Further Studies of Cr - Cr Bond Lengths in Quadruply Bonded Dinuelear G s

X-ray crystallographic studies have been carried out on three compounds containing chromium-chromium quadruple bonds: Cr2(02CCH&(H02CCH&, I, C ~ ~ ( O ~ C C H ~ ) ~ ( C S H , , N ) ~ , 11 and [NHI]~[C~~(CQ~)~(M~~)~](H~~)~-~, III. The results, together with those on nine compounds in the literature, are sufficient to indicate a crude general relationship between the donor ability of the bridging ligands and the length of the quadruple Cr'Cr bond. This trend within dichromium(I1) complexes bridged by 0-C-0 linkages is in keeping with the observation that the shortest quadruple Cr-Cr bonds are obtained with bridging ligands which are very good donors. Crystal data for I: P2'/n, a = 15.123 (2) A, b = 7.775 (1) A, c = 8.302 (1) A, /3 = 102.80 (l)', V = 952.0 (4) A3, Z = 2. Crystal data for 11: C2/m, a i-~ 15.989 (2) A, b =: 10.331 ( 2 ) A, c = 8.035 (1) A, /3 = 116.05 (1)'; V = 1192.4(6) w3, Z = 2. Crystal data for III: P & / c . a = 6.949 (2) A, b = 16.112 (4) A, c = 7.737 (2) A, /3 = 108,14 (2)', V = 823.2 (7) A', Z = 2.