(1) 90.0(1) 90.0(1) 89.8(2)

Co(l)-Co(2) Co(l)-Co(3) Co(2)-Co(3) Mean

2.501(2) 2.489(3) 2.477(3) 2.489(7)

M e a n values Co-P C o - C (terminal C O ) C o - C (bridging C O ) Fe-C C - 0 (terminal) C - 0 (bridging)

2.175(4) 1.756(4) 1.953(6) 1.798(2) 1.147(1) 1.165(1)

Fe-Co(l)-Co(2) Fe-Co(l)-Co(3) Fe-Co(2)-Co(l) Fe-Co(2)-Co(3) Fe-Co(3)-Co(l) Fe-Co(3)-Co(2) Mean

60.6(1) 60.8(1) 60.9(1) 61.1(1) 61.0(1) 61.0(1) 60.9(1)

Co(l)-Fe-Co(2) Co(l)-Fe-Co(3) Co(2)-Fe-Co(3) Mean

58.5(1) 58.2(1) 58.0(1) 58.2(1)

Co(l)-H Co(2)-H Co(3)-H


(Å)

M e a n values P-Co-H C - C o - H (terminal C O ) C - C o - H (bridging C O ) Co-C-Co

91.6(39) 170.5(14) 83.8(6) 79.2(1)

Standard deviations of mean values are calculated as ( Σ ( * , —x) /n(n — l ) ) / , where η is the number of observations. T h e resulting deviations are to be regarded as rough estimates of uncertainty, in cases where η = 3. 2

a

1

2

sentially on the m o l e c u l a r threefold axis, as illustrated i n F i g u r e 3 (bottom). Selected b o n d distances a n d angles are presented i n T a b l e H I . It is interesting to note that the mean carbon-cobalt-hydrogen angle (terminal cobalt) is 170(1)°, so that the h y d r i d e l i g a n d lies about 0.3Å farther f r o m the C03 face than w o u l d be predicted on the basis of undistorted octahedral coordination around the cobalt atoms. In H R u ( C O ) 9 ( C = C - C ( M e ) 3 ) , the analogous m e a n c a r b o n - r u t h e n i u m - h y d r o g e n angle involving the carbonyls trans to the doubly b r i d g i n g hydride l i g a n d is 176.4(5)° (6). H 3 N i ( C p ) 4 . T h e structure of H N i 4 ( C p ) 4 , shown schematically i n F i g u r e 4, consists of a tetrahedral nickel cluster w i t h each n i c k e l atom π-bonded to a C p ring. T h e three h y d r i d e ligands are face b r i d g i n g , as d e d u c e d on the basis of x-ray data (33,34). F i g u r e 5 gives a close-up view of the H3N14 core, w h i c h m a y be envisaged as a tricapped tetrahedron or equivalently as a distorted cube w i t h alternate corners occupied by N i a n d H atoms and one corner vacant. Selected b o n d distances a n d angles are presented i n T a b l e I V . T h e m e a n displacement of the h y d r i d e ligands f r o m the faces of the N14 cluster is 0.90(3) Â. 3

4

3



68

TRANSITION

METAL

HYDRIDES

Discussion In 1968, an unusual structure for H F e C o ( C O ) i w i t h the h y d r i d e l i g a n d 3

2

located inside the cage was proposed by M a y s (36) on the basis of mass spectral evidence a n d electron-counting considerations.

H o w e v e r , this m o d e l was dis-

proved by the x-ray work of H u i e et al. on the tris(trimethyl phosphite) derivative (31 ), i n w h i c h the h y d r i d e l i g a n d was located i n a d i f f e r e n c e - F o u r i e r synthesis and shown to bridge the C03

face.

T h e present neutron d i f f r a c t i o n study has

allowed definitive placement of the hydride ligand and has yielded more accurate b o n d distances a n d angles. One

motivation to c a r r y out a neutron d i f f r a c t i o n investigation of

H3Ni4(Cp)4 was to check the possibility of disorder of the h y d r i d e ligands over all four faces of the N i tetrahedron. 4

x-ray data (33, 34).

T h e hydrides were not located f r o m the

Rather, their positions were i n f e r r e d f r o m the deviations

of the structure from strict tetrahedral symmetry.

T h e observed C p ( i ) - C n - C p ( j )

angles (see T a b l e I V ) are distorted f r o m the tetrahedral value such that Cp(2), Cp(3), a n d Cp(4) are bent away f r o m C p ( l ) .

T h e face defined by N i ( 2 ) , Ni(3),

and Ni(4) therefore could be expected to be vacant.

O u r neutron results indicate

that this is indeed the case, w i t h no evidence for disorder of the h y d r i d e ligands on the nuclear density maps. T h e metal clusters i n H F e C o ( C O ) ( P ( O M e ) ) 3 a n d H N i ( C p ) contain 3

different numbers of electrons.

9

3

3

4

4

T h e former cluster is a closed-shell structure

(60 electrons) w h i l e the latter contains 63 electrons a n d is paramagnetic w i t h S


t

KOETZLE

ET

AL.

Neutron Diffraction Studies

69


5.

(b)

H3M4

Figure 5. The core of Η Μ ( C p ) , drawn with thermal ellipsoids en closing 50 % probability, (a) View approximately along the threefold molecular axis; (b) view approximately normal to the Ni(l)-Ni(2) bond. 3

4

4


70

TRANSITION

Table IV.

HYDRIDES

Selected Distances a n d Angles i n H N i ( C p ) 4 '

Distances Ni(l)-H(l) Ni(l)-H(2) Ni(l)-H(3) Mean


METAL

3

(A) 1.720(8) 1.718(9) 1.711(7) 1.716(3)

û

4

6

Angles (°) Ni(l)-H(l)-Ni(2) Ni(l)-H(l)-Ni(3) Ni(l)-H(2)-Ni(3) Ni(l)-H(2)-Ni(4) Ni(l)-H(3)-Ni(2) Ni(l)-H(3)-Ni(4) Mean

94.0(4) 93.1(4) 93.6(4) 93.9(4) 94.7(4) 93.0(3) 93.7(3)

Ni(2)-H(l)-Ni(3) Ni(3)-H(2)-Ni(4) Ni(2)-H(3)-Ni(4) Mean

94.1(4) 95.7(4) 93.1(4) 94.3(8)

Ni(2)-H(l) Ni(2)-H(3) Ni(3)-H(l) Ni(3)-H(2) Ni(4)-H(2) Ni(4)-H(3) Mean

1.684(7) 1.674(8) 1.674(8) 1.661(9) 1.672(8) 1.704(8) 1.678(6)

Ni(l)-Ni(2) Ni(l)-Ni(3) Ni(l)-Ni(4) Mean

2.490(3) 2.464(3) 2.478(3) 2.477(8)

Cp(l) -Cn -Cp(2) Cp(l)-Cn-Cp(3) Cp(l)-Cn-Cp(4) Mean

117.5(2) 112.3(2) 112.3(2) 114.0(17)

Ni(2)-Ni(3) Ni(2)-Ni(4) Ni(3)-Ni(4) Mean

2.458(3) 2.454(3) 2.471(3) 2.461(5)

Cp(2)-Cn-Cp(3) Cp(2)-Cn-Cp(4) Cp(3)-Cn-Cp(4) Mean

105.0(2) 103.6(2) 105.1(2) 104.6(5)

H(1)...H(2) H(1)...H(3) H(2).»H(3) Mean

2.317(11) 2.305(10) 2.326(9) 2.316(6)

Ni(l)-Cp

Neutron Diffraction Studies of Tetrahedral Cluster ... - ACS Publications

Recommend Documents