2. Metal carbonyls
The metal-carbon bonds in metal carbonyls have both σ
and π characters. A σ bond is formed when the carbonyl
carbon donates a lone pair of electrons to the vacant
orbital of the metal. A π bond is formed by the donation
of a pair of electrons from the filled metal d orbital into
the vacant anti-bonding π orbital (also known as back
bonding of the carbonyl group). The σ bond strengthens
the π bond and vice-versa. Thus, a synergic effect is
created due to this metal-ligand bonding. This synergic
effect strengthens the bond between CO and the metal.
3. Classification of Metal Carbonyls:
1.Classification on the basis of ligands:
Metal carbonyls can be classified into two categories:
(i)Homoleptic carbonyl complexes: The complex in which the metal is bound
to only CO as ligands are known as homoleptic carbonyl complexes.for
example, Ni(CO)4, Cr(CO)6,Fe(CO)5,Co2(CO)8,Mn2(CO)10,Fe3(CO)12,
Ir4(CO)12 etc.
(ii)Heteroleptic carbonyl complexes: The complexes in which a metal is
bound to CO as well as other ligands such as PR3,PPh3,PF3,NO.RNC
etc.For example, Ni(CO)3PPh3,Mo(CO)3(PF3)3,Cr(CO)3(NO)2 etc.
4. 2.Classification on the basis of a number of metal atoms and the
structures of Metal Carbonyls:
(i)Mononuclear Metal Carbonyls: These carbonyls contain only one
metallic atom and these carbonyls do not contain any bridging CO
ligand.For example Ni(CO)4,Cr(CO)6,Fe(CO)5 etc.
(ii)Polynuclear Metal Carbonyls: Polynuclear carbonyls contain only
one or two metal atoms and these are classified as
(a)Homonuclear Metal Carbonyls: These contain metal atoms of only
one element.For example
Fe2(CO)9,Mn2(CO)10,Co2(CO)8,Fe3(CO)12,Co4(CO)12,Rh4(CO)12,I
r4(CO)12 etc.
.
5. (b)Heteronuclear Metal Carbonyls: These carbonyls contain
metals of different elements.For example,MnCo(CO)4
The polynuclear metal carbonyls are also classified as:
(a)Non-bridged Metal Carbonyls: These carbonyls contain
terminal CO ligand and M-M bonds.For example,Co2(CO)8
(In solution),Mn2(CO)10,Ir4(CO)12 etc.
(b)Bridged Metal Carbonyls: These carbonyls contain bridged
as well as terminal CO ligands and M-M bonds.For
example,Fe2(CO)9,Co2(CO)8 (in solid state),Fe3(CO)12 etc
6. Synthesis
The synthesis of metal carbonyls is subject of intense
organometallic research. Since the work of Mond and
then Hieber, many procedures have been developed for
the preparation of mononuclear metal carbonyls as
well as homo- and heterometallic carbonyl clusters.
7. Direct reaction of metal with carbon
monoxide
Nickel tetracarbonyl and Iron pentacarbonyl can be
prepared according to the following equations by reaction
of finely divided metal with carbon monoxide.
Ni + 4 CO → Ni(CO)4 (1 bar, 55 °C)
Fe + 5 CO → Fe(CO)5 (100 bar, 175 °C)
Nickel tetracarbonyl is formed with carbon monoxide.
already at 80 °C and atmospheric pressure, finely divided
iron reacts at temperatures between 150 and 200 °C and a
carbon monoxide pressure of 50–200 bar.[Other metal
carbonyls are prepared by less direct methods.
8. Reduction of metal salts and oxides
CrCl3 + Al + 6 CO → Cr(CO)6 + AlCl3
WCl6 + 6 CO + 2 Al(C2H5)3 → W(CO)6 + 2 AlCl3 + 3 C4H10
Tungsten , molybdenum , Manganese, and rhodium salts may
be reduced with lithium aluminium hydride.Vanadium
hexacarbonyl is prepared with sodium as a reducing agent
in chelating solvents such as diglyme
VCl3 + 4 Na + 6 CO + 2 diglyme → Na(diglyme)2[V(CO)6] +
3 NaCl
[V(CO)6]− + H+ → H[V(CO)6] → 1/2 H2 + V(CO)6
9. Photolysis and thermolysis
Photolysis or Thermolysis of mononuclear carbonyls generates di-
and polymetallic carbonyls such as Fe2(CO)9.
On further heating, the products decompose eventually into the
metal and carbon monoxide.
2 Fe(CO)5 → Fe2(CO)9 + COThe thermal decomposition of
triosmium dodecacarbonyl (Os3(CO)12) provides higher-nuclear
osmium carbonyl clusters such as Os4(CO)13, Os6(CO)18 up to
Os8(CO)23.
Mixed ligand carbonyls of Ru, Os, Rh and Ir are often generated by
abstraction of CO from solvents such as (DMF).
