MÃ¶ssbauer Spectroscopy and Its Chemical Applications - American

15 Covalency of Neptunium(IV)

Organometallics

from Neptunium-237 Mössbauer Spectra

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D. G. K A R R A K E R E . I. du Pont de Nemours & Company, Savannah River Laboratory, Aiken, SC 29808

The isomer shifts in Np Mössbauer spectra arise from the 237

shielding of neptunium's 6s orbitals by the inner 5f orbitals. In covalent bonding, ligand contributions to the 5f electron density increase the shielding, and the

Np

isomer shift

237

reflects differences in bond character among covalently bonded ligands. The large difference in isomer shift (3.8 cm/s)

between ionic neptunium(IV) and neptunium(III)

compounds permits a good determination of ligand bonding differences in neptunium(IV) organometallic compounds. The Mössbauer spectra for about 20 neptunium(IV) organometallic compounds, principally cyclopentadienyl (Cp) compounds of the general composition

Cp NpX x

4x

(x = 1,2,3;

X = Cl, BH , Bu, Ph, OR, acac), show both the differences 4

n

in σbonding among the X ligands, as well as the covalent effect of the Cp ligands.

r-y^he

2 3 7

N p M o s s b a u e r effect has b e e n e s p e c i a l l y v a l u a b l e f o r c h e m i c a l

a n d p h y s i c a l studies of s o l i d n e p t u n i u m c o m p o u n d s . shift i n

2 3 7

N p has a v e r y w i d e r a n g e — f r o m

T h e isomer

—6.9 to + 3 . 5 c m / s , a n d

excellent r e s o l u t i o n c a n b e o b t a i n e d w i t h o u t excessively elaborate e q u i p ment. N e p t u n i u m forms c o m p o u n d s i n five v a l e n c e states, p l u s the m e t a l l i c state, so a w i d e range of c o m p o u n d s a n d i n t e r m e t a l l i c m a t e r i a l s c a n b e p r e p a r e d f o r M o s s b a u e r studies. E x a m p l e s of some past M o s s b a u e r studies are m a g n e t i c p r o p e r t i e s of some n e p t u n i u m c o m p o u n d s

(1-4)

a n d l o c a l i z a t i o n of 5f electrons i n n e p t u n i u m i n t e r m e t a l l i c s ( 5 ) .

This

c h a p t e r outlines t h e t h e o r y a n d e x p e r i m e n t a l p r o c e d u r e f o r b a u e r studies a n d presents t h e a p p l i c a t i o n of t h e

2 3 7

2 3 7

N p Moss

N p M o s s b a u e r effect

to d e t e r m i n e c o v a l e n t effects i n n e p t u n i u m o r g a n o m e t a l l i c c o m p o u n d s .

©

0065-2393/81/0194-0347$05.00/0 1981 American Chemical Society

In Mössbauer Spectroscopy and Its Chemical Applications; Stevens, John G., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1981.

348

MOSSBAUER SPECTROSCOPY A N D ITS C H E M I C A L APPLICATIONS

Np

237

Mossbauer Effect Stone a n d P i l l i n g e r

Description. effect of

2 3 7

discovered

(6)

the

Mossbauer

N p at the S a v a n n a h R i v e r L a b o r a t o r y , a n d the e x p e r i m e n t a l

techniques were further developed

b y t h e g r o u p at A r g o n n e N a t i o n a l

Laboratory, then directed by G . M . Kalvius.

T h e p h y s i c s of

b e e n s u m m a r i z e d i n three excellent r e v i e w s (7,8,9)

2 3 7

N p has

a n d w i l l be discussed

h e r e o n l y briefly.


T h e g a m m a r a y u s e d i n the 5 9 . 5 - k e V , 5 / 2 " - » 5.2

2 3 7

N p M o s s b a u e r effect results f r o m the

E l transition i n

2 3 7

l e v e l is accessible f r o m the a - d e c a y of

2 4 1

+

electron-capture decay from

2 3 7

Pu.

N p (t*

63 n s ) .

T h e 59.5-keV

A m , /?-decay f r o m

T h e h a l f - l i f e of

2 4 1

Am

2 3 7

U , or the

(433

years)

makes i t the o b v i o u s c h o i c e for a M o s s b a u e r source. H y p e r f i n e i n t e r a c t i o n s refer to t h e i n t e r

Hyperfine Interactions. a c t i o n of the

2 3 7

N p n u c l e u s w i t h the s u r r o u n d i n g e l e c t r o m a g n e t i c

( i n t e r n a l or e x t e r n a l ) .

