6
Hydrido
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Group
Complexes of Platinum Metals
L. M. VENANZI State University of New York at Albany, Albany, Ν. Y. 12203
The preparation and properties of hydrido complexes of platinum group metals are reviewed briefly. The reactions of the more common hydrido complexes of iridium are sche matically shown. The reactions of complexes [IrHX (PhP)] (X = Cl, Br, and I), I, with the quadridentate ligand (o-Ph P · C H ) P, QP, are described and rationalized on the basis of the following reaction sequence: 2
3
3
2
6
4
3
I—HX [IrX(Ph P) ] heat [IrHX(0-C H PPh )(Ph P) ] 3
3
6
4
2
3
2
QP [IrH(0-C H PPh )(QP)]X +HX [IrHX(Ph P)(QP)]X Complexes of the latter two types have been assigned seven -coordinate structures on the basis of their proton and phos phorus-31 NMR spectra even though the oxidation number of the iridium atom is 3+. 6
4
2
3
A11 platinum group metals give hydrido complexes of general formula M H XpL (X = anionic ligand and L = uncharged ligand). Al though this field of chemistry is relatively new, its main development occurring over the last 10 years, the number of papers describing new compounds, their properties, and their reactions is very large and is growing at an increasing rate. Hydrido complexes also have been the subject of several reviews (9,11, 23, 24, 25,39), and the present one, like its predecessors, will be sadly out of date by the time it appears in print. As its title implies, this review restricts itself to describing and discuss ing compounds of platinum group metals—i.e., of ruthenium, rhodium, palladium, osmium, iridium, and platinum—although the compounds of the other transition elements and even some post-transition elements are either fully analogous or closely related to those of the platinum metals. n
m
q
66 In Platinum Group Metals and Compounds; Rao, U.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
6.
Hydrido
V E N A N Z I
Table I.
67
Complexes
Some Better Known Types of H y d r i d o Complex of the Platinum Metals' 1
Square
Planar
[PtH L ] [Ir-HX(LL)]
trans-[MBXL ] [IrH.(LL)] [M'(LL),]X
2
2
Five-Coordinate
[IrH L ] [IrH(CO) L ]
[IrH L ]X [M'H(CO)L,] 2
3
3
Octahedral 2
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!
n
2
n
2
2
2
2
2
2
[IrH L 'L ]X [IrH L'L ]X [M'H X _ L ]
2
2
2
2
[PtH X L ] [M HX (CO)L ] [M"H X . (LL) [M"HX(CO)L ] 2
2
3
m
2
3
m
3
3
° M M L m
1
"
M ' = R h or Ir L = monodentate phosphine L L = bidentate phosphine η = 1 or 2
= P d or Pt = R u or Os = C O or monodentate phosphine = 1, 2, or 3
E v e n w i t h i n this l i m i t a t i o n , the subject is vast a n d , therefore, no a t t e m p t is m a d e to give a c o m p r e h e n s i v e d e s c r i p t i o n of the
field.
A s u r v e y of the c o m p o u n d s r e p o r t e d i n the l i t e r a t u r e shows t h a t the f o r m a t i o n of stable h y d r i d o complexes g e n e r a l l y is l i n k e d w i t h the p r e s ence i n the m o l e c u l e of one or m o r e of c e r t a i n types of l i g a n d s , the m o s t frequently occurring being carbon monoxide, phosphorus ( I I I ) senic ( I I I ) d e r i v a t i v e s , a n d the c y c l o p e n t a d i e n e
and ar
group.
I t is u s e f u l to classify the complexes b y t h e i r c o o r d i n a t i o n n u m b e r . T h e m a j o r i t y of t h e m are six-coordinate w i t h o c t a h e d r a l s t r u c t u r e , a l though four- and seven-coordinate
five-coordinate
species are n u m e r o u s , a n d e v e n some
species are k n o w n .
T a b l e I shows a list of the m a i n
types of c o m p o u n d s r e p o r t e d i n the l i t e r a t u r e . P r e p a r a t i v e m e t h o d s for h y d r i d o complexes
vary widely.
