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Metal Carbonyls
CARBON ATOM OF CO HAS LONE
PAIR OF ELECTRONS WHICH CAN BE
DONATED TO TRANSITION METAL
TO FORM COMPLEX COMPOUND
CALLED METAL CARBONYLS.
Metal carbonyls are
complex compound
of transition metals
with CO.
 The number of CO molecules which get
chemically bonded to a metal through
coordination bonds vary with nature of
the metal.
 Example: Number of CO molecules
bonded o nickel carbonyl, Ni(CO) 4 is 4,
while in chromium carbonyl Cr(CO)6 it
is 6
 Carbonyls
are formed readily with
transition metals since they have vacant
d-orbitals which can accept the donated
electron pairs.
 The number of coordination bonds that
can be formed in this manner depend
largely on the number of vacant
orbitals.
 Generally the number of ligands attached to such
that the metal atom appears to possess the same
number of electrons as that noble gas.
 For example: Ni (atomic no.= 28)
 Metal-ligand bond formed by donating one lone pair
from CO
Ni, so four CO donate 8 electrons &
all arising electrons on the central metal atom is 36.
 36 no. of Kr. This number is called Effective
Atomic Number (EAN).
EAN = Atomic no.( Z) of metal atom- oxidation state + 2 x C.N.
[ Fe ( CN) 6 ]
4-
= 26-2+ 2 x 6 = 36
Structure of Ni ( CO) 4
Since the energy of 3d and 4s
electrons are not so very
much
different,
the
4s
electrons can easily go to fill
the 3d orbitals, vacating
thereby the 4s orbitals
 Some
other carbonyl compounds are Fe(CO)5,
V(CO)6, Cr(CO)6, Mo(CO)6. These are mononuclear
carbonyls since there is only one metal in them.
 A large number of polynuclear carbonyls which
contain more than one metal atom are also known.
 Common examples are Co2( CO)8, Mn(CO)10,
Fe2(CO)9, Fe3(CO)12, etc
PROPERTIES OF METAL CARBONYLS
 Solid
at room temperature
and pressure.
 Mononuclear carbonyls are
volatile as well as toxic.
 Colorless or light in color
BONDING IN METAL CARBONYLS
Molecular Orbital Theory
In metal carbonyls initial overlap takes place between the filled
bonding (sigma) orbital of CO with a vacant d-orbital on the
metal atom resulting n sigma bond which is actually coordinate
bond (M
CO).
Thus carbon monoxide CO acts as a ligand with C atom as the
donor atom.
 The next overlap is between filled d-orbital on the
metal atom with anti bonding orbital of carbon
monoxide( back bonding) resulting in additional pi
bond.
 Thus M to C bond in metal carbonyl consist of weak
sigma bond and M
C and M
C pi bond
which result due to back bonding.
 The back bonding between metal and C create a
synergic effect which stabilize the M
C bond