Engineering Chemistry S Datta

146 ENGINEERING CHEMISTRY
To illustrate, let us cite the following examples:
Compounds Central metal Electronic confi-No. of electrons Total no. of
atom with oxida-guration of the donated by electrons in
tion number central metal of ligands (n – 1) d, ns
(n–1) d subshell
and np levels
(i) Ni (CO) 4
Ni(0) 3d 10 2 × 4 (four CO 10 + 8 = 18
groups) = 8 electron
valence shell
configuration
(ii) Fe(CO) 5
Fe(0) 3d 8 2 × 5 (five CO 8 + 10 = 18
groups) = 10 electron
valence shell
configuration
The attainment of 18 electrons in the valence shell of the metal atom or satisfying 18-
electron rule is one of the useful criteria of elucidating the structure of metal carbonyls.
Structure and bonding in organometallic complexes, the 16-and 18-electrons rule:
The role of metals in catalytic cycles during some chemical reactions:
• Hydrogenation using (Ph 3
P) 3
RhCl
The hydrogenation of unsaturated organic compounds i.e., mostly alkenes is an important
industrial reaction. Almost all large scale hydrogenation processes are carried out using Raney
Ni as catalyst, which acts heterogeneously. Homogeneous systems are used for pharmaceutical
industries. Organometallic chemists made the process very popular. The hydrogenations of
alkenes and alkynes have been studied extensively. (Ph 3
P) 3
RhCl, which is generally known as
Willkinson’s catalyst, acts as a homogeneous catalyst for the hydrogenation of alkenes and
alkynes. The catalyst is not able to reduce other organic functional groups, though it is a very
reactive compound. It is dissociated (only 5%) into 14 electron species in pure solvents,thus:
(Ph 3
P) 3
RhCl (Ph 3
P) 2
RhCl + PPh 3
The species (Ph 3
P) 2
RhCl is of low coordinating power.
The action of the Willkinson’s catalyst is represented by the following cycle. In the cycle
there are four co-ordination compounds (II – V). The 14-electron species (II) is formed by
dissociation of phosphene ligand from (I). The structure (II) is seen to possess a vacant
coordination site shown by a square. This is because stable Rh(I) complexes are generally four
coordinated (16-electron species).
The 14-electron species (II) which takes two hydrogen atoms to give (III) by oxidative
addition (III) is still unsaturated and has a vacant site, so it can readily accept π electrons from
the alkene to give (IV). The stage IV is very very significant in the overall hydrogenation
process as in this stage, the substrates alkene, hydrogen are now bonded to the same metal
atom. Next step is the rearrangement of the coordination site to give V. (V) undergoes reductive
elimination to release the alkane and regenerate the 14-electron species.