Some
the m o r e f r e q u e n t l y u s e d of t h e m a r e : (1)
A c t i o n of c o m p l e x h y d r i d e s ,
e.g., LiAlH
4
as-[RuCl (Me PCH CH PMe ) ] 2
2
2
2
2
>
2
imns-[RuHCl(Me PCH CH PMe ),] 2
(2)
2
2
2
A c t i o n of alcohols i n the presence of a base,
(16)
e.g.,
Ph P 3
(NH ) [OsBr ]< 4
2
6
• [ O s H B r ( C O ) ( P h P ) ] (41) MeOCH CH OH 3
2
(3)
P r o t o n a t i o n of complexes,
3
2
e.g., H+
[Ru (x-C H ) (CO) ] 2
5
5
2
6
> [Ru H(x-C H ) (CO) ]+ 2
5
5
2
6
(20)
In Platinum Group Metals and Compounds; Rao, U.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
of
68 (4)
P L A T I N U M
G R O U P
M E T A L S
A N D
C O M P O U N D S
A c t i o n of h y d r a z i n e , e.g., N H 2
· H 0 > i r a r c s - [ P t H C l ( E t P ) ] (17)
4
2
cie-[PrCl,(Et«P)J
3
2
H y d r i d o complexes are also f o r m e d i n some u n u s u a l r e a c t i o n s : MeOCH CH OH > [ P h A s ] [ P d H C l ( C O ) ] (27) at r o o m t e m p . 2
(1) P d C l + C O + [ P h A s ] C l 2
4
2
4
H 0 > [PtHI (Pr P) ]
2
2
(2) [ P t I ( M g I ) ( P r P ) ] 3
2
3
(19)
2
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Some of the p r e p a r a t i v e m e t h o d s are best d e s c r i b e d , at least f o r m a l l y , as i n v o l v i n g a n " o x i d a t i v e a d d i t i o n " of a m o l e c u l e to a c o m p l e x of a m e t a l i o n or a t o m i n a l o w o x i d a t i o n state. E x a m p l e s of this t y p e of r e a c t i o n are shown below. (1)
A d d i t i o n of m o l e c u l a r h y d r o g e n , H
[Rh(Me PCH CH PMe ) ]Cl 2
(2)
2
2
2
e.g.,
2
•aV[RhH (Me PCH CH PMe ) ]Cl
2
2
A d d i t i o n of h y d r o g e n h a l i d e ,
2
2
2
2
2
(12)
e.g.,
HC1 [Rh(Ph PCH CH PPh ) ]Cl 2
(3)
2
2
2
• a s - [ R h H C l ( P h P C H C H P P h ) ] C l (38)
2
2
A d d i t i o n of other h y d r i d e s ,
2
2
2
2
e.g.,
H S [Pt(Ph P) ] — U 3
* m n s - [ P t H ( S H ) ( P h P ) ] (35)
3
3
2
A p a r t i c u l a r l y i n t e r e s t i n g t y p e of 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 is the " h y d r o g e n a b s t r a c t i o n " r e a c t i o n , some examples of w h i c h are g i v e n b e l o w . (1) [ R u ( M e P C H C H P M e ) ] [ R u H ( C H P ( M e ) C H C H P M e ) 2
2
2
2
2
2
(Me PCH CH PMe )] 2
2
2
2
2
2
2
(U)
heat (2) [ I r C l ( P h P ) ] 3
• [ I r H C l ( o - C H P P h ) ( P h P ) ] (7)
3
6
(3) i m n s - [ P t C l ( E t P ) ] + L i C B 2
3
2
1 0
H
1 0
4
2
3
2
C M e -> [ P t ( C B H C M e ) 1 0
1 0
( C H C H P E t ) ( E t P ) ] (8) 2
2
2
3
T h e s e reactions are of p a r t i c u l a r interest as t h e y c o u l d p r o v i d e the basis f o r a n u m b e r of c a t a l y t i c processes i n v o l v i n g s a t u r a t e d h y d r o carbons. T h e p r o p e r t i e s of h y d r i d o complexes v a r y w i d e l y , r a n g i n g f r o m those w h i c h c a n b e d e t e c t e d o n l y s p e c t r o s c o p i c a l l y to those that s h o w r e m a r k a b l e g e n e r a l c h e m i c a l s t a b i l i t y . S t a b i l i t y g e n e r a l l y tends to b e at a m a x i m u m i n c o m p o u n d s of the t h i r d t r a n s i t i o n series. F u r t h e r m o r e , f o r a g i v e n m e t a l a n d t y p e of c o m p l e x , s t a b i l i t y appears to b e at its h i g h e s t w h e n
In Platinum Group Metals and Compounds; Rao, U.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
6.
Hydrido
V E N A N Z I
69
Complexes
p h o s p h i n e l i g a n d s are present i n the m o l e c u l e .
Naturally, it w o u l d
be
f o o l h a r d y to take the a b o v e k i n d of s t a b i l i t y as a g u i d e to t h e r m o d y n a m i c s t a b i l i t y of the m e t a l - h y d r o g e n b o n d , as one w o u l d d r a w a c o r r e l a t i o n b e t w e e n a s t a b i l i t y w h i c h is a c o m p o s i t e of a n u m b e r of t h e r m o d y n a m i c a n d k i n e t i c factors a n d a r e l a t i v e l y s m a l l energy t e r m s u c h as the M - H b o n d energy. I t is regrettable that t h e r m o d y n a m i c d a t a o n h y d r i d o c o m plexes are t o t a l l y l a c k i n g b u t t h e y are almost i m p o s s i b l e to o b t a i n . T h e most c o m m o n l y
quoted physical data on hydrido
are the v i b r a t i o n a l spectra a n d p r o t o n m a g n e t i c resonance
complexes parameters.
