Metal Ligand bonds

1. Assignment 1 Metal-Ligand Bonds
1. a) Draw and label the general molecular orbital diagram for a M-L6
complex ion, [Co(NH3)6]3+
. Identify all the orbitals used for bonding,
nonbonding and antibonding.
Solution:
Figure 1:MO diagram for M-L6 ([Co(NH3)6]3+
complex
i. Bonding Orbitals: a1g, eg(dx2
-y2
, dz2
), t1u
ii. Non-bonding Orbitals: T2g (dxy, dyz,dxz)
iii. Anti-bonding orbitals: eg
*
, A1g
*
, T1u
*
2. Discuss the following M-L bonding under subtopics: Types, Synthesis
and reactions.
(i) Metal-Carbonyls.
Solution:
The carbonyl ligand (CO) distinguishes itself from other ligands in many
respects. For example, unlike the alkyl ligands, the carbonyl (CO) ligand
is unsaturated thus allowing not only the ligand to σ−donate but also to

2. accept electrons in its π* orbital from d metal orbitals and thereby
making the CO ligand π−acidic. The other difference lies in the fact that
CO is a soft ligand compared to the other common σ−and π−basic
ligands like H2O or the alkoxides (RO−), which are considered as hard
ligands. Being π−acidic in nature, CO is a strong field ligand that
achieves greater d−orbital splitting through the metal to ligand π−back
donation. A metal−CO bonding interaction thus comprises of a CO to
metal σ−donation and a metal to CO π−back donation (Figure ).
the extent of the metal to CO π−back donation is almost equal to or even
greater than the extent of the CO to metal σ−donation in metal carbonyl
complexes. This observation is in agreement with the fact that low
valent−transition metal centers tend to form metal carbonyl complexes.
Figure 2: Electronic structure of CO and carbonyl complexes. Shading represents occupied orbitals (a) and (b)
building up CO from C and O, each atom having two p orbitals and two sp hybrids. In (a), the dots represent the
electrons occupying each orbital in the C and O atoms. In (b), only one of the two mutually perpendicular sets of
π orbitals is shown. (c) An MO diagram showing a π bond of CO. (d) Valence bond representations of CO and the
MCO fragment. (e) An MO picture of the MCO fragment. Again, only one of the two mutually perpendicular sets
of π orbitals is shown
Preparations of CO Complexes
The common methods of the preparation of the metal carbonyl compounds are;
i. Directly using CO
The main requirement of this method is that the metal center must be in a reduced
low oxidation state in order to facilitate CO binding to the metal center through
metal to ligand π−back donation.

3. ii. Using CO and a reducing agent
This method is commonly called reductive carbonylation and is mainly used for the
compounds having higher oxidation state metal centers. The reducing agent first
reduces the metal center to a lower oxidation state prior to the binding of CO to
form the metal carbonyl compounds
iii. From carbonyl compounds
This method involves abstraction of CO from organic compounds like the alcohols, aldehydes
and CO2.
Reactions of Metal-Carbonyls
typical reactions are shown in Eqs. 4.8–4.13. All of these depend on the polarization of the CO
on binding, and so change in importance as the coligands and net charge change. For example,
types 1 and 3 are promoted by the electrophilicity of the CO carbon and type 2 by
nucleophilicity at CO oxygen.
i. Nucleophilic attack on carbon
These reactions give carbenes or carbenelike intermediates. The reaction of Eq. 4.10 is
particularly important because it is one of the rare ways in which the tightly bound CO can
be removed to generate an open site at the metal. In this way a ligand L , which would
normally not be sufficiently strongly binding to replace the CO, can now do so.’
ii. Electrophilic attack at oxygen
Protonation of this Re carbonyl occurs at the metal, as is most often the case, but
the bulkier acid, AlMe3, prefers to bind at the CO oxygen
iii.
Migratory insertion reaction

