Formation and Structure of a Monomeric Oxygen Adduct of a Cobalt(II

39 Formation and Structure of a Monomeric Oxygen Adduct of a Cobalt(II)-Ammonia Complex in a Co(II)Y Zeolite Downloaded by UNIV OF ARIZONA on March 12, 2017 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0121.ch039

12,

E. F. VANSANT and J. H. LUNSFORD Department of Chemistry, Texas A&M University, College Station, Tex. 77843 -

2+

The reversible formation of a low-spin [Co(III)(NH ) O ] complex within a Co(II) Y zeolite has been demonstrated by EPR spectroscopy. In this complexnis probably equal to five. A maximum of one cobalt complex per large cavity was formed. The cobalt hyperfine structure shows that the unpaired electro is only 3% on the metal ion. Experiments utilizing Oindi cate thatO enters the coordination sphere of the Co ions and that the unpaired electron is largely associated with the oxyge molecule. The oxygen-17 hyperfine structure reveals that th two oxygen atoms are not equivalent; hence, it is concluded that the oxygen is bonded as a peroxy-type superoxide ion. 3

n

17

2+

2

" e x c h a n g e a b l e c a t i o n s i n a zeolite m a y m o v e f r o m t h e i r u s u a l sites t o f o r m well-defined

t r a n s i t i o n m e t a l complexes i n t h e large c a v i t i e s (1-4)-

S u c h z e o l i t e - t r a n s i t i o n m e t a l complexes are p o t e n t i a l l y t h e heterogeneous analogs of i m p o r t a n t homogeneous c a t a l y s t s .

R e c e n t l y , M i k h e i k i n et al.

(4) a n d V a n s a n t a n d L u n s f o r d (3) h a v e s t u d i e d r e s p e c t i v e l y t h e h i g h - s p i n Co(H 0) 2

lites.

6

and the low-spin C o ( C H N C )

2 +

3

The

low -spin r

cobalt(II)

5 f

6

2 +

complexes i n C o ( I I ) Y zeo

complexes h a v e c h a r a c t e r i s t i c

electron

p a r a m a g n e t i c resonance s p e c t r a w h i c h are v e r y s i m i l a r t o t h e s p e c t r a of analogous complexes f o r m e d i n o t h e r m e d i a (δ, 6). S e v e r a l l o w - s p i n m o n o - a n d dicobalt(II) complexes of s i m p l e amines i n s o l u t i o n s were r e p o r t e d t o b i n d m o l e c u l a r o x y g e n r e v e r s i b l y (7).

The

best k n o w n e x a m p l e of the b i n u c l e a r p e r o x y complexes is [ ( H N ) 5 - C o - 0 8

Co(NH )5] 3

5 +

(8).

2

I n aqueous s o l u t i o n s no d e f i n i t i v e evidence for m o n o -

Centrum voor Oppervlaktescheikunde en Colloidale Scheikunde, Katholieke Universiteit Leuven, De Croylaan 42, B-3030 Heverlee, Belgium. Present address: University of Antwerp, Fort VI-straat, 2610 Wilrijk, Belgium. 1

2

441

Meier and Uytterhoeven; Molecular Sieves Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

2

442

MOLECULAR SIEVES

meric [ C o ( I I I ) ( N H ) 0 - ] 3

5

2

c o m p l e x , w h e r e t h e m o l a r r a t i o of C o t o 0

2 +

1 : 1 , has y e t been o b t a i n e d .

2

is

F u j i w a r a et al. (9) h o w e v e r , r e p o r t e d r e c e n t l y }

a 1:1 a d d u c t p r o d u c e d b y γ-irradiation of t h e [ C o ( N H ) 5 ( N 0 ) ] ( N 0 3 ) 2 3

salt.

I n a l l of these c o b a l t complexes, 0

sphere of t h e C o

2 +

2

3

c a n enter t h e first c o o r d i n a t i o n

i o n s , f o l l o w e d b y a c h a r g e - t r a n s f e r process.

The E P R

m e a s u r e m e n t s s h o w t h a t t h e u n p a i r e d e l e c t r o n of t h e l o w - s p i n C o i o n is l a r g e l y associated w i t h t h e c o o r d i n a t e d o x y g e n . charge t r a n s f e r a l l o w s a r e d u c t i o n b a c k t o C o therefore, t h e o x y g e n a t e d

cobalt (III)

2 +

T h e r e v e r s i b i l i t y of t h e w i t h a release of o x y g e n ;

complexes

are models of

oxygen

carriers i n biological systems.