T h e i n t e r p r e t a t i o n of

d e p e n d s o n t h e analysis of h y p e r f i n e effects.

2 3 7

N p Mossbauer

field

spectra

T h e H a m i l t o n i a n f o r the

M o s s b a u e r effect system has three t e r m s : Hht = where H

I

S

Hi

S

— H

Q

± i /

M

refers to i n t e r a c t i o n s of the c e n t r a l field w i t h the n u c l e u s ;

represents the i n t e r a c t i o n b e t w e e n e l e c t r i c field g r a d i e n t ; a n d ff

M

the quadrupole moment

with

H

Q

the

is the i n t e r a c t i o n of m a g n e t i c fields w i t h

t h e n u c l e u s . T h e s p l i t t i n g of the e x c i t e d a n d g r o u n d states b y h y p e r f i n e fields

is i l l u s t r a t e d i n F i g u r e 1 for single u n s p l i t levels, q u a d r u p o l e - s p l i t

levels, m a g n e t i c a l l y s p l i t levels, a n d c o m b i n e d m a g n e t i c - a n d q u a d r u p o l e - s p l i t levels

(10).

Isomer Shift.

The

central

field

i n t e r a c t i o n is the result of

the

C o u l o m b i n t e r a c t i o n of the e l e c t r o n i c c h a r g e w i t h t h e n u c l e a r c h a r g e . T h i s i n t e r a c t i o n determines the i s o m e r shift.

T h e c e n t r a l field i n t e r

actions, w h i c h l e a d to t h e i s o m e r shift i n the M o s s b a u e r effect,

are

spherically symmetric a n d depend p r i n c i p a l l y on s orbitals. F o r the

Np

2 3 7

M o s s b a u e r effect, 6s o r b i t a l s are s h i e l d e d f r o m the n u c l e u s b y t h e i n n e r 5/ o r b i t a l s ( F i g u r e 2 ) . Increases i n the e l e c t r o n d e n s i t y i n the 5f o r b i t a l s increase the s h i e l d i n g of t h e 6s o r b i t a l s a n d p r o d u c e

a more

positive

i s o m e r shift. T h e 6d a n d 6p electrons c a n also s h i e l d the 65 o r b i t a l s , b u t t h e i r c o n t r i b u t i o n s are s m a l l c o m p a r e d to t h e effect of 5/ s h i e l d i n g . A n o b v i o u s e x a m p l e of the effect of 5 / e l e c t r o n d e n s i t y o n the i s o m e r shift is t h e difference i n t h e isomer shift f o r n e p t u n i u m c o m p o u n d s

of different

valences, w h i c h a m o u n t to 2 - 4 c m / s b e t w e e n consecutive valences a n d a r a n g e over 10 c m / s b e t w e e n N p

7 +

and N p

G i v e n these l a r g e differences

3 +

(Figure 3).

i n i s o m e r shifts b e t w e e n

valences,

c o v a l e n t effects o n b o n d i n g also c a n b e i d e n t i f i e d b y i s o m e r shifts

(II).

C o v a l e n c y r e q u i r e s the o v e r l a p of l i g a n d o r b i t a l s w i t h the 5/ o r b i t a l s ,


15.

Neptunium-237

KARRAKER

Mossbauer


Excited State Energy Levels

±

Mognetic + Quadrupole

Splitting

Mogneti

Splitting

349

Spectroscopy

V2-

Ground State Energy Levels

NPAI 77^< 2

cr-4

0

+4

-4

0 +4

-8 -4

0 +4 +8

V(cm/s)

Figure I .

Splitting

-8 - 4

0 +4

+8

~*

of the ground state and 59.5-keV magnetic and electric fields

— i — i — ' — i — ' — i — i — i

>

i

1

i

level of ** Np in 7

1

r

U (5f ) 4 +

2

r (a ) 0

Figure 2. Radial charge density for U (( ) 5f; ( ) 6s; (— • —) 6p) (courtesy of N. M. Edelstein, Lawrence Berkeley Laboratory, Berkeley, CA) 4+


350


Fluorides

ISOMER SHIFT 4 4 —

NpF


N

Oxides 3+

-

3

Np (in Am 0 ) 2

"

P 4 P

Np0 "

NpF

5

2

3

2

+

«•

Np0

- 4 - — .

