Modern inorganic chemistry

GROUP VI 259

H

\

C

'V

JO, O

H C 2 H 5 X

or a double bond, for example

CH 3 _

\ ^°

O=C=O, C

H

The covalently bonded oxygen atom still has two lone pairs of

electrons and can act as an electron pair donor. It rarely donates

both pairs (to achieve 4 -coordination) and usually only one donor

bond is formed. A water molecule, for example, can donate to a

proton, forming ^O"^1 ", and diethyl ether can donate to an acceptor

such as boron trifluoride :

C 2 H 5

0-.BF3

Sulphur in hydrogen sulphide and its derivatives is a much less
effective simple electron pair donor and the other Group VI elements
show this property to a very minor extent. However, compounds
based on divalent sulphur (for example, dimethylsulphide (CH 3 ) 2 S)
are often found to be effective ligands in transition metal Complexes.
Unlike oxygen, the remaining elements can increase their covalency
to a maximum of six by utilising the low energy d orbitals not
available to oxygen, and 6— coordinate compounds (for example
SF 6 ) are known. However, as the atomic number and size of the
atoms increase from oxygen to polonium, the elements become
more electropositive, the hydrides less stable and the stabilities of
the higher oxidation states decrease. Only polonium can really be
said to show weakly metallic properties, although tellurium oxides
are amphoteric.
There are peculiarities associated with compounds containing
oxygen and hydrogen where hydrogen bond formation gives rise to
many properties which are not shown by the compounds of the other
elements.