Vibrational Spectroscopy of Hydride-Bridged Transition Metal

17

Downloaded by UNIV OF SYDNEY on November 4, 2016 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/ba-1978-0167.ch017

Vibrational Spectroscopy of Hydride-BridgedTransition Metal Compounds C. B. COOPER III, D. F. SHRIVER, and S. ONAKA Department of Chemistry, Northwestern University, Evanston, IL 60201

The fundamental vibrations have been assigned for the M-H-M backbone of HM (CO)10 , M = Cr, Mo, and W. When it is observable, the asymmetric M-H-M stretch occurs around 1700 c m in low temperature ir spectra. One or possibly two deformation modes occur around 850 cm in conjunction with overtones that are enhanced in intensity by Fermi resonance. The symmetric stretch, which involves predominantly metal motion, is expected below 150 cm . For the molybdenum and tungsten compounds, this band is obscured by other low frequency features. Vibrational spectroscopic evidence is presented for a bent Cr-H-Cr array in [PPN][(OC) Cr-H-Cr(CO) ]. This structural inference is a good example of the way in which vibrational data can supplement diffraction data in the structural analysis of disordered systems. Implications of the bent Cr-H-Cr array are discussed in terms of a simple bonding model which involves a balance between nuclear repulsion, M-M overlap, and M-H overlap. The literature on M-H-M frequencies is summarized. 2

-

-1

-1

-1

5

I

5

spectroscopy has p l a y e d a major role i n t h e characterization of hydrogen-bonded compounds and hydride-bridged boron compounds, but similar progress has not been m a d e w i t h the v i b r a t i o n a l spectra of h y d r i d e b r i d g e d m e t a l compounds. O n e of the m a i n problems is that the M - H modes of h y d r i d e - b r i d g e d m e t a l compounds are frequently weak a n d broad i n both the i r a n d R a m a n . Indeed it is often not possible to detect the h y d r i d e stretch of a M - H - M system i n the i r spectrum obtained at r o o m temperature. L o w temperature i r and R a m a n spectroscopy afford significant improvements i n the ease of detecting b r i d g i n g M - H modes. In the present work, these techniques provide sufficient i n f o r m a t i o n to p e r m i t assignment of the major f u n d a m e n t a l vibrational modes of M - H -Bau; M systems n d to supplement the structural i n f o r Transition aMetal Hydrides Advances in American Chemical Society: Washington, DC, 1978. m a t i o n available f rChemistry; o m neutron diffraction. T

17.

233

Vibrational Spectroscopy

COOPER ET AL.

Boron Hydride Bridged Systems T h e three-centered, two-electron hydride bridge, w h i c h is prevalent i n boron h y d r i d e chemistry, has been very w e l l characterized i n the i r a n d R a m a n spec troscopy of diborane a n d certain metal borohydrides.

A brief r e v i e w of these

data w i l l be given at this point because they afford insight into h y d r i d e - b r i d g e d systems. Diborane.

As shown i n T a b l e I, the B - H t e r m i n a l stretching modes of d i

borane occur at an average frequency of 2563 c m bridging B - H stretches (ι/ ,

- 1

, whereas the average of the

*Ί3, and vyj) occurs 28% lower, at 1847 c m

2

- 1

.

The

existing n o r m a l coordinate vibrational analyses display some abnormalities, such


as a near zero H b - B - H b b e n d i n g force constant, but the general features of the force field for diborane are reasonably well understood.

T h e vibrational analyses

indicate that the B - H b force constant is about half of that for B - H (I, 2, 3). t

T a b l e I.