Np0

2

++ 2

-6

NpF

6

-Np(VII) -7

Figure 3.

Isomer shifts of neptunium

fluorides and neptunium

oxides

w h i c h increases the 5 / e l e c t r o n d e n s i t y w i t h a consequent p o s i t i v e increase i n t h e i s o m e r shift. T h e c o m p a r i s o n of i s o m e r shifts b e t w e e n n e p t u n i u m fluorides

and oxygen-bonded

neptunium compounds

the s t r o n g shift t o w a r d l o w e r v a l e n c e of t h e N p 0 p o u n d s c o m p a r e d t o that of N p F

6

t r i b u t e d to t h e 5f o r b i t a l s of t h e N p

2

( F i g u r e 3) 2 +

and N p 0

shows 2

com

+

and N p F .

T h e electron density con

and N p

ions b y the t i g h t l y b o n d e d

5

6 +

5 +

o x y g e n l i g a n d s results i n a n i s o m e r shift t o w a r d l o w e r v a l e n c e .

F o r the

N p , the c o n t r i b u t i o n of o x y g e n l i g a n d s is s m a l l because i t b e c o m e s m o r e 4 +

difficult f o r t h e o r b i t a l s to o v e r l a p i n t h e l a r g e r N p larger N p

3 +

i o n , essentially n o difference

4 +

ion. I n the still

i n the i s o m e r

shifts

occurs

b e t w e e n a fluoride a n d a n o x y g e n l i g a n d e n v i r o n m e n t . Quadrupole Interaction.

T h e i n t e r a c t i o n of t h e n u c l e a r q u a d r u p o l e

tensor a n d the e l e c t r i c field g r a d i e n t tensor f r o m t h e i o n i c e n v i r o n m e n t s u r r o u n d i n g the n e p t u n i u m i o n c a n r e s u l t i n S t a r k ( e l e c t r i c

field)

split

t i n g of t h e n u c l e a r levels, o r q u a d r u p o l e s p l i t t i n g . I n g e n e r a l , q u a d r u p o l e s p l i t t i n g r e q u i r e s t h e a l i g n m e n t of t h e z - c o m p o n e n t of t h e electric g r a d i e n t of t h e u n f i l l e d 5 / e l e c t r o n s h e l l (8).

This alignment must

field be

p r o d u c e d b y a n e x t e r n a l field; o t h e r w i s e , r a p i d r o t a t i o n of the e l e c t r i c field g r a d i e n t a l l o w s o n l y a n average to b e o b s e r v e d . Q u a d r u p o l e i n t e r a c t i o n s c a n b e r e d u c e d to c o m b i n a t i o n s components—the rf (12).

e l e c t r i c field g r a d i e n t a n d t h e a s y m m e t r y

of

two

parameter

F o r a n e p t u n i u m i o n i n a site w i t h a n n - f o l d r o t a t i o n or r o t a t i o n -

reflection axis w i t h n >

2, t h e a s y m m e t r y p a r a m e t e r 17 — 0, a n d the


15.

KARRAKER

Neptunium-237

Mossbauer

351

Spectroscopy

resonance is s p l i t i n t o five e q u a l l y s p a c e d lines. A t t h e other extreme, w h e r e rj =

1, the resonance is s p l i t i n t o three lines. I n t e r m e d i a t e v a l u e s

of rj are r e p r e s e n t e d b y u n e q u a l , five-line spectra. I n a l l cases, the c e n t r a l resonance of the q u a d r u p o l e s p e c t r u m has t h e same isomer shift as t h e u n s p l i t resonance w o u l d h a v e ( F i g u r e 1 ) . T h e i n t e r a c t i o n of the n u c l e a r e n e r g y levels

Magnetic Interaction.

w i t h a m a g n e t i c field ( i n t e r n a l o r e x t e r n a l ) results i n m a g n e t i c s p l i t t i n g of the

2 3 7

N p M o s s b a u e r spectra ( F i g u r e 1 ) .

Normally, a paramagnetic


i o n w i l l h a v e a m a g n e t i c field at the n u c l e u s because of its u n p a i r e d electrons.

A t r o o m t e m p e r a t u r e , the d i r e c t i o n of the field changes too

r a p i d l y f o r the n u c l e u s to r e s p o n d .