10. Salt metathesis
salts such as KCo(CO)4 with [Ru(CO)3Cl2]2 leads
selectively to mixed-metal carbonyls such as
RuCo2(CO)11
4 KCo(CO)4 + [Ru(CO)3Cl2]2 → 2 RuCo2(CO)11 +
4 KCl + 11 CO
11. Metal carbonyl cations and
carbonylates
The synthesis of ionic carbonyl complexes is possible by
oxidation or reduction of the neutral complexes. Anionic
metal carbonylates can be obtained for example by reduction
of dinuclear complexes with sodium. A familiar example is
the sodium salt of iron tetracarbonylate (Na2Fe(CO)4,
1.Fe(CO)5 + 2 Na → Na2[Fe(CO)4] + CO
2. 2 Fe(CO)5 + 2 Na → Na2[Fe2(CO)8] + 2 CO
13. M
C
O
M M
C
O
terminal bridging
2
M
M
M
C
O
bridging
3
2120-1850 cm-1
CO
1850-1700 cm-1 1730-1620 cm-1
Cr
OC
OC CO
CO
CO
CO Fe
Fe
Fe
OC
FeOC
CO
CO
Cp
Cp
Cp
Cp
1620 cm-
1
2018, 1826
cm-1
2000 cm-
1
Terminal versus bridging
carbonyls
14. Determine the total valence electrons (TVE) in the entire molecule (that is, the number of valence electrons of the
metal plus the number of electrons from each ligand and the charge); say, it is A.
Subtract this number from n × 18 where n is the number of metals in the complex, that is, (n × 18) – A; say, it is B.
(a) B divided by 2 gives the total number of M–M bonds in the complex.
(b) A divided by n gives the number of electrons per metal. If the number of electrons is 18, it indicates that there
is no M–M bond; if it is 17 electrons, it indicates that there is 1 M–M bond; if it is 16 electrons, it indicates
that there are 2 M–M bonds and so on.
How to determine the total number of metal -
metal bonds
Fe
Fe
Fe
Co
Co Co
Co
Molecule TVE
(A)
(18 × n) – A
(B)
Total M–M
bonds (B/2)
Bonds per metal Basic geometry of metal
atoms
Fe3(CO)12 48 54 – 48 = 6 6/2 = 3 48/3 = 16; 2
Co4(CO)12 60 72 – 60 = 12 12/2 = 6 60/4 = 15; 3
[η5-CpMo(CO)2]2 30 36 – 30 = 6 6/2 = 3 30/2 = 15; 3 Mo≡Mo
(4-C4H4)2Fe2(CO)3 30 36 – 30 = 6 6/2 = 3 30/2 = 15; 3 Fe≡Fe
Fe2(CO)9 34 36 – 34 = 2 2/2 = 1 34/2 = 16; 1 Fe–Fe
15. PHYSICAL PROPERTIES
State: Majority of the metallic carbonyls are liquids or
volatile solids.
Colour: Most of the mononuclear carbonyls are colourless to
pale yellow. V(CO)6 is a bluish-black solid. Polynuclear
carbonyls exhibit are dark in colour.
Solubility: Metal carbonyls are soluble in organic solvents
like glacial acetic acid, acetone, benzene, carbon tetrachloride
and ether.
16. Toxicity:
Due to low melting points and poor thermal stability, they show toxicity related to
the corresponding metal and carbon monoxide. Exposure to these compounds can
cause damage to lungs, liver, brain and kidneys. Nickel tetracarbonyl exhibits
strongest inhalation toxicity. These compounds are carcinogenic over long-term
exposure.
Magnetic Property: All the metal carbonyls other than vanadium
hexacarbonyl are diamagnetic. The metals with even atomic number form
mononuclear carbonyls. Thus, all the electrons in the metal atoms are paired. In
case of dinuclear metal carbonyls formed by metals with odd atomic number, the
unpaired electrons are utilized for the formation of metal-metal bonds.
17. Thermal Stability:
Most of the metal carbonyls melt or decompose at low temperatures.
Solid carbonyls sublime in vacuum but they undergo some degree of
degradation.
Thermodynamic Stability:
Metal carbonyls are thermodynamically unstable. They undergo aerial
oxidation with different rates. Co2(CO)8 and Fe2(CO)9 are oxidized
by air at room temperature while chromium and molybdenum
hexacarbonyls are oxidized in air when heated
18. REACTIONS OF METAL CARBONYLS
1. Substitution Reactions
Many substitution reactions occur between metal carbonyls and other
potential ligands.
For example,
Cr(CO)6 + 2 py → Cr(CO)4(py)2 + 2 CO
Ni(CO)4 + 4 PF3 → Ni(PF3)4 + 4 CO
Mo(CO)6 + 3 py → Mo(CO)3(py)3 + 3 CO
19. Substitution reactions of metal carbonyls frequently indicate differences in bonding
characteristics of ligands. In the case of Mn(CO)5Br, radiochemical tracer studies
have shown that only four CO groups undergo exchange with 14CO.
Mn(CO)5Br + 414CO Mn(14CO)4(CO)Br + 4 CO
The four CO molecules that undergo exchange reactions are those in the plane,
which are all trans to each other. This indicates that the CO trans to Br is held more
tightly because Br does not compete for π bonding electron density donated from
Mn. In the case of the other four CO groups, competition between the groups,
which are all good acceptors, causes the groups to be labilized
20. 2. Reactions with Halogens
Reactions of metal carbonyls with halogens lead to the
formation of carbonyl halide complexes by substitution
reactions or breaking metal-metal bonds.
