Reaction Schemes for Dinuclear Compounds Containing Metal-Metal

31 Reaction Schemes for Dinuclear Compounds

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Containing Metal-Metal Triple Bonds Illustrated by Recent Findings in the Chemistry of Molybdenum and Tungsten MALCOLM H. CHISHOLM Department of Chemistry, Princeton University, Princeton, N J 08540

The following generalized reactions are proposed for com pounds containing metal-metal triple bonds (M =

Mo, W).

(1) Carbene-like addition across the M-M bond; (2) oxida tive addition with reduction in M-M bond order (three to two);

(3) reductive elimination with an increase in M-M

bond order (three to four); (4) reversible Lewis base associa tion which may or may not change the bond order, depend ing upon the electronic configuration of the M moiety; (5) 2

metal-metal triple bond cleavage by a carbyne-like reagent; and (6) oligomerization of the M = M unit to form a cluster or polynuclear complex.

These generalized

reactions are

discussed in the light of recent experimental observations in the

reactivity

patterns

of

M (OR) 2

6

and

Cp M (CO) 2

2

4

compounds.

he a b i l i t y of t r a n s i t i o n metals to f o r m m u l t i p l e b o n d s w i t h

themselves

is w e l l r e c o g n i z e d , a n d over t h e past d e c a d e a n u m b e r o f s u c h c o m p o u n d s h a v e r e c e i v e d d e t a i l e d e x a m i n a t i o n b y a v a r i e t y of spectroscopic and structural techniques. b o n d s see R e f . I .

F o r a recent r e v i e w of M - M q u a d r u p l e

M o r e recently, certain compounds containing M - M

q u a d r u p l e a n d t r i p l e b o n d s h a v e b e e n t h e subject o f t h e o r e t i c a l treatm e n t s (2).

H o w e v e r , t h e r e a c t i v i t y patterns of these c o m p o u n d s r e m a i n s

t o b e e x p l o r e d ; this s h o u l d p r o v e a r i c h a n d e x c i t i n g n e w area of t r a n s i t i o n metal chemistry.

I t is p o s s i b l e

that o r g a n o m e t a l l i c

reaction

0-8412-0429-2 / 79 / 33-173-396$05.00 / 0 © 1979 American Chemical Society

King; Inorganic Compounds with Unusual Properties—II Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

schemes

31.

CHISHOLM

Metal-Metal

Triple

397

Bonds

e v o l v e d ( 3 ) f o r m o n o n u c l e a r t r a n s i t i o n m e t a l complexes c a n b e a p p l i c a b l e to d i n u c l e a r systems a n d , f u r t h e r m o r e , t h a t d i n u c l e a r c o m p o u n d s c a n p r o v i d e b u i l d i n g b l o c k s f o r the m u c h d e s i r e d systematic of n e w p o l y n u c l e a r a n d cluster c o m p o u n d s (4,5,6). number

of general

modes

of r e a c t i o n

containing metal-metal triple bonds.

syntheses

I n this account, a

are p r o p o s e d

for compounds

T h e s e are t h e n d i s c u s s e d i n l i g h t

of recent e x p e r i m e n t a l observations. T h e notation M =

M is u s e d to represent a n y c o m p o u n d c o n t a i n i n g

a h o m o n u c l e a r m e t a l - m e t a l t r i p l e b o n d i n w h i c h t h e m e t a l atoms are i n v e r y s i m i l a r , i f n o t e q u i v a l e n t , e n v i r o n m e n t s ; t h e y h a v e t h e same n u m b e r of v a l e n c e s h e l l electrons, t h e same c o o r d i n a t i o n n u m b e r , a n d t h e same f o r m a l o x i d a t i o n state.

F i t t i n g these r e q u i r e m e n t s

m o l y b d e n u m a n d tungsten c o m p o u n d s . compounds, where h a l i d e (7).

