3030 zyxwvutsrqp Inorganic Chemistry, zyxwvutsrqp Vol. zyxwvuts 18, No. 11, 1979 Bino, Cotton, and Kaim Contribution from the Department of Chemistry, Texas A&M University, College Station, Texas 77843 Tetraacetanilidodichromium and -dimolybdenum: Another Supershort Chromium-Chromium Bond and an Unexpected Structural Difference AVI BINO, F. ALBERT COTTON,* and WOLFGANG KAIM Received May 29, 1979 The title compounds, Cr2[PhNC(O)CH3I4, zyxwvut 1, and Mo2[PhNC(O)CH3I4, 2, the first tetraamidodimetal compounds with quadruple M-M bonds, are described. They are prepared by reaction of the M2(02CCH3)4 compounds with Li[PhNC(O)CH,]. 1 was obtained as the unsolvated compound and the structure determined by X-ray crystallography. The crystals belong to space group zyxwvutsr Zd with a zyxwvutsrqp = 12.739 (6) A, c = 9.251 (3) A, and Z = 2. Each molecule has a(&) crystallographic symmetry and the Cr-Cr distance is 1.873 (4) A. The molybdenum compound, 2, was also obtained in unsolvated form but crystals large enough for X-ray work could not be obtained. The bis(tetrahydr0furanate)Mo2[PhNC(0)CH,].2THFdid give suitable crystals, in space group P2,/c with a = 9.849 (2) A, b = 11.622 (2) A, c = 17.957 (2) A, /3 = 95.32 (1)O, and zyx Z = 2, in which each molecule resides on a crystallographic inversion center and has an Mo-Mo distance of 2.086 (2) A. The ligand arrangement is different from that in 1; the MoN202 groups have a cis configuration and the overall symmetry of the Mo~(NCO)~ grouping is C2h. This is the first time homologous Cr;+ and M024+ complexes have been found to differ structurally in a qualitative way. Introduction In view of the extraordinary range of Cr-Cr distances so far observed in compounds containing the quadruply bonded Cr-Cr4+ unit, we have been extending our preparative en- deavors to still further ligand types with the goal of elucidating the role played by the ligands in determining these distances. The problem we are addressing may be summarized as follows. All tetracarboxylatcdichromium(I1) compounds' and related ones2 whose structures have so far been determined have Cr-Cr distances in the range 2.29-2.53 A.3 The closely related [Cr2(C03)4]4- ion has a slightly shorter distance,lc 2.22 A. These rather long distances have provoked controversy as to the Cr-Cr bond ~ r d e r . ~ , ~ On the other hand, there are a number of Cr2 species in which the Cr-Cr distances are short enough that there has been no lack of agreement that they contain quadruple bonds. While two of these (with Cr-Cr distances in parentheses), namely, [Cr2(CH3)8]4- (1 -98 A)6 and Cr2(C3H5)4 (1 -97 A),7 have been known for several years they are so different, at least superficially, from the Cr2- (02CR), type that their general significance is (or was) ambiguous. A distinct dichotomy was created with the discovery8 of a series of compounds, of which those in Table I are representative. These contain ligands that are sterically and electronically similar to the carboxylato group, and yet they have vastly shorter Cr-Cr bonds, Le., in a range I1 -90 A, which we informally designate super~hort.~ Our previous attempts to identify the ligand properties re- sponsible for the occurrence of supershort C rkr bonds have involved, inter alia, the following steps. (1) Compound 2 in Table I showed that the presence of Cr-C bonds is not critical, and compound 3 in Table I showed that Cr-0 bonds are not in themselves essential. In short, no one type of ligand atom, C, N, or 0, is critical. (2) Compound 4 in Table I showed that four noncarboxylato ligands are not necessary. (3) Com- pound 5 in Table I showed that it is not necessary for the Cr2L4 species to be a neutral molecule. (4) The one common feature of compounds 1-5 in Table I, namely, the incorporation of some ligand atoms in aromatic rings, was shown by compounds 6 and 7 in Table I not to be essential. We report here a further crucial experimental step in achieving an understanding of what causes (or allows) the formation of supershort C r q C r bonds. We have prepared the first carboxamido-bridged dichromium compound, viz., Cr2- [RNC(O)R'], with R = CH3 and R' = C6H5, and determined its structure X-ray crystallographically. We have also prepared the molybdenum analogue and determined its structure, with 0020- 1669/79/ 13 18-3030$01 .OO/O Table I. Dichromium(I1) Compounds Having Supershort Bonds and Their Cr-Cr Bond Lengths bond no. compd length, A ref 2 L3c'$ky) 1.89 8b 3 ( H ~ C ~ N H ) I I 4 1.87 8c Cr-cr j4 Cr-Cr Cr-Cr 1.86 8d I I 4 the surprising result that, for the first time, the two chemically homologous molecules are not structurally homologous. Experimental Section Preparation of Cr2[PhNC(0)CH3k, 1. Acetanilide (0.54 g, 4 mmol) was dissolved in 20 mL of tetrahydrofuran (THF) which had been dried over Na-K alloy, and 2.6 mL of a 1.6 M solution (4 mmol) of n-butyllithium in hexane was added, to give a solution containing the colorless anion [C6H5NC(0)CH3]-. Anhydrous Cr2(02CCH3)4 0 1979 American Chemical Society