The magnetic moments of octahedral complexes of chromium, manganese, iron, cobalt, and nickel with the chelate groups dipyridyl and a ditertiary arsine are compared. Many of these moments are reported for the first time and the following new compounds are described : [Co(Diarsi,ne),)] (C104)2, [Co( Diarsine),)] (ClO,) 3, [Co(Diarsine) ,(OAc),] (CIO,) , [Co(Dipy),] (C104),3H20, and [Cr(Dipy) ,C1,]C1,2H20, where Diarsine = o-phenylenebisdimethylarsine and Dipy = 2 : 8'-dipyridyl. Also, the co-ordination of tervalent chromium with a tertiary arsine is reported for the first time. The meaning of the terms " ionic " and " covalent '' binding as deduced from magnetic data is discussed. In compounds of the type [MII(Dipy),]X,, the bonds are ionic (4s4p34d2) when MI1 = Mn or Co and covalent (3d24s4p3) when the metal is Fe or Cr. The nature of the bonds in the corresponding nickel compounds is discussed and it is concluded that the paramagnetism indicates ionic rather than covalent bonds. In the dipyridyl complexes of tervalent Fe and Co the bonds are covalent but the magnetic data are not diagnostic when MII1 = Cr. No ditertiary arsine complexes of bivalent Cr or Mn were isolated but Fe, Co, and Ni, in their bi-and ter-valent states, all form stable octahedral complexes with this chelate group ; in every case the moments indicate 3d24s4p3 covalent bonds. Certain hypotheses to account for the observed data are proposed. In all cases the moments are consistent with Pauling's theory relating bond type and stereochemistry with magnetic moments. (Nyholm, J., 1950,851,2061,2071 ; 1951, 38,2602) the complexes formed by iron, cobalt, nickel, and copper with the ditertiary arsine chelate group, o-phenylenebisdimethylarsihe, C,H4(AsMe,),, were described and their magnetic moments reported. The bivalent complexes of iron, cobalt, and nickel with this chelate group may be oxidised, even by air, to the tervalent compounds which contain strong covalent bonds. The tervalent nickel complexes contain one unpaired electron but these compounds, and particularly the diamagnetic complex salt [Ni(Diarsine),](ClO,),, earlier led to some difficulties of interpretation on the simple Pauline; theory relating magnetic moment, bond type, and stereochemistry. Also, in contrast with the stability of the iron, cobalt, and nickel compounds, chromium and manganese were reluctant to co-ordinate with the ditertiary arsine. It was considered that a comparative study of the magnetic behaviour of all five metals with dipyridyl and the ditertiary arsine might throw some light on these problems and, accordingly, a summary of the data available in the literature was made. Surprisingly, although dipyridyl complexes of all of these metals have been known for some time, the magnetic moments of several of them had not been reported. These
IN a recent series of investigations