H y d r i d o complexes e x h i b i t M - H s t r e t c h i n g v i b r a t i o n s i n the r e g i o n 1 7 0 0 Downloaded by GEORGETOWN UNIV on August 26, 2015 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0098.ch006
2250 c m
- 1
a n d M - H b e n d i n g m o d e s i n the r e g i o n 6 6 0 - 8 5 0
cm . - 1
The
absence of a b s o r p t i o n i n the a b o v e regions, h o w e v e r , is not to b e t a k e n as a n i n d i c a t i o n t h a t the m o l e c u l e u n d e r e x a m i n a t i o n does not c o n t a i n M - H bonds.
S e v e r a l h y d r i d o complexes are k n o w n w h e r e M - H b o n d s
w e r e not detectable b y i n f r a r e d spectroscopy
(28, 34, 37).
Proton mag
n e t i c resonance, o n the other h a n d , is a m o r e g e n e r a l l y u s e f u l t e c h n i q u e . P r o t o n resonances i n h y d r i d o complexes h a v e v e r y c h a r a c t e r i s t i c τ-values r a n g i n g f r o m 11 to 42 p p m r e l a t i v e to T M S . T h e l i m i t a t i o n here is i n t r o d u c e d b y s o l u b i l i t y a n d b y fast exchange p h e n o m e n a a l t h o u g h , i n m a n y cases, the f o r m e r difficulty c a n be o v e r c o m e b y s p e c t r u m a c c u m u l a t i o n a n d the latter b y l o w e r i n g the s a m p l e t e m p e r a t u r e . H y d r i d o l i g a n d s e x h i b i t a v e r y strong trans effect w h i c h appears to operate m a i n l y b y a l a b i l i z a t i o n of the b o n d i n trans p o s i t i o n to the M - H b o n d . T h i s l a b i l i z a t i o n g e n e r a l l y is associated w i t h a l e n g t h e n i n g of the b o n d w h i c h has b e e n l a b i l i z e d , a n effect o p e r a t i n g m a i n l y b y trans i n fluence
(36)
c a u s e d b y a selective r e h y b r i d i z a t i o n of the m e t a l σ orbitals
l e a d i n g to the M - H b o n d h a v i n g a large d a n d s c o m p o n e n t .
S u c h effects
are most e v i d e n t i n square p l a n a r complexes, b u t t h e y also h a v e observed i n octahedral
been
complexes.
T h e g e n e r a l r e a c t i v i t y of h y d r i d o complexes is h i g h , a n d i t is r e l a t e d m a i n l y to the c o o r d i n a t i o n n u m b e r of the c e n t r a l m e t a l a t o m . four- a n d
five-coordinate
six-coordinate species.
Thus,
complexes u s u a l l y are m o r e r e a c t i v e t h a n r e l a t e d T h e latter, h o w e v e r , o r d i n a r i l y are m o r e r e a c t i v e
t h a n the c o r r e s p o n d i n g complexes
w h i c h d o not c o n t a i n M - H b o n d s .
S u c h r e a c t i v i t y m a y b e i n d u c e d either b y o p e r a t i o n of the trans effect or, w h e r e possible, b y the r e d u c t i v e e l i m i n a t i o n of a m o l e c u l e of h y d r o g e n h a l i d e w i t h or w i t h o u t assistance of a base. T h e c h e m i c a l b e h a v i o r of M - H b o n d s varies f r o m t h a t of a n a c t i v e h y d r i d e to that of a strong a c i d . T h i s w i d e v a r i a t i o n i n p r o p e r t i e s , h o w ever, m a y be a c h i e v e d w i t h a r e l a t i v e l y s m a l l v a r i a t i o n of c h a r g e d i s t r i b u t i o n (4).
A t t e m p t s to correlate c h e m i c a l shifts a n d c o u p l i n g constants
w i t h b o n d distances a n d electron d i s t r i b u t i o n i n M - H b o n d s h a v e l e d
In Platinum Group Metals and Compounds; Rao, U.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
70
P L A T I N U M
G R O U P
M E T A L S
A N D
C O M P O U N D S
t o t h e c o n c l u s i o n that s u c h correlations m a y b e v a l i d o n l y w h e n v e r y s m a l l changes are m a d e to the l i g a n d s
(3).
T h e f e v e r i s h interest i n h y d r i d o complexes
has, as its m a i n cause,
the t r e m e n d o u s p o t e n t i a l of these reactions i n c a t a l y t i c systems.
