On the Arsenic-Arsenic Double Bond To the Editor: The recent article, "The Structure of Salvarsan and the Arsenic-Arsenic Double Bond," by A. S. Levinson [J.CHEM. EDUC., 54,98(1977)] clearly shows that erroneous structural assignments made by early organoarsenic chemists are unfortunately still carried through present day literature despite crystdllo:raphir evidence to thccontrary.However, the conclurling statement ". . . that thesimplearsenic-arsenicdouble bond is not known a t this timeWmavverv well be an incorrect .. statement. In order to specify what length corresponds to a double bond, a (bond length) - (valence-bond order) correlation would have to exist. Such a correlation is not available for the arsenic system. The correlation is incomplete in the sense that, although a mean bond length of approximately 2.43 A has been ascribed to the As-As sinele bond ( I ) . no hond length data is available for the neutral, free ~sz'molecule which presumably would have a triple bond. This is unfortunate since the existence of Xz molecules (where X = P, As, Sb, and Bi) has been known for some time from earlv vapor density and mass spectroscopy investigations (2):~s ;consequence, the range in length for the As-As douhle bond cannot be specified with any certainty. The need for this information becomes crucial when considering the stereochemical features of the Coz((CO)sP(C6H& As2 molecule (I) which contains the shortest As-As hond known to date: 2.273(3) A (3). The question: "Could this be a double bond?" becomes relevant here. A comparison of this molecule with two other compounds having similar crystal structures, Fe2(CO)& (11) (4) and Co2(CO)6(0Cz0)(111) (5)is revealing. The free A s 2 entity (like acetylene) should possess a triple bond whileneutral SI (like 02) Should possess a double bond. However, the bond: lengths of the S2 and Cz moieties in I1 and 111.. res~ectivelv. . are~consistentwith a decrease from the free-molecule bo; order of approximately one in each case (i.e., from 2 to 1for Sz, and from 3 to 2 for Cz). The presumption is that the bond order for the AS?moiety in I has also (like the Cp moiety in 111) drcrcased to approximarply two. That there i; a suhitantial ir-hond orrlrr in I is reflected in the sign~firnntlyshortened As-As hond distance (as compared to the typical AS-AS single bond distance of 2.43 A). Although hond length data on the neutral, free AS? molecule is rewired to satisfactorilv resolve the issue, the bond length of t i e As2 moiety found i n I may conceivablv be the first evidence for the existence of the elusive arsenic-arsenic double bond. Literature Clted (11 Smith, L. R.andMarfin, J.L.,J Orgonometo1 Chem., 84,1(1975).
(2) Carlson,K.D..Kohl.F.J..andUy,O,M.in"MassSpeetrometrymIno'ganicCbrnispy:' Advances in Chemistry Solien, No. 72, (EditorMargrave. J. L.1 AmericanChernd Soeiety, Wsrhington, D.C., 1968,pp245-60, and references citodtherein. (31 Fou8t.A. S.,Fmter,M.S.,andDehl,L. F., J Amer Chem. Soe., 91,5633(1969). (41 Wai, C.H. sndDah1.L. F.,Inorg. Cham., 4,1(1%51. (51 Sly, W . G..J. Amer Chrm. Sor., 81.18(1959).
Harold C. Nelson The University of Texas at Austin Austin, 78712
718 I Journal of Chemical Education
To the Editor: The principle point of my article [J. Chem. Educ., 54,98 (1977)] was that organo-arsenic compounds which have been written with an arsenic-arsenic double bond (the arseno group) have been shown to be either polymers or cyclopolyarsines. This should have been clear from the text and especially from the statement in the penultimate paragraph, "Extensive structure studies using modern tools of analysis have failed to reveal the existence of an arsenic-arsenic double bond in a simple organo-arsenic compound". I now regret having avoided redundancy in the last sentence by omitting the proviso "in organo-arsenic compounds" so that the closing statement would have read, "But it is clear that the simple arsenic-arsenic double bond is not known in organo-arsenic compounds at this time." Nelson has taken the last sentence of mv article out of context and has made several points that I &st comment on. First, in regard tu his discussion of the length of an arsenicarsenic double bond, I have emphasized rhat structures of organo-arsenicals that have been written with a simple arsenic-arsenic double bond have been unambiguously shuwn to be something altogelher different. In this context, how one mieht calculate the arsenic-arsenic double hond leneth is of peripheral interest. Notwithstanding the discussion above. renorts bv Foust. Foster and Dahl(lY2,and hy Foust and &hi(3)areerer). in: wresting and, in a broader context than orieinallv intended. certainl; merit discussion. Briefly, Foust, Foster and Dahl orepared AsnCo(COhand characterized i t (1). The molecufe is tetrahedral with the Co(C0)s occupying a comer of the tetrahedron, in effect substituting for anarsenic atom in tetrahedral As4. An interesting feature of the molecule is the arsenic-arsenic hond length of 2.372 (5)A. This is shorter than the arsenic-arsenic bond length of 2.388 A in Fe(C0)4 (AsCgF5)z.In a companion communication, Foust, Foster and Dahl reported the preparation of Co2(CO)6Asz in which two arsenic atoms of tetrahedral As4 are replaced by Co(C0)s groups (2). Because the compound is a liquid, X-ray examination was performed on the crystalline triphenylphosphine derivative. The arsenicarsenic bond length was found to be extremely short, 2.273 (3) A. Foust and Dahl have summarized their conclusions regarding the nature of this extremely short hond as follows, "The recent weparation and characterization of CodCO)EAs9 in our laborstory led to the postulationof theenist&ceo"f an &-As multiple bond of r order nrw one in order to rationabe the unusualiy short &-As internuclear separation of 2.273 (3) A found in an X-ray structural examination of the structurally analogous triphenylphosphine derivative Co2 I(CO)6P(C,jH5)3)As2" (3). The recent review of cyclopolyarsines by Smith and Mills uses the value of 2.27 A obtained by Foust, Foster and Dahl as the value for an As-As "double" hond (quotation mark theirs) (4). These analogs of As4are clearly a different class of material than I dealt with. ~
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Literature Cited
Portland State University Portland, Oregon 97207
Alfred S. Levinson