Y =

are t w o classes of

C l a s s I are M X 2

and M X _ ; Y

6

2

6

n

R(alkyl), N R , O R , 0 C N R , 0 C O R , and 2

C l a s s I I are C p M ( C O ) 2

2

4

2

2

2

compounds ( M = C r , M o , and W )

i n w h i c h the m e t a l atoms are f o r m a l l y i n t h e + 1 o x i d a t i o n state a n d a t t a i n a n 18-valence s h e l l e l e c t r o n i c c o n f i g u r a t i o n b y t h e f o r m a t i o n o f t h e metal-metal triple bond ( 8 , 9 ) . valent ( M

3 +

I n class I t h e metals a r e f o r m a l l y t r i -

) a n d , e v e n after f o r m i n g a m e t a l - m e t a l t r i p l e b o n d , d o n o t

a t t a i n a n 18-valence s h e l l e l e c t r o n i c c o n f i g u r a t i o n .

B o t h o x i d a t i o n state

a n d v a l e n c e s h e l l electronic c o n f i g u r a t i o n are e x p e c t e d t o influence t h e r e a c t i v i t y of t h e m e t a l - m e t a l t r i p l e b o n d . T h e p r o p o s e d reactions i n v o l v e t h e s y m m e t r i c a l a d d i t i o n / e l i m i n a t i o n of substrate m o l e c u l e s to M =

M

c o m p o u n d s ; t h e p r o d u c t s are c o n s i d e r e d to h a v e e q u i v a l e n t m e t a l atoms. R e a c t i o n s w h i c h m i g h t l e a d to a n o d d n u m b e r of electrons i n t h e p r o d u c t s are n o t c o n s i d e r e d .

T h i s is n o t m e a n t

to i m p l y that

such

reactions c a n n o t o c c u r n o r to i m p l y that o d d e l e c t r o n i n t e r m e d i a t e s a r e not i n v o l v e d i n some o f t h e p r o p o s e d reactions. M-M

K Mo (S0 ) -3.5H 0 3

(Compounds containing

b o n d s of f r a c t i o n a l o r d e r are w e l l d o c u m e n t e d . 2

4

4

2

2

( i i i ) [ C p C o ( C O ) ] " (12).)However, 2

2

E . g . , as i n ( i )

( J O ) , ( i i ) M o W ( 0 C B u ^ ) 4 ( C H C N ) (I)

2

(11),

3

thus f a r i n o u r studies w e h a v e

n e i t h e r o b t a i n e d as p r o d u c t s n o r d e t e c t e d as i n t e r m e d i a t e s o d d e l e c t r o n d i n u c l e a r species. A d d i t i o n of X : to M =

M i n R e a c t i o n 1 represents a

carbene-like

a d d i t i o n to a t r i p l e b o n d . T h e m o i e t y X : c o u l d i n d e e d b e a c a r b e n e o r X / M ^ M

+ X : *±M

=

\ M

(1)

an organic molecule capable of reacting w i t h a metal-to-metal triple b o n d i n this w a y , e.g., c a r b o n m o n o x i d e o r a n i s o - n i t r i l e . X : c o u l d also b e a n i n o r g a n i c / o r g a n o m e t a l l i c substrate s u c h as F e ( C O ) , C p N b H , S n R , o r 4

2


2

398

INORGANIC

a d

COMPOUNDS

WITH

square p l a n a r t r a n s i t i o n m e t a l c o m p l e x .

s

m e r e l y that i t is c a p a b l e

UNUSUAL

PROPERTIES

II

T h e r e q u i r e m e n t of X : is

of e x p a n d i n g its c o o r d i n a t i o n n u m b e r a n d

o x i d a t i o n state b y t w o . A n i n t e r e s t i n g e x a m p l e of R e a c t i o n 1 has just b e e n d i s c o v e r e d i n a s t u d y of the r e a c t i o n b e t w e e n M o ( O R ) 2

compounds and carbon mon-

6

o x i d e ( 1 3 ) . T h e c o m p o u n d M o ( O B u * ' ) reacts r e v e r s i b l y w i t h c a r b o n e r

2

6

m o n o x i d e i n h y d r o c a r b o n solvents at r o o m t e m p e r a t u r e a n d 1 a t m to g i v e a deep purple crystalline c o m p o u n d M o ( O B u 2

t e r t

) CO, v(CO) = 6

c m " . T h e m o l e c u l a r structure d e d u c e d f r o m x-ray studies (13) 1

i n F i g u r e 1. T h e m o l e c u l e has v i r t u a l C

symmetry.