Some F u n d a m e n t a l s for Gas Phase B 2 H 6 U

2524 cm 2104

stretch stretch stretch (ring breathing)

"6

B-Hb

stretch (ring stretch)

1768°

Biu

"8

B-H

t

stretch

2612

B

"11

B-H

t

stretch

2591

"13

B-Hb

t

v

2

"4

2g

B

2u

Bzu

stretch (ring stretch) stretch stretch (ring stretch)

B-H B-Hb

"16

t

"17 a

(8)

B-H B-Hb B-B

"ι

A,

This

794

1915 2525 1602

Corrected for Fermi resonance.

result fits w i t h the simple b o n d i n g models that distribute two b o n d i n g electrons between B - H i n the t e r m i n a l case a n d an average of one b o n d i n g electron be tween each B - H i n a h y d r i d e bridge.

Also, a large B - B force constant is c a l c u

lated (ca. 2.7 m d y n / A ) , w h i c h the authors interpret as evidence for significant B - B b o n d i n g (2).

This interpretation is based on the reasonable assumption that

the B - H - B bending force constant is very low and can be neglected.

However,

it is important to point out that if a significant resistance to B - H - B b e n d i n g is introduced, the B - B force constant w o u l d be calculated lower. U n f o r t u n a t e l y , i n c y c l i c systems such as these, the r e d u n d a n c y between the various stretching a n d b e n d i n g coordinates makes it impossible to prove the extent of direct B - B bonding. Metal Borohydrides.

Several covalent borohydrides, B e ( B H 4 ) (4, 5), 2

A l ( B H ) (6), Z r ( B H ) (7), a n d H f ( B H ) 4 , a n d U ( B H ) 4 (7), have been the topic 4

3

4

4

4

4

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

234

TRANSITION M E T A L HYDRIDES

of detailed ir and R a m a n spectroscopic investigation. F o r one of these, A l ( B H ) 3 4

(4), the c o m b i n e d v i b r a t i o n a l a n d electron d i f f r a c t i o n data indicate an overall s y m m e t r y , w i t h each B H bonded by a double h y d r i d e b r i d g e (Structure

DM

4

2) to the central a l u m i n u m atom.

Cook and N i b l e r make a case that the A l - H - B

bridge stretching vibrations are best described as i n d i v i d u a l B - H b a n d A l - H b stretches rather than r i n g stretches a n d expansions similar to those i n diborane. T h e i r argument is based on the presence of four equivalent B - H bridge bonds i n diborane as opposed to the large inequivalence between B - H a n d A l - H i n A l ( B H ) , yielding two bands that are primarily B - H b stretches above 2000 c m 4

3

and two p r e d o m i n a n t l y A l - B b stretches below 1600 c m

- 1

.

These frequencies

are significantly lower than typical B - H and A l - H stretching modes.


t

- 1

t

The A l B

3

s y m m e t r i c stretch i n this c o m p o u n d is assigned to a p o l a r i z e d R a m a n b a n d at 495 c m cm

- 1

- 1

, a n d the doubly degenerate a s y m m e t r i c stretch is f o u n d at 596

. As opposed to the double h y d r i d e bridges (Structure 2) discussed for

A l ( B H ) 3 , single h y d r i d e bridges (Structure 1) appear to exist i n Β2Ηγ~, 4

H(B(CH3)3)2~, and H(B(C2H5)3)2~, for w h i c h rather broad i r bands of moderate intensity (2050, 2100, a n d 1915 c m " h y d r i d e stretching modes (6).

respectively) are assigned to the b r i d g i n g

1

T h e compounds B e ( B H ) 4

2

(gas phase) (7),

Z r ( B H ) (9), H f ( B H ) (JO), and U ( B H ) (9) all contain the triple hydride bridge 4

4

4

(Structure 3).

4

4

4

F o r H f ( B H ) , the symmetric H f - B stretch is assigned to a R a m a n 4

4

feature at 552 c m " , a n d the s y m m e t r i c B - H a n d B - H b stretches are at 2572 1

and 2192 c m "

1

t

respectively.

T h i s pattern of higher t e r m i n a l than b r i d g e B - H

stretching modes first was discussed i n detail by P r i c e ( I I , 12) and is most pro nounced when the M - H b bond is strong; thus, weakening the B - H b bond.

