Shapes of Molecules
119
categories: (I) bond pairs that are used to bind ligands to the central atom, or (2)
lone pairs that are held by the central atom but not used for bonds. Each pair,
regardless of the type, will occupy & molecular orbital, not an atomic orbital.
The key to success in prediction hinges on finding the correct number of valence electrons associated with M.
A summary of all the procedural steps for shape prediction is given here
because it will be useful for later reference; the application of the steps is
illustrated in detail on the following pages.
1. Find the number of valence electrons around the central atom by drawing
an "electron-dot formula" for the molecule or ion whose shape is
desired. See pp 120-121. Follow these rules.
(a) The central atom must be the least electronegative atom in the
molecule or ion. H can never be a central atom; it can form only
one bond.
(b) Every ligand must obey the octet rule (or duet rule for H).
(c) If the shape involves an ion (but not a complex ion), take account
of the charge on the ion by adding one valence electron to M for
each negative charge, or subtracting one valence electron from M
for each positive charge.
(d) If possible, also make M obey the octet rule. Manipulation will be
possible only if one or more ligands can form multiple bonds. The
four or six electrons involved in double or triple bonds count as
belonging to both the ligand and the central atom, just as do the two
electrons in a single bond. See pp 121 and 132.
(e) For complex ions you canignore the electrons originally possessed
by M (usually a metal ion) and the charge on the ion. You do not
need to draw an electron-dot formula. You need to know only that
each ligand contributes a bond pair to M; there are no lone pairs.
The number of ligands therefore directly tells you the number of
pairs of valence electrons around M. See p 141.
2. Calculate the number P of electron pairs around the central atom (M), as
follows.
(a) If the electron-dot formula shows no multiple bonds (double or
triple bonds) between M and the ligands, divide the total number of
electrons around M by 2 to get the number/
3 of electron pairs.
(b) If the electron-dot formula shows one or more multiple bonds between M and the ligands, only two electrons are counted for each
multiple bond (double or triple). Add these electrons to those involved in single bonds or lone pairs, then divide the total by 2 to get
the number/
3 of electron pairs around M. See pp 121, 132, and 138.
(c) If the number of valence electrons in (a) or (b) is an odd number,
then (for the purpose of this calculation only) increase the number of
electrons by one to make it even before proceeding as in (a) or (b).
119
categories: (I) bond pairs that are used to bind ligands to the central atom, or (2)
lone pairs that are held by the central atom but not used for bonds. Each pair,
regardless of the type, will occupy & molecular orbital, not an atomic orbital.
The key to success in prediction hinges on finding the correct number of valence electrons associated with M.
A summary of all the procedural steps for shape prediction is given here
because it will be useful for later reference; the application of the steps is
illustrated in detail on the following pages.
1. Find the number of valence electrons around the central atom by drawing
an "electron-dot formula" for the molecule or ion whose shape is
desired. See pp 120-121. Follow these rules.
(a) The central atom must be the least electronegative atom in the
molecule or ion. H can never be a central atom; it can form only
one bond.
(b) Every ligand must obey the octet rule (or duet rule for H).
(c) If the shape involves an ion (but not a complex ion), take account
of the charge on the ion by adding one valence electron to M for
each negative charge, or subtracting one valence electron from M
for each positive charge.
(d) If possible, also make M obey the octet rule. Manipulation will be
possible only if one or more ligands can form multiple bonds. The
four or six electrons involved in double or triple bonds count as
belonging to both the ligand and the central atom, just as do the two
electrons in a single bond. See pp 121 and 132.
(e) For complex ions you canignore the electrons originally possessed
by M (usually a metal ion) and the charge on the ion. You do not
need to draw an electron-dot formula. You need to know only that
each ligand contributes a bond pair to M; there are no lone pairs.
The number of ligands therefore directly tells you the number of
pairs of valence electrons around M. See p 141.
2. Calculate the number P of electron pairs around the central atom (M), as
follows.
(a) If the electron-dot formula shows no multiple bonds (double or
triple bonds) between M and the ligands, divide the total number of
electrons around M by 2 to get the number/
3 of electron pairs.
(b) If the electron-dot formula shows one or more multiple bonds between M and the ligands, only two electrons are counted for each
multiple bond (double or triple). Add these electrons to those involved in single bonds or lone pairs, then divide the total by 2 to get
the number/
3 of electron pairs around M. See pp 121, 132, and 138.
(c) If the number of valence electrons in (a) or (b) is an odd number,
then (for the purpose of this calculation only) increase the number of
electrons by one to make it even before proceeding as in (a) or (b).