Ch 6: Carbonyl Complexes and Hydrido Complexes

Ch 6: Carbonyl Complexes and
Hydrido Complexes
The metal centers of metal carbonyl compounds (M-CO) are more reactive then
the CO ligands themselves. The metal is especially reactive towards oxidizing and
reducing agents but a little less so towards electrophiles and nucleophiles.
The M-CO anion is a reducing metal and thus reduces back donation. There are
not many cation metal complexes.
Analyses of these clusters were initially very difficult because of the difficulty of
the method employed, X-ray diffraction. Not much better is NMR analysis, which
provides results that are much too vague. X-ray crystallography, in which crystals form
via the layering solvent method, is a useful approach.
W. Hieber discovered that if a reaction were run with a metal carbonyl and
NaOH, this would result in an oxidation of the CO and a reduction of the metal. Thus,
making Transition metal hydrides
In essence:
M-CO  MH2
This led him to produce the first transition metal hydrido complexes, H-Co(CO)4
and H2Fe(CO)4:
NOTE: Some MCO compounds are quite basic and will take up a proton from water, in
this case it has not yet been determined what the product is.
How acidic or how basic are these transition metal hydrido compounds?
mp (C) Dec.
1 pKa = 4.7, 2
-10 ppm
-11 ppm
-7.5 ppm
pKa = 14 H2O
It is important to note that –10
and –11 ppm are outside the usual range
for the 1H-NMR spectrum, thus, when
running the test the range must be increased.
Also note, that with NMR spectrums,
TMS is a good standard reference because
nothing is to the right of it.
With compound #1,
as with many of the
compounds, it is
evident by the pKa
that they can be very
A second method of obtaining carbonyl hydride complexes did so via a formylcomplex intermediate. This method involves a reaction between an M-CO with a hydride
donor such that a decarboxylation occurs:
Formyl-complexes are extremely unstable and were never isolated however, in
1975 C. Casey characterized the complexes.
Metal hydrido complexes are important in terms of their role in catalytic
processes. For example, in hydroformylation reactions, a metal-hydrido catalyst
HCo(CO)4, is used. (This catalyst is formed in situ from Co2(CO)8 and H2).
Reactions in which metal-metal bonds are cleaved are usually thermodynamically
favored. However, high temperatures and pressures are required:]
M—M + H2  M—H + M—H
More indirect methods can be applied:
M—M + I2  M—I + M—I + H-  M—H + M—H
(C )
M—M + 2e-  M- + M- + H+  M—H + M—H
Note: reduction of the halo carbonyl complexes are usually performed with NaBH 4, LiBH4, or BH3
Not all TM hydrido compounds are unstable, K2ReH9 is stable even at ~200C.
The compound is made from the reduction of potassium perrhenate with metallic
potassium in wet ethylenediamine. The product was first published as K[Re(H2O)4].
From this one can conclude the Golden Rule of Chemistry: One Method is No Method
It took five methods to deduce the correct structure
the reduction worked and it was concluded and published that
oxidation state was +1.
- This was incorrect as Re is a heavy metal and below the
favored coordination number 5, is very rare.
- It was a wrong publication
- IR method used, found coordination number of Re to be 8, this was
wrong as well
- elemental analysis produced wrong results as well
- x-ray diffraction, produced a bad structure, the crystallographer was
experienced only in this and therefore believed this was the structure
- Neutron diffraction, found the correct structure
In 1976, Ashworth and Singleton postulated the complexation of dihydrogen at
transition metal centers. The bonding is as such
The H—H have 2e- that are donated to the M empty orbital, the 
orbital of the ligand.
Nature’s way of dihydrogen compounds are in enzymes.
The first paper was published in 1984, in which Kubas synthesized
a series of Mo(o)-complexes that reversibly add H2. Found that the
compound obtained 18VE by picking the H2 up.
Evidence for the 2-coordination of H2 came from single crystal Xray crystallography of the complex (Cy3P)2W(CO)3(H-D). With NMR, if
the molecule is symmetric, it will not pick up coupling. This is the case
with H-H, however, a complex with H-D will produce coupling ~33.5Hz.
An IR band at 2360 cm-1 also indicates the H-D bond. (Particularly since
there are not many peaks in this region to confuse the cause of this one)
Dihydrogen complexes require both -donation and -back
donation thus electron rich metal centers provide this electron density.
The dihydrogen complexes can be acidic with pKa values of 0-10.
Most complexes have the transition metal in the d6 configuration.
It is still possible to add to the square planar metals in configuration d8
but these reactions lead to classical complexes. These reactions are the
elementary step of homogeneously catalyzed hydrogenations.
The H2 molecule seems to add in a way that eliminated trans
For the Vaska complex, the addition of H2 can be along the Cl
axis or the PPh3 axis. However, only one isomer is observed. The one
resulting from addition of the Cl-Ir-CO axis.
(These results are not normally the case)
Tremendous diversity of these complexes arises from the stability of the
M-M bonds (particularly Fe, Co and Ni) and the compatibility with many other
CO-ligands. For example, we can have mono-, di-, tri-, and polynuclear
complexes (obtained by heating MCO’s until CO evolution starts).
With Triiron-dodecacarbonyl, it was estimated that the compound was
symmetric but it is not.