Owing

to this frequency decrease, it is generally easy to detect M - H - B linkages i n metal borohydrides.

V i b r a t i o n a l spectroscopy also offers a convenient m e t h o d of

distinguishing double (see

Structure 2) a n d triple (see Structure 3) h y d r i d e -

bridged borohydrides (13).

O n e of the most characteristic features is the presence

of two BH2t stretches i n the former a n d a single B H stretch for the latter. t

\

/

— Β — Η — Β — / \

χ

\ κ

Η

Μ \

/ H

Transition Metal Hydride-Bridged

Η

Β

/

\

M — H — Β — H \ H

Systems

T e r m i n a l M - H stretching modes of transition metal hydrides are readily identified i n the i r around 1900 ± 300 c m " , w i t h intensities that usually are 1

stronger than C H stretching modes (14).

W i t h b r i d g i n g hydrides, however, the

bands often are not observed i n the ir. As demonstrated i n the laboratories of both Jones (15) and Kaesz (14), M - H modes of bridging hydrides are more readily


17.

235


COOPER E T AL.

observable i n the R a m a n spectrum, but even w i t h this technique the bands are broad at room temperature.

As w i l l be described later, low temperatures lead

to greatly decreased bandwidths.

Unfortunately, most synthetic chemists have

not used the R a m a n technique w i t h n e w l y prepared transition metal hydrides so there is, at present, a paucity of data i n this area.

Also, the assignments are

not entirely clear for the series of R a m a n bands observed between 800-900 c m

- 1

.

M o r e discussion of this p r o b l e m w i l l be given later. As with diborane and the metal borohydrides, the M - H - M stretching modes are lower i n frequency than their terminal counterparts.

O t h e r chapters i n this

v o l u m e describe the various environments for b r i d g i n g hydrides w h i c h range


f r o m nearly linear (Structure 4) a n d strongly bent (Structure 5) single h y d r i d e bridges (16,17,18,19)

to double (20), Structure 6, triple (21), Structure 7, a n d

q u a d r u p l e (22), Structure 8, bridges between two metals.

In a d d i t i o n , h y d r i d e

bridges i n metal clusters can either bridge a triangular face (14) or be present at the center of an octahedral site (23).

In this chapter, we discuss three of these

b r i d g i n g h y d r i d e configurations, Structures 4, 5, a n d 6. D a t a for b r i d g i n g h y d r i d e modes are collected i n T a b l e II.

O n e of the

striking aspects of the R a m a n data is the large number of bands i n the 8 0 0 - 1 1 0 0 cm

region for some of the hydrides.

- 1

In several instances, the n u m b e r of bands

greatly exceeds the n u m b e r of expected fundamentals T a b l e II.

(14).

Vibrational Data on Metal H y d r i d e Bridged Complexes Vibrational Frequencies Associated with Hydride (Deuteride) Motion Ramam Ir (cm' ) (cm )

Complex

1

μ Hydrides HFeCo (CO) DFeCo (CO)i [HCoi^CsHsJU

- 1

Ref.

3

3

1 2

3

2

M2 H y d r i d e s HZn(C H ) DZn(C H ) HZn(C F ) DZn(C F ) [H Fe ((CH )C(CH P0 ) ) ]+ [D Fe ((CH )C(CH P0 ) ) ]+ [HTi(r 5C H5)(C H4)] [HTi(77 C H ) ] [DTi(77 C H ) ] 6

5

2

6

5

2

6

5

2

6

5

2

3

2

3

3

2

3

7

2

5

5

5

5

2

5

5

2

[H Re (CO) ] 2

2

8

D Re (CO) 2

2

8

2

2

5

5

24 24 25

1114 813 1052 950 890

2

2

3

3

2

2

26 26 27 27 21,28 21,28 29 30 30

1250-1650 900-1200 1300-1700 1000-1200 1048 790 1230 1450 1260 1060 1382 1275 973 922