A t l o w t e m p e r a t u r e s , the r e l a x a t i o n

t i m e is g r e a t l y decreased, often to the p o i n t w h e r e the

2 3 7

N p Mossbauer

s p e c t r u m is m a g n e t i c a l l y split. W h e n the a b s o r b i n g c o m p o u n d b e c o m e s f e r r o m a g n e t i c or a n t i f e r r o m a g n e t i c , the r e l a x a t i o n t i m e becomes infinite o n the

2 3 7

N p M o s s b a u e r t i m e scale, a n d a l a r g e m a g n e t i c s p l i t t i n g n o r m a l l y

results. P u r e m a g n e t i c s p l i t t i n g has a 16-line p a t t e r n ( F i g u r e 1 ) , b u t as t h e energies of some of the resonances are n e a r l y the same, t h e s p e c t r u m u s u a l l y shows o n l y eight to t e n lines. T h e average of t w o s y m m e t r i c a l l y s p l i t lines d e t e r m i n e s the i s o m e r shift i n m a g n e t i c a l l y s p l i t O c c a s i o n a l l y , q u a d r u p o l e s p l i t t i n g also m a y b e i m p o s e d o n

spectra. magnetic

s p l i t t i n g , b u t since the m a g n e t i c s p l i t t i n g is n o r m a l l y m u c h greater t h a n q u a d r u p o l e s p l i t t i n g , no serious c o m p l i c a t i o n is i n t r o d u c e d i n i n t e r p r e t i n g the spectrum.

Where

quadrupole

e q u a l , as for s o m e N p 0

2

2 +

and magnetic

or N p 0

2

+

splitting

compounds,

are n e a r l y

assignment of

the

resonances b e c o m e s v e r y difficult. Intermediate Relaxation Effects.

T h e d i s c u s s i o n of h y p e r f i n e i n t e r

actions i n v o l v e s t h e a s s u m p t i o n t h a t the h y p e r f i n e fields are t i m e i n d e pendent.

F o r some c o m p o u n d s ,

time-dependent

effects

(intermediate

r e l a x a t i o n ) influence t h e M o s s b a u e r s p e c t r u m . T h e electrons i n the u n f i l l e d s h e l l of the n e p t u n i u m i o n c a n considered

to alternate b e t w e e n

be

" s p i n - u p " a n d " s p i n - d o w n " positions

u n d e r the influence of e x t e r n a l fields o n the i o n . T h e e l e c t r o n i c r e l a x a t i o n t i m e is t h e p e r i o d r e q u i r e d for the s p i n flip. I f the r e l a x a t i o n t i m e is s l o w , c o m p a r e d to t h e L a r m o r p r e c e s s i o n f r e q u e n c y of the n u c l e u s a n d the l i f e t i m e of the e x c i t e d state, t h e n u c l e u s w i l l see a static field, a n d t h e s p e c t r u m w i l l b e m a g n e t i c a l l y split. field

F a s t r e l a x a t i o n t i m e s average t h e

o n the n u c l e u s to zero, a n d s i n g l e - l i n e or q u a d r u p o l e spectra w i l l

result. I n t e r m e d i a t e r e l a x a t i o n t i m e s are of the same o r d e r of m a g n i t u d e as t h e L a r m o r p r e c e s s i o n f r e q u e n c y a n d t h e excited-state l i f e t i m e , a n d r e s u l t i n a loss of r e s o l u t i o n of t h e M o s s b a u e r s p e c t r u m , often so m u c h so t h a t t h e s p e c t r u m becomes a n u n i n t e r p r e t a b l e smear ( F i g u r e s 4 a n d 5).

R e l a x a t i o n is c o n s i d e r e d to o c c u r p r i n c i p a l l y t h r o u g h s p i n - l a t t i c e

and

s p i n - s p i n i n t e r a c t i o n s , a l t h o u g h other m e c h a n i s m s h a v e b e e n c o n -


352


RELAXATION TIME Fast

Intermediate

Slow


(Magnetic)

Figure 4. sidered

(13).

Relaxation effects on ^Np Mossbauer

P h y s i c a l methods

of a v o i d i n g

spectra

intermediate

relaxation

effects d e p e n d o n c h a n g i n g t h e e x p e r i m e n t a l c o n d i t i o n s to f a v o r faster (higher temperatures) peratures)

or slower

r e l a x a t i o n times.

(external magnetic

fields,

lower

tem

I n p r a c t i c e , s u c h m e t h o d s a r e n o t easy t o

a p p l y a n d n o t necessarily successful.