Downloaded by UNIV OF ARIZONA on March 12, 2017 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0121.ch039

I n t h i s w o r k , 1:1 o x y g e n a t e d l o w - s p i n c o b a l t ( I I ) - a m m o n i a complexes w e r e s y n t h e s i z e d w i t h i n t h e zeolite f r a m e w o r k b y t h e a d s o r p t i o n of N H and 0

2

i n C o ( I I ) Y zeolites w i t h d i f f e r i n g c o b a l t ( I I ) c o n t e n t .

3

Spin densi

ties a n d t h e n a t u r e of t h e superoxide a n i o n ( 0 ~ ) were e s t i m a t e d b y i n t r o 2

d u c i n g oxygen-17

i n the ammoniated Co(II)

zeolites.

Questions

con

c e r n i n g t h e e q u i v a l e n c e of t h e t w o o x y g e n a t o m s h a v e a r i s e n i n studies o n o x y g e n a d d u c t s of C o (II) Schiff base c o m p o u n d s (7), a n d i t w a s of i n t e r e s t t o s t u d y t h i s p r o b l e m i n c o b a l t ( I l ) - a m m o n i a complexes. Experimental T h r e e C o ( I I ) Y zeolites w i t h different c o b a l t c o n c e n t r a t i o n s were p r e p a r e d f r o m a L i n d e N a Y z e o l i t e (lot n o . 13544-76) b y c o n v e n t i o n a l i o n exchange. A c a t i o n a n a l y s i s of t h e C o ( I I ) Y zeolites i n d i c a t e d c o n c e n t r a t i o n s of 0.8, 5, a n d 16 C o ions per u n i t cell. T h e C o ( I I ) Y zeolite samples were a c t i v a t e d b y h e a t i n g t o 400° C i n i n c r e m e n t s of 1 0 0 ° C p e r h o u r u n d e r a v a c u u m of 1 0 ~ t o r r . A m m o n i a w a s a d s o r b e d i n t h e d e h y d r a t e d C o ( I I ) Y zeolites a t r o o m t e m p e r a t u r e . T h e a m m o n i a t e d C o ( I I ) Y zeolites were o x i d i z e d b y e x p o s i n g t h e s a m p l e t o o x y g e n (3 m m ) a t - 7 0 ° C for 10 m i n . The N H , 0 , and a 0 0 mix t u r e e n r i c h e d t o 4 4 . 5 % 0 were o b t a i n e d f r o m c o m m e r c i a l sources a n d were u s e d w i t h o u t f u r t h e r p u r i f i c a t i o n . T h e E P R s p e c t r a , r e c o r d e d a t - 1 9 6 ° C or a t 2 5 ° C , were t a k e n w i t h V a r i a n E 6 S a n d V 4 5 0 2 s p e c t r o m e t e r s for X - b a n d (9.1 G H z ) , a n d Q - b a n d (35 G H z ) m e a s u r e m e n t s , r e s p e c t i v e l y . T h e g v a l u e s were e v a l u a t e d b y u s i n g a 2 , 2 - d i p h e n y l - l - p i c r y l h y d r a z y l ( D P P H ) s t a n d a r d , w i t h a g v a l u e of 2.0036. S p i n c o n c e n t r a t i o n s were o b t a i n e d b y u s i n g a single c r y s t a l of f r e s h l y r e c r y s t a l l i z e d C u S 0 - 5 H 0 as a s t a n d a r d . T h e e s t i m a t e d e r r o r i n s p i n c o n c e n t r a t i o n is ± 3 0 % . 2 +

5

3

1 6

2

1 7

1 8

1 7

4

2

Results and Discussion U p o n a d s o r p t i o n of excess a m m o n i a i n a C o ( I I ) Y zeolite a w h i t e , h i g h - s p i n c o b a l t ( H ) - a m m o n i a c o m p l e x w i t h a s p i n c o n f i g u r a t i o n of (fc^) 5

(e ) g

2

is f o r m e d .

A c c o r d i n g t o studies of c o b a l t ( I I ) complexes i n s o l u t i o n s ,

salts, a n d i n zeolites, a h e x a c o o r d i n a t e C o ( I I ) - a m m o n i a c o m p l e x is t h e m o s t l i k e l y f o r m w h e n a n excess of a m m o n i a is present (3, 4, 6).