2v

1670

is s h o w n

T h e coordination

p o l y h e d r o n a b o u t e a c h m e t a l a t o m is a p p r o x i m a t e l y a square p y r a m i d w i t h the b r i d g i n g c a r b o n y l c a r b o n at t h e c o m m o n apex. T h e short m e t a l m e t a l distance, 2 . 4 9 8 ( 1 ) A (cf. M o — M o —

2.222(1) A i n M o ( O R ) ) , 2

6

the d i a m a g n e t i c nature of the c o m p o u n d , a n d e l e c t r o n c o u n t i n g r e q u i r e the existence of a m e t a l - m e t a l d o u b l e b o n d . R e a c t i o n s 2 a n d 3 represent o x i d a t i v e a d d i t i o n a n d r e d u c t i v e e l i m i n a t i o n sequences. Reactions 1, 2, a n d 3 a l l i n v o l v e a reversible a d d i t i o n / e l i m i n a t i o n of a substrate m o l e c u l e that contributes t w o electrons to a d i n u c l e a r center. H o w e v e r , since the p r o p o s e d reactions p r o c e e d w i t h a c h a n g e i n M - M b o n d order, t h e n u m b e r of m e t a l v a l e n c e s h e l l electrons

Mo 0 (CO) 2

6

Skeleton

of

Mo (0-t-Bu) (CO) 2

6

Figure 1. A view of the coordination geometry of Mo (OBu ) CO showing the main internuclear distances. Each atom is represented by its thermal ellipsoid of vibration, scaled to enclose 40% of the electron density. The tertiary butyl groups are omitted for clarity. 2

tert

6


31.

CHISHOLM

Metal-Metal M = M

+

Triple X — X +±

X — M = M — X

is n o t i n c r e a s e d .

399

Bonds

(2)

X — M = M — X

?± M E = M

(3)

+ X — X

T h i s contrasts w i t h the reactions of m o n o n u c l e a r t r a n -

s i t i o n m e t a l c o m p l e x e s w h e r e t h e metals c h a n g e t h e i r n u m b e r of v a l e n c e s h e l l electrons b y t w o . A t present t h e r e are n o w e l l - d o c u m e n t e d examples of R e a c t i o n s

2

a n d 3 a l t h o u g h several k n o w n c o m p o u n d s c o u l d serve as excellent m o d e l s f o r these types of reactions.

F o r example, the compounds M M e ( 0 2

2

2

C N R ) 4 , w h e n h e a t e d to > 1 5 0 ° C i n v a c u o , e l i m i n a t e ethane a n d y i e l d 2

residues w h i c h , b y e l e m e n t a l analyses, c a n b e f o r m u l a t e d as M ( 0 C N R ) 2

compounds. Both M o ( C H S i M e ) 2

2

3

2

2

4

a n d M o ( O P r ) h a v e b e e n f o u n d to

G

i

2

6

react w i t h acetic a c i d to y i e l d , u p o n v a c u u m s u b l i m a t i o n ( 2 0 0 ° C , 10~

4

c m H g ) , M o ( O A c ) . H e r e a M - M triple-to-quadruple b o n d transforma2

4

t i o n is a c h i e v e d , R e a c t i o n 3, b u t t h e d e t a i l e d r e a c t i o n p a t h w a y a n d t h e nature of the e l i m i n a t e d o r g a n i c c o m p o u n d s are n o t k n o w n . T h e s i m p l e o x i d a t i v e a d d i t i o n of X - X across a M - M t r i p l e b o n d t o y i e l d a n u n b r i d g e d M - M d o u b l e b o n d has y e t to b e s t r u c t u r a l l y establ i s h e d a l t h o u g h there are a n u m b e r of reactions i n w h i c h this m i g h t o c c u r , e.g.