In a
r e l a t i v e l y short s p a n of t i m e , h y d r i d o complexes h a v e b e e n f o u n d to p l a y a r o l e i n a significant n u m b e r of c a t a l y t i c processes ( 6 , 9)—e.g., o l i g o m e r i z a t i o n of olefins ( r h o d i u m ) , d e c a r b o x y l a t i o n reactions
(rhodium),
a n d hydrogénation reactions ( r u t h e n i u m , o s m i u m , r h o d i u m , i r i d i u m , a n d p l a t i n u m ) . D i s c u s s i o n of these a p p l i c a t i o n s w o u l d go b e y o n d the scope of the present treatment. Downloaded by GEORGETOWN UNIV on August 26, 2015 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0098.ch006
T h e r e m a i n d e r of this r e v i e w is d e v o t e d to g i v i n g a closer l o o k at the h y d r i d o complexes of i r i d i u m a n d t h e i r reactions. I r i d i u m gives the most extensive a n d versatile range of h y d r i d o complexes of a n y p l a t i n u m metal (29).
T h e m a i n types of c o m p o u n d s
k n o w n a n d some of t h e i r
reactions are s h o w n i n T a b l e I I . T o this, one m u s t a d d the
hydrogen
a b s t r a c t i o n r e a c t i o n m e n t i o n e d earlier. T h e w i d e r a n g e of h y d r i d o complexes f o r m e d b y i r i d i u m is a c l e a r i n d i c a t i o n t h a t this m e t a l has a p a r t i c u l a r l y strong t e n d e n c y to f o r m I r - H bonds.
T h e reason for this affinity is n o t a p p a r e n t as yet, b u t this c o n -
c l u s i o n is r e i n f o r c e d constantly b y n e w d a t a . T h u s , one c a n isolate stable seven-coordinate
complexes
of i r i d i u m ( I I I ) .
T h e quadridentate ligand
( o - P h P · C H ) P , Q P , reacts w i t h [ I r H X ( P h P ) ] ( X = 2
6
4
3
2
3
CI, Br, and
8
I ) i n r e f l u x i n g c h l o r o b e n z e n e to give [ I r H X ( P h P ) ( Q P ) ] X , I , w h i c h is 3
a seven-coordinate h y d r i d o c o m p l e x of i r i d i u m ( I I I ) .
(21)
The infrared
s p e c t r u m does n o t s h o w a n I r - H s t r e t c h i n g v i b r a t i o n , b u t t h e presence of a h y d r i d o l i g a n d is c l e a r l y v i s i b l e i n the p r o t o n m a g n e t i c
resonance
spectrum. T h e f o r m a t i o n of these c o m p o u n d s is p r e c e d e d b y that of a species of c o m p o s i t i o n
[ I r ( P h P ) ( Q P ) ] X , I I , w h i c h reacts w i t h N a [ B P h ] 3
to
4
g i v e the same c o m p l e x i r r e s p e c t i v e of the n a t u r e of the a n i o n X i n the s t a r t i n g m a t e r i a l , [ I r H X ( P h P ) ] . T h e i r N M R s p e c t r a (22) 2
3
[IrHX(Ph P)(QP)]X. 3
indicate
3
the presence of I r - H b o n d s a n d t h e i r r e a c t i o n w i t h H X gives
compounds
C o m p l e x e s I I are, therefore, t e n t a t i v e l y f o r m u -
l a t e d as [ I r H ( o - C H P P h ) ( Q P ) ] X . 6
4
2
I n a n a t t e m p t to g a i n a better u n d e r s t a n d i n g of the factors c a u s i n g t h e f o r m a t i o n of the a b o v e seven-coordinate h y d r i d o complexes, the r e a c t i o n of [ I r H C l ( P h P ) ] w i t h r e f l u x i n g c h l o r o b e n z e n e 2
3
3
was e x a m i n e d .
P r e l i m i n a r y experiments suggest that the p r i m a r y step is t h e e v o l u t i o n of
hydrogen
chloride
and
formation
of
the
unstable
intermediate
[ I r C l ( P h P ) ] , w h i c h reacts f u r t h e r to g i v e a n u m b e r of p r o d u c t s . 3
3
has the c o m p o s i t i o n I r C l ( P h P )
3
III,
compound
3
of
Bennett
and
Milners
One
a n d is l i k e l y to b e a n i s o m e r i c f o r m , (7),
[IrHCl(o-C H PPh )6
4
2
( P h P ) ] . T h e others, s o m e p u r e l y o r g a n i c a n d some i r i d i u m - c o n t a i n i n g , 3
2
In Platinum Group Metals and Compounds; Rao, U.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
6.
Hydrido
V E N A N Z I
Table II.
Y
Some Iridium H y d r i d o Complexes and T h e i r Reactions
2
3
(44)
HX
->
[IrH L ](C10