The reaction
[Mn(CO)5]2 + Br2 → 2 Mn(CO)5Br
involves the rupture of the Mn–Mn bond, and one Br is
added to each Mn.
21. In the reaction
Fe(CO)5 + I2 → Fe(CO)4I2 + CO
one CO is replaced on the iron by two iodine atoms so that
the coordination number of the iron is increased to 6. The
formulas for these carbonyl halides obey the EAN rule.
The reaction of CO with some metal halides results in the
formation of metal carbonyl halides directly, as illustrated in
the following examples:
PtCl2 + 2 CO → Pt(CO)2Cl2
2 PdCl2 + 2 CO → [Pd(CO)Cl2]2
22. 3. Reactions with NO
The nitric oxide molecule has one unpaired electron residing in an antibonding
π * molecular orbital. When that electron is removed, the bond order increases
from 2.5 to 3, so in coordinating to metals, NO usually behaves as though it
donates three electrons. The result is formally the same as if one electron were
lost to the metal,
NO → NO + e
followed by coordination of NO+, which is isoelectronic with CO and CN.
Because NO+ is the nitrosyl ion, the products containing nitric oxide and
carbon monoxide are called carbonyl nitrosyls . The following reactions are
typical of those producing this type of compound:
Co2(CO)8 + 2 NO → 2 Co(CO)3NO + 2 CO
Fe2(CO)9 + 4 NO → 2 Fe(CO)2(NO)2 + 5 CO
[Mn(CO)5]2 + 2 NO → 2 Mn(CO)4NO + 2 CO
It is interesting to note that the products of these reactions obey the 18-electron rule
23.
24. The electrons are partially transferred from a d-orbital of the metal to anti-
bonding molecular orbitals of CO (and its analogues). ... Many ligands other than
CO are strong "backbonders". Nitric oxide is an even stronger π-acceptor than is
CO and νNO is a diagnostic tool in metal–nitrosyl chemistry.
25. 4. Disproportionation
A number of metal carbonyls undergo disproportionation reactions in the
presence of other coordinating ligands. For example, in the presence of amines,
Fe(CO)5 reacts as follows:
2Fe(CO)5 +6 Amine → [Fe(Amine)6]2+ [Fe(CO)4]2- + 6 CO
This reaction takes place because of the ease of formation of the carbonylate
ions and the favorable coordination of the Fe2+ produced. The reaction of
Co2(CO)8 with NH3 is similar.
Co2(CO)8 + 6 NH3 → [Co(NH3)6][Co(CO)4]2
Formally, in each of these cases the disproportionation produces a positive
metal ion and a metal ion in a negative oxidation state. The carbonyl ligands
will be bound to the softer metal species, the anion; the nitrogen donor ligands
(hard Lewis bases) will be bound to the harder metal species, the cation.
26. These disproportionation reactions are quite useful in the preparation of a
variety of carbonylate complexes.
For example, the [Ni2(CO)6]2 ion can be prepared by the reaction
3 Ni(CO)4 + 3 phen → [Ni(phen)3][Ni2(CO)6] + 6 CO
The range of coordinating agents that will cause disproportionation is rather
wide and includes compounds such as isocyanides, RNC:
Co2(CO)8 + 5 RNC → [Co(CNR)5][Co(CO)4] + 4 CO
27. Metal carbonyl hydride or Carbonylate
Anions
Several carbonylate anions such as Co(CO)4
– , Mn(CO)5
– , V(CO)6
–, and
[Fe(CO)4]2– obey the EAN rule.
One type of synthesis of these ions is that of reacting the metal carbonyl
with a reagent that loses electrons readily, a strong reducing agent. Active
metals are strong reducing agents, so the reactions of metal carbonyls with
alkali metals should produce carbonylate ions.
The reaction of Co2(CO)8 with Na carried out in liquid ammonia at 75°C is
one such reaction.
Co2(CO)8 + 2 Na → 2 Na[Co(CO)4]
Similarly,
Mn2(CO)10 + 2 Li → 2 Li[Mn(CO)5]
28. Although Co(CO)4 and Mn(CO)5 do not obey the 18-electron rule, the
anions Co(CO)4
– and Mn(CO)5
– do
A second type of reaction leading to the formation of carbonylate anions is
the reaction of metal carbonyls with strong bases.
For example,
Fe(CO)5 + 3 NaOH → Na[HFe(CO)4] + Na2CO3 + H2O
Cr(CO)6 + 3 KOH → K[HCr(CO)5] + K2CO3 + H2O
With Fe2(CO)9 , the reaction is
Fe2(CO)9 + 4 OH– → Fe2(CO)8
2– + CO3
2– + 2H2O
29. Properties
The neutral metal carbonyl hydrides are often volatile and can
be quite acidic. The hydrogen atom is directly bounded to the
metal. The metal-hydrogen bond length is for cobalt 114 pm,
the metal-carbon bond length is for axial ligands 176 and
182 for the equatorial ligands.