(8), C p M ( C O ) 2

2

+

4

I

- » C p M ( C O ) I . There

2

2

2

4

2

are, h o w e v e r ,

k n o w n examples of w h e r e a n X - X a d d i t i o n to a c o m p o u n d c o n t a i n i n g a m u l t i p l e b o n d occurs w i t h t h e f o r m a t i o n of m e t a l - l i g a n d b r i d g e s . r e a c t i o n of M o ( O P r ) i

2

G

The

to g i v e M o ( O P r ) , w h i c h is d i s c u s s e d later, is i

2

8

representative of this t y p e of M - M t r i p l e - t o - d o u b l e b o n d t r a n s f o r m a t i o n since i n t h e p r o d u c t , M o ( O P r ) , there are b r i d g i n g a l k o x y l i g a n d s . T h e 1

2

a d d i t i o n of X ( X = 2

8

I o r B r ) to M o ( S C O E t ) , w h i c h contains a M - M 2

2

quadruple bond, yields M o X ( S C O E t ) 2

2

2

4

compounds having

4

Mo-Mo

s i n g l e b o n d s ( M o - M o = 2.72 A ) as a result of a s u r p r i s i n g r e a r r a n g e m e n t i n the b o n d i n g m o d e of the xanthate l i g a n d (14).

Clearly the reactivity

of c o m p o u n d s c o n t a i n i n g M - M m u l t i p l e b o n d s t o w a r d s o x i d a t i v e a d d i t i o n / r e d u c t i v e e l i m i n a t i o n reactions is g o i n g to b e as c o m p l e x a n d e v e n less p r e d i c t a b l e t h a n analogous reactions i n v o l v i n g m o n o n u c l e a r t r a n s i t i o n m e t a l complexes

(15,16).

T h e r e are s e v e r a l examples of L e w i s base association reactions o f t y p e 4.

MEEEEM +

2 L : *±

L—M=M—L

H e r e f o u r electrons are d o n a t e d to t h e M M-M

2

(4)

c e n t e r w i t h r e t e n t i o n of t h e

t r i p l e b o n d . L e w i s base association s h o u l d b e a p p l i c a b l e o n l y t o

m e t a l - t o - m e t a l t r i p l e - b o n d e d c o m p o u n d s i n w h i c h t h e m e t a l atoms 16 o r less v a l e n c e s h e l l e l e c t r o n i c

configurations.

have

T h i s is t h e case f o r


400

INORGANIC COMPOUNDS W I T H UNUSUAL PROPERTIES

II

Figure 2. An ORTEP view of the Mo (OSi) (NC ) portion of the Mo (OSiMe ) (HNMe ) molecule looking directly down the Mo-Mo bond with Mo(l) eclipsed by Mo(2). Atoms labelled in smaller print are bonded to Mo(l). All atbms are represented by 50% probability ellipsoids. Some important interatomic distances and angles are: Mo-Mo = 2.242(1) A, Mo-O(av) = 1.95 A, Mo-N(av) = 2.28 A, MoMo-O(av) = 102°, Mo-Mo-N(av) = 95°. * 2

2 2

Mo (OR) 2

2

rr

3 G

2 2

c o m p o u n d s , a n d these react r e v e r s i b l y w i t h amines to g i v e

G

adducts M o ( O R ) ( a m i n e ) 2

G

Mo (OSiMe ) (HNMe ) 2

3

c

2

2

A v i e w of t h e c e n t r a l core of t h e

(17).

m o l e c u l e is s h o w n i n F i g u r e 2. T h e M o - M o

6

distance is 2 . 2 4 2 ( 1 ) A . O t h e r examples i n w h i c h m e t a l atoms i n M =

M compounds expand

t h e i r c o o r d i n a t i o n n u m b e r a n d n u m b e r of v a l e n c e s h e l l electrons are seen i n t h e reactions of M o ( O R ) , (18), 2

(19)

W M e ( N E t ) (19),

6

2

2

2

and W ( N M e )

4

2

2

6

compounds w i t h C 0 . T h e products M o ( O R ) ( 0 C O R ) , W M e 2

2

4

2

2

2

2

( 0 C N E t ) , a n d W ( 0 C N M e ) p r o v i d e examples of c o m p o u n d s c o n 2