However,

m e d i a t e r e l a x a t i o n effects c a n b e a v o i d e d

i n some cases, i n t e r

b y changing the chemical

c o m p o u n d to a s i m i l a r c o m p o u n d that retains t h e features u n d e r s t u d y . S u b s t i t u t i o n of a b u l k i e r l i g a n d o r c a t i o n [ M e C p f o r C p , N ( C H ) 2

5

4

+

for

C s ] often r e d u c e s r e l a x a t i o n effects w i t h o u t affecting t h e object of t h e +

study.

B u l k i e r l i g a n d s increase t h e distance b e t w e e n

neptunium ions,

thus r e d u c i n g t h e s p i n - s p i n interactions that affect r e l a x a t i o n . 144.95

o144.75

144.70

L

-8

-4 Velocity,

Figure 5.

0 cm/sec

Mossbauer spectrum of

4

Np(MeCp) Cl s

Experimental Techniques. T h e experimental apparatus a n d techniques for Np M o s s b a u e r s p e c t r o s c o p y h a v e b e e n i n c l u d e d i n earlier r e v i e w s ( 7 , 8 , 1 4 , 15,16). T h e i n s t r u m e n t a t i o n u s e d i n M o s s b a u e r studies has a l w a y s b e e n s i m i l a r a m o n g different w o r k e r s , b u t there w e r e s o m e differences i n sources, detectors, s t a n d a r d i z a t i o n o f spectra, etc., p a r t i c u l a r l y b e f o r e 2 3 7


15.

KARRAKER

Neptunium-237

Mossbauer

353

Spectroscopy

1972. I n recent years some t e c h n i q u e s h a v e b e c o m e s t a n d a r d ; t h e y w i l l be e m p h a s i z e d i n t h e next section. Instrumentation. N p M o s s b a u e r experiments u s u a l l y h a v e u s e d a c o n v e n t i o n a l c o n s t a n t - a c c e l e r a t i o n spectrometer i n t r a n s m i s s i o n g e o m etry (17). T h e spectrometer m u s t a c h i e v e r e l a t i v e l y h i g h velocities ( ± 2 0 c m / s ) w h i l e o p e r a t i n g at l i q u i d - h e l i u m t e m p e r a t u r e s . T h e recoilless fraction for N p M o s s b a u e r effect is decreased severely a b o v e h e l i u m t e m p e r a t u r e s for m o s t absorbers, so the source-absorber e q u i p m e n t r e q u i r e s a l i q u i d - h e l i u m d e w a r , p r e f e r a b l y one c a p a b l e of m a i n t a i n i n g h e l i u m t e m p e r a t u r e s for 3 - 4 days. A c o n v e n i e n t v e l o c i t y c a l i b r a t i o n c a n b e o b t a i n e d f r o m a n N p A l absorber, w h i c h has a w e l l - c h a r a c t e r i z e d , m a g n e t i c a l l y s p l i t s p e c t r u m at 4.2 K . Sources. T h e m o s t c o n v e n i e n t source is ^ A m m e t a l as a 5% a l l o y i n c u b i c t h o r i u m m e t a l m a t r i x . A 433-year A m source lasts i n d e f i n i t e l y ; one s u c h source has b e e n u s e d satisfactorily at the S a v a n n a h R i v e r L a b o r a t o r y for a b o u t t e n years. A b o u t 3 m g A m i n a source y i e l d s c o u n t i n g rates a b o v e 1 0 c o u n t s / c h a n n e l - m i n . Sources i n earlier w o r k used U (6.75 d ) , a n d the use of P u (44.6 d ) has b e e n c o n s i d e r e d , b u t the h a l f - l i v e s of b o t h isotopes are q u i t e i n c o n v e n i e n t . T h e Am-Th source has the n a r r o w e s t e x p e r i m e n t a l l i n e w i d t h yet a c h i e v e d , a b o u t 0.1-0.2 c m / s (14). 2 3 7


