606 6. ARSENICALS 



The phenylarsonates form fairly insoluble salts with various metal ions 

 (Portnov, 1948), as do the inorganic arsenates and arsenites. Perhaps 

 this should be borne in mind in biological work, although no thorough 

 investigation of this has been made. The complexes formed between tri- 

 phenylarsine oxide and ions such as Ni++ and Hg++ in organic solvents 

 (e. g., ether or alcohol) have been studied by Phillips and Tyree (1961), 

 but there is some question whether such complexes occur in aqueous media. 



The arseno compounds (R — As=As — R), which include the previously 

 used antiluetics, arsphenamine and neoarsphenamine, are unstable in so- 

 lutions near neutrality in the presence of oxygen, and spontaneously split 

 into the corresponging arsenoso compounds (E, — As=0), which are bio- 

 logically active (Eagle, 1939 a). There is thus very little reason for using 

 these compounds either therapeutically or in enzyme studies. Substances 

 such as tetraarsenoacetate ("OOC — CHg — As=As — As=As — CHg — COO") 

 are likewise active only after splitting to arsenite and arsenosoacetate 

 (Barbour et al, 1925). 



The preparations and chemical properties of the organic arsenicals 

 are well covered in the book by Raiziss and Gavron (1923). 



Atomic Properties and Bond Characteristics of Arsenic 



Crystallographic studies of arsine, arsenomethane, lewisite, arsenoben- 

 zene, phenylarsonic acid, the cacodyl halides, and various arsenicals 

 (Nielsen, 1952; Shimada, 1960; Hedberg et al, 1961) have provided the 

 mean bond lengths shown in the accompanying tabulation (the As — C 



distance seems to be nearer 1.90 A in aromatic compounds and nearer 

 1.98 A in alkyl derivatives). Bond angles are generally not so well known. 

 Arsine has a structure similar to ammonia but the bonds are apparently 

 more purely p bonds, since the angles between the As — H bonds are 91^35', 

 but in the pentavalent arsonic acids the angles are nearly tetrahedral in 

 orientation. Bond energies have been calculated to be 32.1 kcal/mole for 

 As — As, 58.6 kcal/mole for As — H, and 68.9 kcal/mole for As — CI (Pauling, 

 1960). The van der Waals' radius of arsenic is 2.0 A. The electronegativity 

 value is 2.0 so that arsenic assumes a positive fractional charge in most of 

 its bond dipoles. 