2

4

2

2

2

6

t a i n i n g m e t a l - m e t a l t r i p l e b o n d s b e t w e e n m e t a l atoms that are c o o r d i n a t e d to f o u r , five, a n d six l i g a n d atoms, r e s p e c t i v e l y . I n contrast, L e w i s base association to a M = the m e t a l atoms h a v e a n 18-valence

M compound in which

shell electronic configuration w i l l

p r o c e e d w i t h r e d u c t i o n i n M - M b o n d o r d e r as s h o w n i n R e a c t i o n 5. T h e reversible reaction between C p M o ( C O ) ( M o - M o = 2

2

4

2.40 A ) a n d C O

w h i c h gives C p M o ( C O ) ( M o - M o = 3.27 A ) p r o v i d e s a g o o d e x a m p l e 2

2

6

of R e a c t i o n 5 ( 8 ) . M = M + 2L: ^

L—M—M—L


(5)

31.

CHISHOLM

Metal-Metal

Triple

401

Bonds

T h e c o m p o u n d s C p M ( C O ) 4 also h a v e b e e n f o u n d to b e r e a c t i v e 2

2

t o w a r d s a n u m b e r of u n s a t u r a t e d m o l e c u l e s , u n , g i v i n g s i m p l e a d d i t i o n products C p M ( C O ) 4 ( u n ) . 2

PhC =

2

CPh, EtC =

a n d M e N C N (24)

T h e c o m p o u n d s w h e r e M == M o a n d u n

CEt, H C =

C H (20,21),

C H =

C — C H

2

=

(22,23),

2

h a v e b e e n s t r u c t u r a l l y c h a r a c t e r i z e d . I n a l l cases, t h e

2

u n s a t u r a t e d o r g a n i c m o l e c u l e spans the M o b o n d (see 2

increases i n l e n g t h f r o m 2.40 A i n C p M o ( C O ) 2

8.117 A w h e r e u n =

HC =

2

C H , M e N C N , and C H = 2

F i g u r e 3)

which

to 2.974, 3.015, a n d

4

C — C H , respec-

2

2

t i v e l y . T h e o r g a n i c m o l e c u l e s act as f o u r e l e c t r o n donors to t h e M

2

group

a n d c a n be c o n s i d e r e d as f u r t h e r examples of p r o d u c t s f o r m e d i n reactions of t y p e 5.

T h e c o m p o u n d C p M o ( C O ) ( a l l e n e ) has C 2

2

4

thus e q u i v a l e n t m o l y b d e n u m atoms.

2

symmetry and

H o w e v e r , the c o m p o u n d s C p M o 2

2

( C O ) ( R C R ) a n d C p M o ( C O ) ( N C N M e ) a d o p t structures i n w h i c h 4

2

2

2

4

2

the m o l y b d e n u m atoms are i n e q u i v a l e n t . I n C p M o ( C O ) ( R C == C R ) 2

2

4

c o m p o u n d s , the a s y m m e t r y is associated w i t h the c a r b o n y l b o n d i n g a n d p r e s u m a b l y arises f r o m i n t e r n a l c r o w d i n g . I n C p M o ( C O ) ( N C N M e ) , 2

V

OC

4

2

/'

i\

l/ (A)

2

co

Mo_

M

c o

o

-

M

Cp

M e ,v e

;

M e M e . A

'Mo' C

(C)

N Mo:

Cp

*c

M o -

M o

0

Figure 3. Representations for the molecular structures of Cp Mo (CO) (un) compounds: (a) un=RC=CR, (b) un=CH =C=CH , and (c) un=Me NCN 2

2

h

2

2

2


o

402

INORGANIC

COMPOUNDS

WITH

UNUSUAL

PROPERTIES

II

the b r i d g i n g M e N C N g r o u p donates a n i t r o g e n l o n e p a i r to one m o l y b 2

d e n u m a t o m a n d a C N ^-electron p a i r to the other. indicate

that

C p M o ( C O ) ( P h C P h ) and 2

2

4

1 3

C N M R studies

Cp Mo (CO) (NCNMe )

2

2

2

4

2

c o m p o u n d s a d o p t structures i n s o l u t i o n a k i n to those f o u n d i n t h e s o l i d state a n d that l o w energy processes cause t h e t w o m e t a l centers to b e c o m e e q u i v a l e n t o n the N M R t i m e scale a b o v e — 4 0 ° C . I n R e a c t i o n 6 the m e t a l - m e t a l t r i p l e b o n d is c l e a v e d a n d r e p l a c e d by a metal-ligand triple bond. M = M + 2 X i -> 2 M = X

(6)

T h e r e is therefore, n o o v e r a l l change i n t h e n u m b e r of m e t a l v a l e n c e s h e l l electrons.

