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Diarsenic, As,, is a versatile ligand. While it is side-on coordinated as a 4r ligand in [Cp(C0)2M]2As2 (M = Mo (4a), W (4b)), it acta as a six-electron donor in [Cp(C0)2M]2AszCr(CO)5 (M = Mo (3a) or W (3b)) or an eight-electron donor in [Cp(CO)zM]2Asz[Cr(CO)5], (M = Mo (2a) or W (2b)), respectively, by additional end-on coordination via ita lone pairs. Another form of an eight-electron donor, As2, is found for [Cp(CO)2Mn]2As-As[Mn(C0)2Cp]2 (6a), prepared by reductive -coupling of two arsinidene complexes [Cp(CO)2Mn]AsC1. In 6a Asz is bonded in a diarsinidene, &-As, valence state. As,, side-on coordinated as a six-electron donor is present in the star-type compound [(CO)&o]& (7a) obtained from Na2Mq(CO)lo and AsC13. Ita tungsten analogue [(C0)5W]3As2 reacts with iodine to give [ (CO),W,I](~-I)(p-~2-As,) (8)with a side-on coordinated four-electron donor, AS,. Syntheses and properties including structures as determined by X-ray methods are discussed.As a higher homologue of dinitrogen, diarsenic, As2, is a major constituent of arsenic vapor at high temperatures.'0276-7333185 f 2304-0326$0l.50 f 0 Even though the binding energy of this binuclear tenelectron species is quite respectable (3.93 eV),2 it is 0 1985 American Chemical Society Diarsenic as an Electron Donor Ligand

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d) S. Ofner, V. Rasetti, B. Zehnder, A. Eschenmoser, Helu. Chim. Acta 64 (1981) 1431. 161 A B,C,D-hexadehydrocorrinate is formed as by-product (fa. 4%) in the anaerobic cyclization of a secoporphyrinogen to nickel( 11)-C,D-tetradehydrocorrinates [2c].