One Fe has 4 terminal ligands
Another Fe has only 3 terminal ligands but there is a bridging CO
ligand, which shortens the Fe-Fe bond distance
(I): X-Ray Crystallography
The M-C and O-C bond distances in carbonyl complexes do not vary
much but can be determined by single X-ray crystallography. The M-C-O
fragment is rarely linear, mostly bent.
X-ray crystallography provides information about the number of ligands
around the metal and the presence of unusual coordination modes
MCO-s are typically reactive, especially if UV light is shone on them. The
light causes a charge transfer, the transport of e—density from M to the CO ligand,
which goes into the anti-bonding levels.
2 Re(CO)5Br
Here lone pairs on Bromine allows it to dimerize forming a strong lewis acid
Here the M=M double bond allows the two to share electrons and gain a
valence electron count of 18.
Reacting Fe(CO)5 with Na, which forms Na2Fe(CO)4, can obtain this
compound. This compound, which is the dianion of Fe(CO)4, is not very stable
but can be stabilized as a complex with dioxane.
The procedure allows you to replace an organic halide with a formyl group
in an extremely selective manner.
Two types of reactions can proceed:
In this reaction, CO inserts itself into the C-Halogen bond and is then
hydrogenated with an acid. As can be seen, Br is more reactive then Cl, however
both are much better then I.
Another reaction can be performed to produce a ketone. Instead of
protonating with an acid, use another organic compound
This reaction is a multi component, 3, condensation and produces a ring. It
is one of the only good multi-component reactions because this can make many
useful compounds. For example, a family run business in Switzerland, Butaco,
makes fragrances through this three-component reaction.
The reaction takes CO + alkene + Alkyne and produces cyclopentanones.
Chapter 7: Alkyl Complexes,
Carbene Complexes, and Carbyne
Organic compounds of the transition elements differ from those of the main group
elements for many reasons, most obvious is the difference in thermal stability. The low
thermal stability of transition metal alkyl complexes is due to the existence of a number
of low energy decomposition pathways that are specific to transition metals.
For example
The instability of this
compound can be accounted for by
its highly electron deficient character
(8 VE instead of 18). Decomposing
at -70C.
How can these compounds be stabilized? Since they decompose through a
certain mechanism. In this case the reaction proceeds through a -elimination.
M-H + ethylene.
Thus, a good method would be the blockage of this mechanism by making
sure the alkyl C does not have a -hydrogen . Wilkinson set out to do this by
building compounds with more bulk that are higher in stability.
the substituents, CH2SiMe2,
increase the VE count (to 14 VE)
and therefore stabilize the Ti-Me
bond. This compound sublimes at
You can tell if there is a -hydrogen because of the characteristics of this
type of interaction, Agostic Interaction. This was discovered by M. L.
H.Green, and is common with early transition metals, or electron deficient metals;
Ti, V, Zn, Nb etc.
Has unusual 1H shifts because
Of strange shielding
Also has low (C-H) which is
~3000cm-1 usually but drops in this case ~2600-2900cm-1.
Other effective stabilizers are covalent ligands with lone pairs.
Stability is not restricted to singly bound ligands, doubly bound oxygen
and nitrogen and triply bound nitrogen also greatly increase the stability of the
Monomolecular pathways do not require an encounter of two different
molecules, like bimolecular pathways. As such, they are difficult to block by the
introduction of bulky auxiliary ligands
As demonstrated by J. Okuda, it is likely that bimolecular pathways are
important, particularly for complexes of type CpTiMe3.
Mp = +58C, dec > 120C
Sublimes w/o decomposition
A number of factors are involved with stability. These are
Stability of Alkyl Complexes
5d > 4d > > 3d
late TM > early TM
M in high CN > M in low CN
18 VE > 16 VE > 17, 19 VE
cyclic > open chain
Os , Re, Pt > others
Combining these rules increases the stability of complexes.
For example, G. Whitesides investigated the stable cyclic platinum
complex formed in this reaction:
In this reaction alkynes react with the Schwartz Reagent Cp2Zr(Cl)H, and
produces vinyl zirconium, these compounds favor 1,4-addition to Michael
Note: Under certain conditions the Zr can migrate.
You can basically use these reagents as vinyl anions “vinyl -”
R – Li
Can also perform hydrometalations:
Or Decarbonylations:
Or Decarboxylations:
Hg2+ (CF3COO-)2
 / -CO2
Note: this is limited to Cd, Bi, (As), (Sb), Pt and Au
The first compounds with a double bond between a transition metal and a
carbon, in the Carbene Complex, first isolated by E. O. Fischer and Maasbol in
Note, that the product has the same valence electron count because the
substituent :C(OCH3)2, donates 2 e- just like the CO does.
A number of interdependent characterizations of the general Fischer
Carbene Complexes are:
L: CO, isonitril, Cp, PR3 and other ligands that act as –M groups
M: Cr > Mn > Fe triads with oxidation states < 2
R: At least one donor group is required (-OR, SR, NR2)
Reactvity: Dominated by electrophilic M=C carbon
Note: that reactivity has to do with the mesomeric structure associated
with the double bond, which inhibits rotation:
A new carbene complex was discovered by R. Schrock in 1974 in an
attempt to obtain pentakis(neopentyl)tantalum(V), which was discovered to be the
intermediate. This formed an “Alkylidene”, where the substituent is bound by a
double bond:
By: Marsha Youash
Prof. Denk
March 16, 2000