A l t h o u g h n o t m a n y substrates m e e t t h e r e q u i r e m e n t of

b e i n g c a r b y n e - l i k e , the reactions b e t w e e n n i t r i c o x i d e a n d a m e t a l - t o - m e t a l t r i p l e b o n d c a n b e v i e w e d as examples of R e a c t i o n 6. Cp Mo (CO) 2

2

4

c o m p o u n d s react r e a d i l y w i t h N O ( 2 e q u i v a l e n t s ) t o

g i v e t h e m o n o n u c l e a r complexes C p M o ( C O ) ( N O ) (25). S i m i l a r l y , M o 2

(OR)

2

c o m p o u n d s react w i t h N O ( 2 equivalents) to g i v e [ M o ( O R ) ( N O ) ]

6

3

2

c o m p o u n d s (26). H e r e there is a p a i r of b r i d g i n g a l k o x i d e l i g a n d s , w h i c h

Mo (0-i-Pr) 2

Mo (0-i-Pr) (NO) 2

8

Skeleton

6

2

Skeleton

Figure 4. Coordination geometries of (A) Mo (OPf ) and (B) Mo (OPr ) (NO) showing some pertinent bond distances. Distances shown for B are averaged over two independent molecules. In both A and B the molecules possess rigorous C and virtual C symmetry. i

2

8

2>

{

2h


2

l

6

31.

CHISHOLM

Metal-Metal

Triple

403

Bonds

leads to a 14-valence s h e l l e l e c t r o n i c c o n f i g u r a t i o n f o r m o l y b d e n u m . T h e M o - M o distance is 3.325 A , w h i c h p r e c l u d e s a n y d i r e c t m e t a l - m e t a l b o n d . T h e d i m e r c a n be c l e a v e d b y t h e a d d i t i o n of a d o n o r l i g a n d s u c h as pyridine,

and a mononuclear

compound W ( O B u *

recently been structurally characterized (OPr ) NO] i

3

2

is s h o w n i n F i g u r e 4.

c r

' ) ( N O ) (py)

has

3

T h e structure of

(27).

[Mo-

I n W ( O B u * * ) s ( N O ) ( p y ) there is e r

also a l i n e a r M - N - O m o i e t y i n a n a x i a l p o s i t i o n of a t r i g o n a l b i p y r a m i d ; the p y r i d i n e l i g a n d is i n the other a x i a l p o s i t i o n (27).

T h e v a l u e of the

N O s t r e t c h i n g f r e q u e n c y , 1555 c m " , is l o w for a l i n e a r M - N - O g r o u p 1

( f o r a recent r e v i e w of m e t a l - n i t r o s y l complexes see

R e f . 28),

which

i n d i c a t e s v e r y extensive W-to-NO?r* b o n d i n g a n d the significance of t h e resonance f o r m M =

N - O ; (the t w o other resonance structures f o r a

l i n e a r M - N O g r o u p are M =

N =

O

and N - N =

O:).

A p o t e n t i a l source of a n X : substrate is, of course, a n X == X t y p e of m o l e c u l e . reaction.

Reaction

6 would

then

s i m p l y represent

a

metathesis

Since t r a n s i t i o n m e t a l c a r b y n e complexes are w e l l k n o w n , it is

not i n c o n c e i v a b l e that R e a c t i o n 7 c a n o c c u r . A l t e r n a t i v e l y a n X = M = M

2X:

substrate

+

RCEEBCR ?±

X or (7)

2ME=CR

c o u l d react to f o r m a p l a n a r M X 2

2

m o i e t y of the

type

shown below.