Indeed,

39.

Cobalt (II)-Ammonia Complex

VANSANT AND LUNSFORD

443

w h e n a n excess of N H , C H C N , C H N C , or H 0 i n C o ( I I ) solutions a n d C H N C or H 0 i n C o (II) Y zeolites was present, s i x - c o o r d i n a t e d C o (II) c o m plexes were a l w a y s observed. B e c a u s e of t h e s h o r t r e l a x a t i o n t i m e , n o E P R s p e c t r a of t h e ϋ ο ( Ν Η ) complexes c a n be detected a t — 1 9 6 ° C ; h o w ever, w h e n 0 was a d s o r b e d i n t h e a m m o n i a t e d C o ( I I ) Y zeolite, E P R spectra attributed to a low-spin oxygen-carrying c o b a l t - a m m o n i a com p l e x were observed a t r o o m t e m p e r a t u r e a n d a t — 1 9 6 ° C . F i g u r e 1 show^s a t y p i c a l X - b a n d E P R s p e c t r u m of a n o x y g e n a t e d C o ( I I ) - a m m o n i a Y 3

3

3

3

2

2

3

6

2 +


2

Figure L EPR spectrum, at —196°C, of an oxygenated Co(II)-ammonia complex in a Co(II)Y zeolite zeolite. T h e 16 h y p e r f i n e lines i n t h e o b s e r v e d s p e c t r a m a y be a t t r i b u t e d t o t h e s u p e r p o s i t i o n of t w o sets of 8 lines c o r r e s p o n d i n g t o t h e p a r a l l e l a n d p e r p e n d i c u l a r d i r e c t i o n s of t h e s y m m e t r y axis w i t h respect t o t h e e x t e r n a l m a g n e t i c field. S u c h a s p e c t r u m is c h a r a c t e r i s t i c of t h e h y p e r f i n e i n t e r action from a C o monomeric complex. A Q-band experiment was carried o u t t o ensure t h e p r o p e r d e t e r m i n a t i o n of a g v a l u e , since a t t h e h i g h e r frequencies t h e m a x i m u m c o r r e s p o n d i n g t o g\\ is b e t t e r resolved. A s s h o w n i n T a b l e I , t h e m a g n e t i c p a r a m e t e r s of t h e o x y g e n a t e d C o ( I I ) - a m m o n i a c o m p l e x i n t h e zeolite are c o m p a r a b l e w i t h o t h e r m o n o n u c l e a r C o ( I I ) - 0 complexes, regardless of t h e n a t u r e of t h e c o b a l t ( I I ) l i g a n d s . T h e v e r y s i m i l a r s p e c t r u m o b s e r v e d b y F u j i w a r a et ah (9) f o r 5 9

2


MOLECULAR SIEVES

444

Table L

Magnetic Parameters of Some Cobalt(II) Monomeric Oxygen Adducts in Solutions, Salts, and Co (II)Y Zeolites 9± 9\\ Band ± 0.004 ± 0.004

Compound

0

[Co(III)(NH ) 0 -p+Y Zeolite

X

[Co(NH ) N0 ](N0 ) Co (acacen) py(0 ) Vitamin B (0 ) TPP-L-Co(0 ) Co-TsPc(0 )

Q X X X X X

3

3

B

n

3

2

2

2

2

2

±

Ref.

2.084 2.083 2.081 2.082 2.07 2.07 2.075

2.000 1.996 1.995 1.999 2.004 2.00 2.004

17.8

12.5

17.7 19.6 15 18.3 15.9

12.2 10.7 13 14.2 8.5

—

this work this work (9) (11)

—

(12)

(7d) (7c)

Py pyridine, T P P - L = tetraphenylporphine-4 aminopyridine, acacen = [CH C(0~)=CHC(CH8)=NCH2-)2], andTsPc = tetrasulfophthalocyanide. a


C o

2

3

12r

\a *\ ± 1 G

hi l ± 1 G

=

3

7-irradiated [ C o ( N H ) ( N 0 ) ] ( N 0 ) 3

(ΝΗ ) 0 -] 3

5

2

2 +

5

3

3

2

w a s also a t t r i b u t e d t o a

[Co(III)-

complex.