31 31

236


T a b l e II. Continued Vibrational Frequencies Associated with Hydride (Deuteride) Motion Raman Ir (cm- ) (cm~ )

Complex

1

HRe (CO) Cl 2

1449 1311 1236 1213 1175 1051 958 925 907 881

8

DRe (CO) Cl Downloaded by UNIV OF SYDNEY on November 4, 2016 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/ba-1978-0167.ch017

2

8

HMo X DMo X H Re (CO) 3

2

8

2

8

3

a

3

α

3

D Re (CO) 3

3

1250 910

3

1 2

1 2

2

2

3

1 2

2

3

1100 1076 1000 792 752 692

1 2

H Os (CO) (terminal + μ ) D Os (CO) (terminal + μ ) HRe (CO) (possibly μ ) 2

4

H Ru (CO)i 4

4

31

32 32 15, 31, 33

15, 31, 33

34 34 1258 1184 1097 1041 952 904 850 1122 825 742

2

3

31

1930 1525 1410 1110

1 4

DRe (CO)i

Ref.

l

1605

2

31

31

35, 36 1585 1290

D Ru (CO)i 4

4

1272 1153

2

35, 36

1095 909 H D Ru (CO) 2

2

4

H Re (CO) 2

3

1 2

895 1 2

1587 1291 1156 909 1102 1052


36

31,37

17.

T a b l e II. Continued Vibrational Frequencies Associated with Hydride (Deuteride) Motion Raman (cm~ ) Ir (cm~ )

Complex

l

l

803 740 632 1165 1125 832 1442

D Re (CO) 2

3

1 2

H Re (CO) 6

4

2

D Re (CO) H Ru (CO) (possibly μ and/or μ ) 6

4

3


1 2

2

1 2

4

237


COOPER ETA L

1 2

2

Ref. 31,37

31 31 38

3

Group V I B M - H y d r i d e s [Et N][HCr (CO) ] [Et N][DCr (CO) ] [PPN][HCr (CO) ] 2

4

2

4

2

1 0

2

1 0

-1750 -1274 -1755

10

b

6

835 785

1274

[PPN][DCr (CO) ] 2

39 39

10

550 940 855 795 760 785 643 540 985 915 692 645 576

[Et N][HMo (CO) ] 4

2

1 0

[Et N][DMo (CO) ] 4

2

10

[PPN][HMo (CO) ] 2

1 0

[PPN][DMo (CO) ] 2

10

4

2

1 0

960 905 870 830 783 709 613 603

[Et N][DW (CO) ] 4

2

10

2

1 0

960 731 720 628

[PPN][DW (CO) ] 2

a

b

10

X = C l " , Br". Et N+ = ( C H C H ) 4 N , PPN+ = [ ^ Η ) Ρ ] 2 Ν . 4

3

2

+

6

5

3

39

39 39

39 39

1702

[PPN][HW (CO) ]

39

39, 40

1683

[Et N][HW (CO) ]

39

+


39

238


M

M

H — M 4

M

M'

M

5 6

( O C ) 5 M - H - M ( C O ) 5 - SALTS.

T h e single hydride bridged anions of C r , M o ,

and W , having the formula indicated above, were chosen by us as a starting point in the detailed investigation of h y d r i d e - b r i d g e d metal systems.

T h e structures

of these complexes i n the solid state are strongly cation dependent.

Room

temperature neutron d i f f r a c t i o n data for the P P N , bis(triphenylphosphine) Downloaded by UNIV OF SYDNEY on November 4, 2016 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/ba-1978-0167.ch017

+

i m i n i u m , salt of H C r ( C O ) i o ~ display a rather large t h e r m a l motion of the h y 2

dride ligand i n a plane perpendicular to the C r - C r axis, and consequently, it was not possible to determine if the C r - H - C r backbone is truly linear or is a composite of slightly bent structures (16).