2 3 7

2

2 4 1

2 4 1

4

2 3 7

2 3 7

2 4 1

Standards. T h e s i n g l e - l i n e s p e c t r u m of N p A l at 77 K is the r e c o m m e n d e d zero of i s o m e r shift (18). T h i s zero c a n be d e t e r m i n e d w i t h m o r e p r e c i s i o n t h a n a zero b a s e d o n the single resonance l i n e of N p 0 . T h e N p 0 resonance is b r o a d e n e d b y a w e a k m a g n e t i c s p l i t t i n g b e l o w 25 K , a n d is thus m u c h less satisfactory, t h o u g h often u s e d i n e a r l y w o r k . I n p r a c t i c e , the c e n t r o i d of t h e N p A l s p e c t r u m at 4.2 K is n o r m a l l y u s e d as z e r o isomer shift. A s n o t e d before, the m a g n e t i c a l l y s p l i t spectra o f N p A l can provide simultaneously a convenient velocity calibration. 2

2

2

2

2

Detectors. T w o detectors are i n u s e — N a l ( T l ) s c i n t i l l a t i o n crystals a n d G e ( L i ) s e m i c o n d u c t o r s , a n d b o t h are satisfactory. T h e detector i n c u r r e n t use at S a v a n n a h R i v e r L a b o r a t o r y is a n N a l ( T l ) s c i n t i l l a t i o n counter. T h i s c o u n t e r has b e e n q u i t e satisfactory o v e r several years of service a n d a v o i d s the l i q u i d - n i t r o g e n c o o l i n g necessary f o r G e ( L i ) detectors. T h e w i n d o w of the s i n g l e - c h a n n e l a n a l y z e r is a d j u s t e d to a c c e p t t h e 59.54 k e V p h o t o p e a k f o r a l l three detectors. Absorbers. N p c a n b e o b t a i n e d i n g r a m q u a n t i t i e s to p r e p a r e absorbers for s t u d y . T h e specific a c t i v i t y of N p is 1.57 X 1 0 a d / m i n m g , a n d q u a n t i t i e s greater t h a n a f e w m g s h o u l d be h a n d l e d i n a g l o v e box or other f o r m of c o n t a i n m e n t to p r e v e n t c o n t a m i n a t i o n of t h e l a b o r a t o r y w i t h a l p h a a c t i v i t y . M o s t n e p t u n i u m o r g a n o m e t a l l i c c o m p o u n d s are d e c o m p o s e d b y w a t e r a n d o x y g e n ( o f t e n v i o l e n t l y ) , so p r e p a r a t i o n of t h e c o m p o u n d s , absorbers, etc., is p e r f o r m e d i n a n i n e r t - a t m o s p h e r e g l o v e box. T h e absorbers are u s u a l l y p r e p a r e d b y ( 1 ) p a c k i n g t h e p o w d e r e d s a m p l e i n a p l a s t i c h o l d e r , ( 2 ) c o v e r i n g the p o w d e r w i t h a p l a s t i c p l u g , a n d ( 3 ) w r a p p i n g t h e a s s e m b l y w i t h a d h e s i v e p o l y e s t e r tape. T h e a b s o r b e r is r e m o v e d f r o m the g l o v e box a n d w r a p p e d w i t h a n a d d i t i o n a l l a y e r of p l a s t i c t a p e to p r e v e n t the s p r e a d of a l p h a a c t i v i t y . N o r m a l l y , a b o u t 50 m g N p / c m is a d e q u a t e f o r most m a t e r i a l s . C r y s t a l l i n e solids, s u c h as N p C l , N p B r , N p C l , etc., h a v e l a r g e r recoilless fractions t h a n the essentially a m o r p h o u s n e p t u n i u m o r g a n o m e t a l l i c c o m p o u n d s , w h i c h c a n r e q u i r e 3-4 days to d e v e l o p a n a c c e p t a b l e s p e c t r u m . 2 3 7

2 3 7

6

2

4

4

3


354


Compounds. The neptunium organometallic compounds used i n this s t u d y w e r e p r e p a r e d ( m o s t f o r the first t i m e ) b y u s i n g t h e g e n e r a l p r o c e d u r e s d e v e l o p e d i n t h e synthesis of t h e analogous u r a n i u m c o m p o u n d s . S o m e m o d i f i c a t i o n of t h e p r o c e d u r e s w a s necessary since N p is m o r e easily r e d u c e d t h a n U . T h e b a s i c r e a c t i o n u s e d i n n e a r l y a l l p r e p a r a t i o n s is 4 +

4 +

N p halide + M ligand - » N p ligand + M halide w h e r e M is a n a l k a l i or T l . I n g e n e r a l , m e t a l l a t e d l i g a n d s are s t r o n g r e d u c i n g agents, so often a n N p o r g a n o m e t a l l i c is t h e m a j o r p r o d u c t r a t h e r t h a n the d e s i r e d N p o r g a n o m e t a l l i c . A d j u s t m e n t of e x p e r i m e n t a l c o n d i t i o n s o c c a s i o n a l l y w a s successful i n o b t a i n i n g t h e d e s i r e d N p product.