4. Bridging CO Groups
CO has a high tendency to bridge two metals (see the example below):
The electron count remains unchanged on going from 4.4 to 4.5. The 15e CpFe(CO)
fragment is completed in 4.4 by an M−M bond, counted as a 1e contributor to each metal,
and a terminal CO counting as 2e. In 4.5, on the other hand, we count 1e from each of the
two bridging CO (µ2-CO) groups and 1e from the M−M bond. The bridging CO is not entirely
ketonelike because an M−M bond seems almost always to accompany a CO bridge. The CO
stretching frequency in the IR spectrum falls to 1720–1850 cm−1 on bridging.
(ii) Metal-Hydrides
Solution:
Metal hydrides occupy an important place in transition metal organometallic chemistry
as the M−H bonds can undergo insertion reactions with a variety of unsaturated organic
substrates yielding numerous organometallic compounds with M−C bonds. Not only the
metal hydrides are needed as synthetic reagents for preparing the transition metal
organometallic compounds but they also are required for important hydride insertion
steps in many catalytic processes.
The metal hydride moieties are easily detectable in H NMR as they appear high field of
TMS in the region between 0 to 60 ppm, where no other resonances appear. The
hydride moieties usually couple with metal centers possessing nuclear spins. Similarly,
the hydride moieties also couple with the adjacent metal bound phosphine ligands, if at
all present in the complex, exhibiting characteristic cis (J = 15 − 30 Hz) and trans (J = 90 −
150 Hz) coupling constants. In the IR spectroscopy, the M−H frequencies appear
between (1500 − 2200) cm but their intensities are mostly weak. Crystallographic
detection of metal hydride moiety is difficult as hydrogen atoms in general are poor
scatterer of X−rays. Located adjacent to a metal atom in a M−H bond, the detection of
hydrogen atom thus becomes challenging and as a consequence the X−ray
crystallographic method systematically underestimates the M−H internuclear distance
by ~ 0.1 Å.
Synthesis
The main synthetic routes to hydrides are shown in Eqs. 3.27– 3.33:
a. Protonation Reaction
For this reaction to occur the metal center has to be basic and electron rich.

5. b. From Hydride donors
Generally for this method, a main group hydride is reacted with metal halide.
c. From H2:
This method involves oxidative addition of H and thus requires metal centers that
are capable of undergoing the oxidative addition step.
d. From a ligand
This method takes into account the β−elimination that occur in a variety of metal
bound ligand moieties, thereby yielding a M−H bond.
Reactions
Metal hydrides are reactive species kinetically and thus participate in a variety of
transformations. Hydride transfer and insertion are closely related; the former
implies that a hydridic hydride is attacking an electrophilic substrate like the ones
discussed below. (Eqs 3.34-3.37)
a. Deprotonation
The deprotonation reaction can be achieved by a hydride moiety resulting in the
formation of H gas as shown below
b. Hydride transfer and insertion:
In this reaction a hydride transfer from a metal center to formaldehyde resulting
in the formation of a metal bound methoxy moiety is observed as shown below
c. H atom transfer
An example of hydrogen atom transfer reaction is given below
Bridging Hydrides
The metal hydrides usually show two modes of binding, namely terminal and
bridging. In case of the bridging hydrides, the hydrogen atom can bridge
between two or even more metal centers and thus, the bridging hydrides often
display bent geometries.

6. σ−complexes
σ−complexes are rare compounds, in which the σ bonding electrons of a X−H
bond further participate in bonding with a metal center (X = H, Si, Sn, B, and P).
The σ complexes thus exhibit an askewed binding to a metal center with the
hydrogen atom, containing no lone pair, being more close to the metal center
and thereby resulting in a side−on structure. Many times if the metal center is
electron rich, then further back donation to the σ* orbital of the metal bound
X−H moiety may occur resulting in a complete cleavage of the X−H bond.
(iii) Metal-Halides
Solution:
(iv) Metal-Carbenes
Solutions
Carbenes are highly reactive hexavalent species that exist in two spin
states, i.e. (i) in a singlet form, in which two electrons are paired up and (ii) in a
triplet form, in which the two electrons remain unpaired. Of the two, the singlet
form is the more reactive one. The instability of carbene accounts for its unique
reactivity like that of the insertion reaction. The singlet carbene and the triplet
carbene bind differently to metals, with the singlet one yielding Fischer type
carbene complexes while the triplet one yielding Schrock type carbene
complexes.
Synthesis
Carbene complexes can be prepared by the following methods.
i. by the reaction with electrophiles
ii. by H−
/H+
abstraction reactions as shown below
iii. from low−valent metal complexes
3. The formation of metalacycles from alkenes and carbenes is the key
reaction in alkene metathesis. Explain the mechanism of this reaction.