T h e s p i n c o n c e n t r a t i o n s of t h e o x y g e n a t e d c o b a l t - a m m o n i a complexes w e r e e s t i m a t e d t o b e 0.75, 4 . 1 , a n d 6.5 spins p e r u n i t c e l l f o r t h e C o ( I I ) Y zeolites w i t h r e s p e c t i v e l y 0.8, 5, a n d 16 C o zeolite c o n t a i n e d less t h a n one C o

2 +

2 +

ions p e r u n i t c e l l .

i o n per large c a v i t y ( < 8 C o

c e l l ) , t h e n u m b e r of s p i n s w a s i n agreement w i t h t h e C o experimental error. (>8Co

2 +

2 +

When the 2 +

per u n i t

content, w i t h i n

However, for t h e high-exchanged C o ( I I ) Y

zeolite

p e r u n i t c e l l ) , t h e s p i n c o n c e n t r a t i o n i n d i c a t e d o n l y 6.5 c o b a l t

complexes p e r u n i t c e l l w h i c h i s s l i g h t l y less t h a n one p e r large c a v i t y . T h e s e r e s u l t s suggest t h e presence of i s o l a t e d [ C o ( I I I ) ( N H ) 0 ~ ] 3

5

2

2 +

com

plexes i n t h e l a r g e c a v i t i e s of t h e zeolite w h e r e t h e m o l a r r a t i o of C o t o 0

2

is 1 : 1 . According to Table I, the small C o

2 +

hyperfine splitting constants

indicate t h a t t h e unpaired electron must be largely localized o n t h e co ordinated oxygen molecule.

I f t h e unpaired electron is localized i n only

one d o r b i t a l , t h e h y p e r f i n e tensor c a n b e r e s o l v e d i n t o a n i s o t r o p i c a n d anisotropic part i n the f o r m :

an

- *2 +

-β

(D

-β

+2/3

T h e F e r m i c o n t a c t t e r m , Α? °, i s p r o p o r t i o n a l t o t h e 4s c h a r a c t e r of t h e Β

w a v e f u n c t i o n whereas t h e a n i s o t r o p i c t e r m 2β i s p r o p o r t i o n a l t o t h e 3 d c h a r a c t e r of t h e w a v e f u n c t i o n . a

±

S i n c e o n l y t h e a b s o l u t e v a l u e s of an a n d

m a y be determined f r o m the E P R spectrum, various sign combinations

are possible w h i c h r e s u l t i n A

i80

= ± 2 . 4 o r ± 14.3 G .

T h e l a t t e r absolute

v a l u e agrees w e l l w i t h a n i s o t r o p i c c o b a l t s p l i t t i n g of |l3.3| G w h i c h w a s m e a s u r e d d i r e c t l y f o r t h e m o n o m e r i c o x y g e n a d d u c t of N,N ' - e t h y l e n e b i s ( a c e t y l a c e t o n i m i n a t o ) c o b a l t ( I I ) , a b b r e v i a t e d C o (acacen) (11).


39.

445

Cobalt (II)-Ammonia Complex


A p o s i t i v e sign for A-

corresponds t o 2β =

lso

sign corresponds t o 20 = —3.5 G .

+ 3 . 5 G , and a negative

T h e a c t u a l sign of 2β is d e t e r m i n e d b y

t h e 3d o r b i t a l w h i c h c o n t a i n s t h e u n p a i r e d e l e c t r o n ; 2β > 0 whereas for t h e o t h e r d o r b i t a l s 2β < 0 (18).

for t h e d * o r b i t a l z

Based on the model

s h o w n i n F i g u r e 2 H o f f m a n a n d c o - w o r k e r s (11), h a v e p o i n t e d o u t t h a t t h e 2p7r* m o l e c u l a r o r b i t a l o n t h e o x y g e n w h i c h c o n t a i n s the u n p a i r e d electron mixes o n l y w i t h t h e Sd

yg

n e g a t i v e sign for A


iso

o r b i t a l of c o b a l t .

T h i s , of course, suggests a

a n d 2β.

