_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

The main Groups IV and V
Acid
H 2 PO(OH)
Me 2 PO(OH)
Me 2 AsO(OH)
HPO(OH) 2
MePO(OH) 2
MeAsO(OH) 2
PhAsO(OH) 2
PhSbO(OH) 2
PO(OH),
AsO(OH),
sb.o, aq.
pK,
1.1
3.1
6.3
1.3
2.3
3.6
3.5
Very weak
2.2
2.2
Very weak
6.7
7.9
8.3
8.5
7.2 12.4
6.9 11.5
one of its modifications. Arylstibonic acids ArSbO(OH) 2 may be prepared
similarly. The latter are amphoteric as they dissolve in concentrated hydrochloric
acid forming the anions ArSbCl;, which may be precipitated as pyridinium salts.
Some acid dissociation constants of oxo-acids of P, As and Sb are listed in
Table 4.6. While the phosphorus and arsenic compounds are of comparable
strength, the antimony derivatives are very weak acids. Phenylstibonic acid,
nominally PhSbO(OH) 2 , is polymeric and insoluble in water, although it dissolves
as a monomer in acetic acid. It probably forms the octahedral anion PhSb(OH); in
alkaline solution. In this it resembles not only Sb 2 0 5 which forms Sb(OH); and
telluric acid, which in Te( OH) 6 rather than H 2 TeO 4 , but also related tin complexes
which form ions Me 2 Sn(OH)~-and MeSn(OH); in strongly alkaline solutions
(p.117). There is a general trend for oxoanions to adopt octahedral rather than
tetrahedral structures on moving from the First to the Second Long Period.
Dimethylarsinic acid crystallizes as hydrogen bonded dimers, similar to those
formed by carboxylic acids. In solid propylarsonic acid, C 3 H 7 As0(0H) 2 , the
molecules are linked together in chains through hydrogen bonds. Arsonic and
arsinic acids are readily reduced, in contrast to the phosphorus analogues
(Fig. 4.10). By suitable choice ofreagent MeAsO(OH) 2 yields MeAsX 2 , (MeAs)n or
MeAsH 2 •
Me
Table 4.6. Acid dissociation constants of some organooxo-acids
+-2.57-
' _/O-H------0"' /
1 91 .As 1.62 As
Me..
Me
"'-o------H-o/ ···· ... Me
Apart from their use as pharmaceuticals (p. 142) organoarsenicals such as
alkyl- or arylarsonic acids have found some application as herbicides, fungicides
and bactericides. They are being superseded by the less toxic tin compounds. All
arsenic compounds should be treated as poisonous. The halides cause severe
burns on the skin which heal only with difficulty. This property was put to evil use
132