R o o m temperature neutron data for the E t 4 N

+

salt, however, clearly shows a bent backbone with a M - H - M angle of 158.6° (16). S i m i l a r l y , low temperature (14°K) neutron data indicates that the M - H - M backbone of ( E t N ) ( H W ( C O ) i ) is strongly bent w i t h an angle of 137.2° (46). 4

2

0

A n a l o g y between room temperature x-ray data for ( P P N ) ( H W ( C O ) i o ) a n d 2

neutron data for H W ( C O ) ( N O ) indicates that the P P N + salt has an angle on 2

the order of 150° (18> 19).

9

T h e wide variation in M - H - M angles for these species

is indicative of a h i g h l y c o m p l i a n t M - H - M

backbone.

As a first a p p r o x i m a t i o n to the influence of structure on the vibrational frequencies, we shall concentrate on the M - H - M tri-atomic array. F o r a linear M - H - M unit, the n o r m a l modes are very simple.

T h e r e is a R a m a n - a c t i v e

symmetric M - H - M stretch (Structure 9) that only involves motion of the massive metal and therefore occurs at m u c h lower frequencies than the B - B a n d M - B stretching modes discussed i n a previous section.

In an intermediate frequency

region, a doubly degenerate M - H deformation mode occurs that involves h y drogen motion (Structure 10), a n d at h i g h frequencies, an a s y m m e t r i c ir-active stretching vibration (Structure 11) should be observed. If the M - H - M array is bent, the selection rules are relaxed f r o m the linear case; a l l three bands are allowed i n the ir a n d R a m a n .

Also, the f o r m of the

n o r m a l modes changes, i n that the two s y m m e t r i c modes (those associated w i t h v\ a n d i> ) are m i x e d . 2

A qualitative view of the influence of the M - H - M angle on v\ v% a n d v$ y

can be d r a w n f r o m a simple valence force field calculation i n w h i c h the force 9 10 11


17.

239


COOPER ET AL.

constants are fixed a n d only the M - H - M angle, Θ, is varied f r o m linear (180°) to strongly bent (90°) ( F i g u r e 1). T h e frequencies should not be taken literally because some variation i n force constants with angle is to be expected, and because the force field does not i n c l u d e interactions between bond stretching or defor mation coordinates (i.e., no off-diagonal force constants are included).

One

feature to note in Figure 1 is the small change i n all frequencies between the linear (180°) and slightly bent cases (160°); thus, molecules possessing these geometries should display similar vibrational frequencies.

As the M - H - M angle, 0, decreases


(i.e., the backbone becomes more strongly bent), a r a p i d change is noted for the

1600

1200 ν (cm ) 1

800

400

180*

150*

120°

θ

90·

Figure 1. Results of a simple valence force field calcuhtion for the Mo-H-Mo system shounng the relationship among the frequencies of the three hydride modes and the Mo-H-Mo angle, θ a s y m m e t r i c stretch v$ a n d the deformation *>. 2

As θ approaches 9 0 ° , it w i l l be

noted that the frequencies of i> a n d v$ cross; thus, for molecules w i t h a h i g h l y 2

bent backbone, unambiguous assignment of i> and v§ w i l l be difficult. 2

It is clear,

however, that strongly bent M - H - M systems should display significantly lower t>3 a n d higher f frequencies than their linear counterparts. 2

Next we shall discuss specific assignments for the H M ( C O ) i o ~ ions. 2

The

low frequency v\ mode is considered first, then the h i g h frequency v§ M - H stretch, and finally the complex intermediate region w h i c h we believe to include "2·



240


200

150

-I

100

50

cm

Figure 2. Low-frequency Raman spectra for solid (Et NXHCr (CO) ) (upper) and (Et N) ( C r ( C O ) i o ) (lower) 4

2

w

4

2

2

A good set of reference points for establishing the v\ fre-

The v\ Region.

q u e n c y range is p r o v i d e d by the simple m e t a l - m e t a l b o n d e d analogs of the h y drides w h i c h we are considering, n a m e l y C r 2 ( C O ) i o ~ , M o 2 ( C O ) i o ~ , and 2

W2(CO)io ~ and 115 c m

2

T h e M - M frequencies i n these compounds are found at 160, 140,

2

- 1

for R a m a n spectra of the respective t e t r a e t h y l a m m o n i u m salts

i n the solid state at room temperature (19).