^ 3

Co

+a

NH-

Figure 2. Structure of the [Co(III)(NH ) 0 -] complex in Co (II) Y zeolites z

5

2

+2

Figure 3. Typical EPR spectrum, at —196°C, of ammoniated Co(II) Y zeolite after absorption of 0 enriched with 44.5% 0 2


17

446

MOLECULAR SIEVES

R e g a r d l e s s of t h e s i g n choice, t h e 4s a n d 3d c h a r a c t e r of t h e w a v e function m a y be evaluated b y comparing the experimentally determined c o u p l i n g c o n s t a n t w i t h t h e v a l u e s of 1350 G (14) a n d — 1 7 3 G (13) f o r t h e u n p a i r e d e l e c t r o n i n a p u r e 4s or 3d o r b i t a l o n C o ( I I I ) . T h i s c o m p a r i s o n confirms t h a t t h e u n p a i r e d e l e c t r o n is o n l y a b o u t 1 % l o c a l i z e d i n t h e 4s o r b i t a l a n d 2 % l o c a l i z e d i n t h e 3d o r b i t a l . Since n o n i t r o g e n hyperfine s p l i t t i n g w a s observed f o r t h e N H l i g a n d s , t h e u n p a i r e d e l e c t r o n m u s t b e almost completely localized (>97%) o n the 0 ligand. vz

3

2

T o s t u d y f u r t h e r t h e n a t u r e of t h e c o o r d i n a t e d 0 molecule, a d s o r p t i o n experiments w i t h 0 0 were c a r r i e d o u t . F i g u r e 3 shows a n E P R s p e c t r u m of t h e a m m o n i a t e d C o ( I I ) Y zeolites, t r e a t e d w i t h 0 0. S i n c e t h e n u c l e a r s p i n of 0 is / , p a r a m a g n e t i c species w i t h one 0 h a v e 21 + 1 o r six lines. T w o sets of s i x h y p e r f i n e lines ( 0 0 ) c a n b e o b s e r v e d i n a d d i t i o n t o t h e c o b a l t hyperfine lines. T h e p a r a m a g n e t i c p a r a m e t e r s of 0 species a r e g i v e n i n T a b l e I I . T h e s e v a l u e s , i n c l u d i n g t h e g tensor, a r e c o m p a r a b l e w i t h those observed i n several studies of t h e superoxide i o n o n v a r i o u s oxides (16). 2

1 7

1 8

1 7


1 7

B

1 8

1 7

2

1 7

1 8

1 7

T h e 0 hyperfine s t r u c t u r e i n d i c a t e s t h a t t h e t w o o x y g e n a t o m s a r e n o t e q u i v a l e n t . T h i s o b s e r v a t i o n tends t o s u p p o r t t h e m o d e l of t h e C o ( a c a c e n ) 0 complexes proposed b y C r u m b l i s a et al. (7a) u s i n g I R d a t a a n d H o f f m a n et al. (11) u s i n g E P R d a t a . T h e 0 h y p e r f i n e s t r u c t u r e w a s n o t a v a i l a b l e i n t h e l a t t e r case. C o n s i d e r a t i o n s of t h e g e o m e t r y suggest a s y m m e t r y as s h o w n i n F i g u r e 2, where t h e 0 - 0 i n t e r n u c l e a r axis (ζ') is l a r g e l y a l o n g t h e χ axis. T o a first a p p r o x i m a t i o n a ^ ~ a > > ~ 0 (16). 1 7

2

1 7

y

Table II.

z

E P R Data for the 0 Hyperfine Interactions 17

a v, aj,v, a v, x

2

0(1)

0(2)

-80G ~0G ~0G

-60G ~0G ~0G

^iiso,

-27G

2/3,

-53G 0.016

p

8

0.51

P2 Tc * P

z

xf

-20G

-40G 0.012 0.38

T h e e x p e r i m e n t a l h y p e r f i n e tensor f o r each o x y g e n c a n be r e s o l v e d i n t o i t s i s o t r o p i c a n d a n i s o t r o p i c components i n t h e f o r m g i v e n b y E q u a t i o n 1. B e c a u s e t h e n u c l e a r m a g n e t o g y r i c r a t i o f o r o x y g e n is n e g a t i v e , Ai < O , a n d 2β > Ο. A n a n a l y s i s of t h e e x p e r i m e n t a l h y p e r f i n e tensor s i m i l a r t o t h a t c a r r i e d o u t for c o b a l t reveals t h a t t h e u n p a i r e d e l e c t r o n o n t h e o x y g e n is m a i n l y l o c a l i z e d ( ~ 9 0 % ) i n a 2ρπ * m o l e c u l a r o r b i t a l . T h e values of Ai , 2β, a n d t h e electron densities o n t h e different o x y g e n a t o m s are s u m m a r i z e d i n T a b l e I I . %Q

χ

SO

A n e x a m i n a t i o n of t h e r e v e r s i b i l i t y of t h e o x y g e n a t i o n shows t h a t u p o n e v a c u a t i o n of t h e 0 t h e h i g h - s p i n c o b a l t ( I l ) - a m m o n i a c o m p l e x w a s r e 2


39.

