Introduction - Organic Synthesis Using Transition Metals - page 11

shell with eight electrons and obtaining an inert-gas configuration. As they have to fill up an s or-
bital and three p orbitals, this means acquiring eight electrons, including the electrons that they already
possess.
Transition metals have to fill an s orbital, three p orbitals and five d orbitals. This requires eighteen electrons.
This is the eighteen-electron rule. These electrons must either belong to the metal atom already or must be
supplied by the ligand. We must also adjust for the charge.
There are two methods for adding up electrons, both are based on counting the electrons contributed to the
complex from the metal and the ligands. The methods have been referred to as the “covalent” and “ionic”
methods as they differ in the notional origin of the electrons. 31 It has to be clearly understood that this is the
notional origin, not the actual origin. A hydride ligand is assigned as bringing 1 or 2 electrons to the complex
respectively, whether its actual origin was from LiAlH 4 ,H 2 or HCl. The same answer is obtained whichever
method is used. The important thing is to not get the two methods mixed up! Examples of both methods are
given in Figures 1.15-1.18.

1.3.1 Method 1: Covalent

Electrons from the metal: This is equal to its group number. Just count from the far left-hand column
(group 1) of the periodic table (Table 1.2).
Electrons from the ligands: this depends, naturally on the ligands. For hydrocarbon ligands, the number
is equal to the hapto number. Single-bonded ligands (hydride, halide etc) count as 1 (although a bridging
halide counts as 2 - a lone-pair donor), while carbenes and carbynes count as 2 and 3, respectively. Lone-pair
donors, such as phosphines and CO, count as 2.
Charge: electrons have a negative charge. A positive charge on your complex means a missing electron,
so subtract one. A negative charge means an extra electron, so add one.

1.3.2 Method 2: Ionic

Electrons from the metal: first, the oxidation state of the metal must be assigned. Oxidation state is a
formalism, but a useful formalism. The assignment can be done by the notional stripping off of ligands
to reveal a notional metal ion. Ligands that are donors of pairs of electrons, or multiple pairs of electrons
are removed with their pair(s) of electrons and do not effect the charge of the metal. Examples include
alkenes, dienes and arenes (all of which have an even hapto number), CO, phosphines and carbenes. Ligands
with a sigma bond are stripped off as anions even if this makes no chemical sense. Examples are alkyl,
allyl, dienyl and even acyl ligands (all of which have an odd hapto number), hydride, halide and carbynes.
The number of electrons contributed by the metal is then its group number (count from the far left-hand
column (group 1) of the periodic table) minus the oxidation state. This is also the number of d electrons,
d x . This number is useful for comparing metals with different oxidation states across groups of the periodic
table.

Tab l e 1 . 2 Transition metals and numbers of electrons

3 4 5 6 7 8 9 10 11

Sc Ti V Cr n Fe Co i Cu

Y Zr b o Tc Ru Rh Pd Ag

La Hf Ta W Re Os Ir Pt Au