C o m p o u n d s c o n t a i n i n g b r i d g i n g c a r b y n e l i g a n d s , e.g., (/x-CSiMe )] 3

M C 2

2

2

where M =

N b , T a (29),

and W

(30),

[Me SiCH ) M3

2

2

contain planar

moieties of this t y p e .

A n o t h e r m o d e of r e a c t i o n f o r a three-electron s h o w n i n R e a c t i o n 18.

d o n o r substrate

is

T h i s is closely r e l a t e d to t h e s i m p l e o x i d a t i v e -

a d d i t i o n r e a c t i o n s h o w n i n R e a c t i o n 2 a n d c a n b e e x p e c t e d to

occur

w h e n e v e r the X m o i e t y has one or m o r e l o n e p a i r s of electrons, p r o v i d i n g that the f o r m a t i o n of the t w o M - X - M b r i d g e s does not r e q u i r e r u p t u r e

(8)


404

INORGANIC

COMPOUNDS

WITH

UNUSUAL

PROPERTIES

II

of t h e M - M d o u b l e b o n d as i n R e a c t i o n 5. A l t e r n a t i v e l y t h e a d d i t i o n o f 2 X • or X - X across a M - M t r i p l e b o n d c a n o c c u r as i n d i c a t e d i n R e a c t i o n 2, a n d one o f t h e other l i g a n d s c a n t h e n take u p a b r i d g i n g p o s i t i o n , as seen i n R e f . 14. T h e f o r m a t i o n of M o ( O P r )

i n the r e a c t i o n b e t w e e n

Mo (OPr ) ,

a n d p r o t o n sponge ( 2 e q u i v a l e n t s )

i n isopropyl

i

2

A g P F ( 2 equivalents), 6

8

a l c o h o l c a n b e v i e w e d as a n e x a m p l e of R e a c t i o n 8 (31). of M o ( O P r ) i

2

i

2

6

T h e structure

is s h o w n i n F i g u r e 4, w h e r e a s i m p l e c o m p a r i s o n is m a d e

8

w i t h the r e l a t e d c o m p o u n d M o ( O P r ) i

2

C )

(NO)

2

(26). I n b o t h c o m p o u n d s

t h e r e is essentially t r i g o n a l b i p y r a m i d a l c o o r d i n a t i o n a b o u t e a c h m o l y b d e n u m atom, a n d there is a p a i r of P r O b r i d g i n g l i g a n d s w h i c h f o r m j

alternately l o n g ( a x i a l ) a n d short ( e q u a t o r i a l ) M o - O b o n d s .

T h e most

s t r i k i n g differences b e t w e e n t h e t w o structures a r e : ( i ) t h e M o - M o d i s tances, w h i c h a r e 3 . 3 3 5 ( 2 ) a n d 2 . 5 2 5 ( 1 ) A f o r M o ( O P r ) ( N O ) i

2

6

M o ( O P r ) , r e s p e c t i v e l y ; a n d ( i i ) the angles of the M o (/x-O) 1

2

8

2

2

and

2

moiety.

T h e s e differences are r e a d i l y a c c o u n t e d f o r b y s i m p l e l i g a n d field c o n siderations.

A t r i g o n a l b i p y r a m i d a l field splits the m e t a l d orbitals i n t o

three sets e'(d *. *, d ), x

y

e"(d , d ), a n d %!(&#) w i t h the degenerate p a i r ,

xy

xz

yz

d ^ a n d d , l y i n g lowest i n energy.

I n M o ( O P r ) ( N O ) , each m o l y b 2

yz

i

G

2

d e n u m a t o m c a n b e assumed, f o r m a l l y , to h a v e f o u r Ad electrons after the f o r m a t i o n of o--bonds to e a c h o f the five l i g a n d s . T h i s f o r m of electron c o u n t i n g uses t h e c o n v e n t i o n a l , t h o u g h p u r e l y f o r m a l , d e s c r i p t i o n o f t h e l i n e a r M o - N - O g r o u p as M "

Reaction Schemes for Dinuclear Compounds Containing Metal-Metal

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