447

Cobalt (II)-Ammonia Comptez


stored many times. This indicates a reversible charge transfer between the central cobalt ion and the coordinated oxygen molecule. We may conclude that the divalent cobalt ions move out into the large cavities upon adsorption of N H to form a hexacoordinate cobalt(II)ammonia complex. Following adsorption of 0 in the ammoniated Co (II) Y zeolites, oxygen enters the coordination sphere of the C o ions. This is accompanied by a charge-transfer process to form a [Co(III) ( Ν Η ) 0 ί Γ ] complex. The general intermolecular redox process can be approximated by the reactions 3

2

2 +

2 +

3

Co

2+

2+

+ 6NH *± [Co(II)(NH ) ] 3

3

(2)

6

2+

2+

[Co(II)(NH ) ] + 0 [Co(III)(NH ) 0 -] + NH Downloaded by UNIV OF ARIZONA on March 12, 2017 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0121.ch039

3

e

2

3

5

5

2

(3)

3

The reversibility of the charge transfer makes this complex useful as an oxygen carrier or perhaps as an oxidation catalyst. This monomeric complex forms in solution instead of the dimer in solutions because each peroxy complex is restricted from motion by the zeolite framework. There fore, the formation of binuclear [ ( Η Ν ) - Ο ο - 0 - Ο ο - ( Ν Η ) ] complex is slow in the zeolite. 5 +

3

5

2

3

δ

Literature Cited 1. 2. 3. 4.

Naccache, C., Ben Taarit, Y., Chem. Phys. Lett. (1971) 11, 11. Vansant, E. F., Lunsford, J. H., J. Phys. Chem. (1972) 76, 2860. Vansant, E. F., Lunsford, J. H., Chem. Commun. (1972) 830. Mikheikin, I. D., Brotikovskii, O. I., Zhidomirov, G. M., Kazanskii, V. B., Kinet.Katal.(1971) 12, 1279. 5. Symons, M . C. R., Wilkinson, J. G., J. Chem. Soc. A (1971) 2069. 6. Maher, J. P., J. Chem. Soc. A (1968) 2918. 7. See, for example, (a) Grumbliss A. L., Basolo, F., J. Amer. Chem. Soc. (1970) 92, 55; (b) Walker, F. Α., ibid. (1970), 92, 4235; (c) Abel, E. W., Chem. Commun. (1971) 449; (d) Yamamoto, K., Kwan, T., J. Catal. (1970) 18, 354. 8. Sykes, A. G., Weil, J. Α., Progr. Inorg. Chem. (1970) 13, 1. 9. Fujiwara, S., Watanabe, T., Tadano, H., J. Coord. Chem. (1971) 1, 195. 10. Cotton, F. Α., Wilkinson, G., Advan. Inorg. Chem. (1966) 2, 863. 11. Hoffman, B. M., Diemente, D. L., Basolo, F., Amer. Chem. Soc. (1970) 92, 61. 12. Bayston, J. H., Kelso, N., Looney, F. D., Winfield, M . E., J. Amer. Chem. Soc. (1969) 91, 2775. 13. Goodman, Β. Α., Raynor, J. B., Advan. Inorg. Chem. Radiochem. (1970) 13, 135. 14. McGarvey, B. R., J. Phys. Chem. (1967) 71, 51. 15. See, for example, (a) Tench, A. J., Holroyd, P. H., Chem. Commun. (1968) 471; (b) Symons, M . C. R., Phys. Chem. (1972) 76, 3095; (c) Lunsford, J. H., Catal. Rev., in press. RECEIVED November 17, 1972. Work supported by National Science Foundation Grants GP-35662X and GP-29898 as part of a cooperative program with J. Uytterhoeven, University of Leuven, Belgium, and by an Aspirant grant from the N.F.W.O. (Belgium) to E. F. V.


Formation and Structure of a Monomeric Oxygen Adduct of a Cobalt(II

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