3 +

4 +

4 +

Np

237

Isomer Shift and Covalency T h e first a c t i n i d e o r g a n o m e t a l l i c

N p C p X Compounds. 3

compound

prepared was tris(cyclopentadienyl) u r a n i u m ( I V ) chloride, U ( C H ) C 1 5

(hereafter C H " is a b b r e v i a t e d C p ) 5

5

c e e d e d b y the p r e p a r a t i o n of C p c o m p o u n d s of the 3 + a c t i n i d e ions u p to C f

(20).

3 +

5

3

( 1 9 ) , a n d its p r e p a r a t i o n w a s suc and

W i t h f e w exceptions, t h e

4+-valent

organometallic

c o m p o u n d s of t h e a c t i n i d e s i n c l u d e the C p l i g a n d i n t h e i r s t r u c t u r e ; b y reacting U C p C l (or N p C p C l ) w i t h an alkali metal ligand 3

compound,

3

s u c h as L i B u , L i P h , N a O B u , etc., the l i g a n d replaces the c h l o r i d e i o n , w

forming N p C p

3

n

B u , N p C p P h , N p C p O B u , etc. 3

3

Q u i t e a v a r i e t y of c o m

p o u n d s h a v e b e e n p r e p a r e d i n this m a n n e r f r o m U C p C l , most of w h i c h 3

c a n also be p r e p a r e d f r o m N p C p C l .

S u b s t i t u t e d C p l i g a n d s , s u c h as

C H — C 5 H 4 " ( M e C p ) , f o r m analogous

compounds.

3

3

T h e i s o m e r shifts i n

2 3 7

N p M o s s b a u e r spectra of these c o m p o u n d s

n o r m a l l y are d i s p l a c e d f r o m a n i o n i c N p i s o m e r shift.

d e r i v a t i v e s of the N p C p comparison from their

4 +

i s o m e r shift t o w a r d t h e N p

3 +

U s i n g the v a r i e t y of c o m p o u n d s t h a t c a n b e p r e p a r e d as 3

+

moiety a n d selecting appropriate standards, a

of the b o n d i n g p r o p e r t i e s o f t h e l i g a n d s c a n b e

2 3 7

N p M o s s b a u e r spectra.

C p ligands per N p

i o n also h a v e b e e n p r e p a r e d .

4 +

obtained

C o m p o u n d s c o n t a i n i n g one or t w o

afford a m e a s u r e of t h e effect o n t h e

2 3 7

These

compounds

N p i s o m e r shift of a d d i n g one,

t w o , or three C p l i g a n d s . U n f o r t u n a t e l y , the are s t r o n g l y affected counter

2 3 7

N p M o s s b a u e r spectra of N p C p X c o m p o u n d s

by

3

i n t e r m e d i a t e r e l a x a t i o n effects.

r e l a x a t i o n effects

by

Attempts

synthesizing compounds that

to

substitute

M e C p for C p or C H C H 5 f o r — C H , etc., often s u c c e e d e d i n o b t a i n i n g 6

4

2

6

5

i n t e r p r e t a b l e , a l t h o u g h n o t necessarily i d e a l , spectra. F i g u r e 6 shows t h e spectra of

NpCp BH 3

4

and

N p ( M e C p ) B H , illustrating 3

4

a

successful

e x a m p l e w h e r e a n i s o m e r shift c o u l d b e o b t a i n e d f r o m the s p e c t r u m of the s u b s t i t u t e d c o m p o u n d b u t not f r o m t h e p a r e n t c o m p o u n d .

Analysis

of the results i n terms of t h e l i g a n d ' s p r o p e r t i e s i n v o l v e s t h e a s s u m p t i o n


15.

Neptunium-237

KARRAKER

Mossbauer

355

Spectroscopy

125, 35 125. 30

NpCp BH 3

125. 25 —

4

.-•v%.;. ".**.*'..•;.' ..v.

£ g 125. 201 ' 15 — , 2 5

c I 261, 95 o 90 Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 11, 2016 | http://pubs.acs.org Publication Date: July 1, 1981 | doi: 10.1021/ba-1981-0194.ch015

Q.

w 261.85 c 3 261. 80

Np(MeCp) BH 3

4

261, 75 261, 70 261,,65

-4

0

4

Velocity, cm/sec

Figure 6.