These frequencies should be upper

bounds on the m e t a l - m e t a l stretches for the ( O C ) 5 M H M ( C O ) s ~ complexes of C r , M o , and W .

C a r e f u l inspection of the low frequency region of the R a m a n

spectra for the hydrides reveals a m e d i u m strong peak at 144 c m

- 1

for the E t 4 N

+

salt of the c h r o m i u m c o m p o u n d , w h i c h is tentatively assigned as the v\ mode. In F i g u r e 2, this feature is c o m p a r e d w i t h the M - M stretch of its simple M - M bonded analog.

It should be noted that the low frequency region of the hydrides

(below about 115 c m ) is intense and complex. - 1

T h e presence of these strong

bands can be responsible for our inability to locate the v\ frequency for the h y -


17.

241


COOPER E T AL.

dride-bridged molybdenum and tungsten compounds.

E v e n at 10° Κ where these

features are sharpened considerably, it was not possible to p i c k out the v\ band. A n o t h e r point of some importance is that v\ i n [ E t N ] [ D C r ( C O ) i o ] is i d e n t i c a l 4

to that i n its h y d r i d o counterpart.

2

Therefore, this h y d r i d e - b r i d g e d metal system

does not appear to exhibit the very large anharmonicity of the type seen i n some hydrogen-bonded systems, where the X - - X stretching frequency can be quite different between X - H - · - X a n d X - D - · - X . The i>3 Region. array.

1

and P P N amined.


T h i s m o d e w i l l be ir-active for a linear or bent M - H - M

A c c o r d i n g l y , low temperature spectra for a l l 12 combinations of Et^N" " +

w i t h h y d r i d o - a n d deuterio-bridged C r , M o , a n d W anions were ex F o r spectra recorded at 8°K, the H : D isotope shift allows positive

identification of the v$ mode i n the four c h r o m i u m salts and two tungsten hydrides

1

1

1700

1680

1

1

1660

1640

f*

1620

cm'

1

Figure 3. Variable temperature ir spectra of (Et NXHW2(CO)io). The sample temperatures are probably higher than the measured temperatures shown here. 4


242


(see T a b l e I).

These modes shift a n d broaden w i t h increasing temperature, as

illustrated i n F i g u r e 3.

H a r r i s a n d G r a y previously have observed this feature

for ( E t N ) ( H C r ( C O ) i o ) (40). 4

2

W h y these bands are so strongly temperature

dependent and w h y they were not observed i n all the salts is not known; however, failure to observe some of these modes, p a r t i c u l a r l y i n the deuterides, can be attributed to interferences from cation bands.

T h e Christiansen effect degraded

the q u a l i t y of some spectra a n d m a y be responsible for obscuring some of these modes.

T h e lack of a significant shift i n i> between E t N 3

4

and P P N

+

+

salts of


[ H C r 2 ( C O ) i o ~ ] implies that these compounds have s i m i l a r C r - H - C r angles.

1

1

KXX)

1

1

900

>

1

t

I

800

i

700

I

I

1

600

cm

Figure 4. Raman spectra of (PPNXHMo2(CO)io) (upper) and (PPNXDMO (CO)IQ) flower). The connection lines suggest an approximate \/2 shift in the frequencies. 2

Simple valence force field calculations predict a difference of 20 c m

- 1

between

a truly linear C r - H - C r array and the slightly bent array (159.8°), w h i c h has been observed by neutron d i f f r a c t i o n for the E t N 4

The v Region. 2

+

salt.