Mossbauer spectra of NpCp BH s

and

4

Np(MeCp) BH^ 3

that the effect of s u b s t i t u t e d l i g a n d s a n d u n s u b s t i t u t e d l i g a n d s o n the i s o m e r shifts is the same; d a t a g i v e n l a t e r i n this c h a p t e r s u p p o r t this assumption.

The

2 3 7

N p M o s s b a u e r s p e c t r u m of N p C p O C H ( C H ) 3

s h o w n i n F i g u r e 7, a n d spectra for N p ( a c a c ) C l 2

C H 3 C O C H C O C H 3 ) and N p ( a c a c ) ( M e C p ) 2

2

3

is

2

• T H F (acac

2

=

are s h o w n i n F i g u r e s 8 a n d

9, r e s p e c t i v e l y . T h e latter t w o s p e c t r a a l l o w a c o m p a r a t i v e i s o m e r shift for

Cp NpX 2

NpCp X . n

4

n

2

compounds

to

be

derived.

Mossbauer parameters

for

c o m p o u n d s are s h o w n i n T a b l e I.

T o c o m p a r e the c o n t r i b u t i o n s of the l i g a n d s , the a s s u m p t i o n is m a d e that C I " a n d B H " ions m a k e n o c o v a l e n t c o n t r i b u t i o n s , a n d t h a t N p C p C l 4

3

or N p ( M e C p ) B H a n d N p C ^ c a n b e u s e d as reference c o m p o u n d s . 3

4

d e t e r m i n e the effect of t w o C p l i g a n d s , N p ( a c a c ) C l 2

2

To

• T H F was used

as a reference, since the acac l i g a n d s m a d e a s t r o n g n e g a t i v e c h a n g e i n the n o r m a l N p

4 +

i s o m e r shift. T h e i s o m e r shifts assigned to e a c h l i g a n d

are s h o w n i n T a b l e I I .

-4

0

4

Velocity, cm/sec

Figure 7.

Mossbauer

spectrum of

NpCp OCH(CH ) 3

3 2


356

M O S S B A U E R SPECTROSCOPY A N D ITS C H E M I C A L A P P L I C A T I O N S


134.4

-4

-8

Figure 8.

Mossbauer

0 Velocity, cm/sec

2

Isomer Shift"

3

4

2

5

3

3

3

2

3

3

2

2

2

Parameters

S(cm/s)

: 1.0 1.4 :0.4 1.45 0.07 0.27 0.28 0.42 0.93 : 0.07 0.86 0.2 0.79 0.3 0.86: 0.3 -0.31 : 0.07 -1.47 : 0.07 -0.53 : 0.07 -0.35 : 0.05 3.54 : 0.05

4

3

e

• THF

g

Mossbauer

NpCpaCl Np(MeCp) BH NpCp "Bu NpCp C H C H Np(MeCp),0*Pr NpCp 0*Pr NpCp OCH(CF ) NpCp 'Bu Np(MeCp)Cl • 2THF Np(acac) Cl • T H F * Np(MeCp) (acac) NpCV NpCV 3

12

spectrum of Np(acac) Cl Table I.

Compound

4

2

' Referred to NpAI = 0. 2

Table II. Bond

2 3 7

N p Isomer

Compound

Cp-NpZ Cp2-NpZ Cpr-NpX

Np(MeCp)Cl • 2THF Np(acac) (MeCp) NpCp,Cl Np(MeCp) BH NpCp "Bu NpCp C H C H Np(MeCp) 0*Pr NpCp 0 Pr NpCp OCH(CF ) NpCp 0'Bu

3

3

2

R-NpCp Ar-NpCp RO-NpCp

2

2

3

3

4

3

3

3

3

6

4

2

5

3

3

3

4

3

2

3

(acac)2-NpCl

2



Figure 9. for N p - C p - X _ n

4

n

Mossbauer spectrum of

±

Magnetic Hyperfine Constant gow H e / / (cm/s)

1.0

0.97 ± 0.15 -0.42 0.49 6

± ±

2

Compounds

Quadrupole Coupling Constant eqQ/4 (cm/s)

5.0

Np(acac) (MeCp)

MÃ¶ssbauer Spectroscopy and Its Chemical Applications - American

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