As discussed earlier, there should be one mode i n this i n -

termediate frequency range for a linear or bent M - H - M

array.

However, in

these more complex metal c a r b o n y l complexes, the bent f o r m of ( O C ) s M - H M ( C O ) 5 ~ should exhibit two d e f o r m a t i o n frequencies because the presence of the c a r b o n y l ligands transforms what w o u l d have been a rotational m o t i o n for an isolated M

M into an internal v i b r a t i o n a l mode.

Therefore, a m a x i m u m

of two modes w o u l d be expected i n the intermediate frequency region.


By

17.

243


COOPER ET AL.

contrast, a large n u m b e r of features are seen between 700 a n d 960 c m m o l y b d e n u m a n d tungsten salts.

- 1

for the

As illustrated i n F i g u r e 4, the substitution of

D for H i n ( P P N ) ( H M o ( C O ) i ) shifts several bands roughly by the \Î2 factor. 2

0

T h e existence of more than two bands can arise f r o m F e r m i resonance w i t h overtones a n d combinations of the M C O a n d M - C modes. ls

Experiments utilizing

O - s u b s t i t u t e d compounds are b e i n g pursued to c l a r i f y this point.

vation of features i n the 7 0 0 - 9 0 0 c m

region (see

- 1

T h e obser-

T a b l e I) for ( P P N ) -

( H C r 2 ( C O ) i o ) w o u l d be anomalous if this anion were truly linear because v

2

should be R a m a n inactive.

T h e observation of v i n the R a m a n along w i t h the 2

similarity i n v% values for the Et^N" " a n d P P N + salts is, i n our o p i n i o n , good e v i 1

dence that i n both salts, H C r 2 ( C O ) i o ~ is bent.

As previously mentioned, the


neutron diffraction data for the hydrogen i n the P P N + salt show a large thermal a m p l i t u d e p e r p e n d i c u l a r to the C r - C r axis.

H o w e v e r , observation of similar

i>3 values for the Et^N" " a n d P P N + salts, along w i t h similar temperature d e p e n 1

dence for the linewidth of v& indicate that the C r - H - C r array i n both salts is bent to approximately the same degree. the P P N

+

A p p a r e n t l y the large t h e r m a l ellipsoid for

salt is the result of a composite of bent M - H - M arrays i n the crys-

tal. T h e bent structure for the M - H - M moiety i n [ E t N ] [ H C r ( C O ) i ] a n d 4

2

0

particularly that i n [ P P N ] [ H C r ( C O ) i o ] where there is little or no externally 2

imposed distortion of the anion, indicate that there m a y be inherent stability i n a bent array.

In previous discussions of related bent systems, it has been postu-

lated that an energetic advantage is achieved by b e n d i n g , w h i c h allows closer approach of the metal atoms and thus stronger M - M interaction (16,19,22).

It

is our purpose here to point out the central role that nuclear repulsion must play in such a b o n d i n g argument. Since hydrogen possesses no inner-core electrons, the central proton i n a three-center, two-electron b o n d w i l l exert only a simple electrostatic repulsion for the metal cores.

Close approach of the metal atoms, w h i c h w o u l d m a x i m i z e

M - M overlap, w i l l be resisted by this electrostatic repulsion.

T h u s , it can be

energetically favorable for the proton to m o v e off the M - M axis simultaneously w i t h the closer approach of the m e t a l atoms. excursion w i l l reduce M - H overlap.

H o w e v e r , a very large off-axis

Thus, the nuclear repulsion, the M - M

overlap, and the M - H overlap may strike a delicate balance leading to an off-axis hydrogen position. In s u m m a r y , the assignments favored by us for these single h y d r i d e bridged-carbonyl anions are ca. 1700 c m

- 1

for i / , ca. 850 c m 3

for v% a n d

Vibrational Spectroscopy of Hydride-Bridged Transition Metal

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