Sensitivity of the Cr-Cr Quadrupole Bond to Axial Interactions in Dichromium(II) Compounds

Article abstract of DOI:10.1021/ja00530a026

X-ray crystallographic studies have been made on five Cr24L2-type compounds, R = C6H5, 2,6-(CH3)2C6H3; 4-(Me2N)C6H4; R' = CH3, NHPh; L = THF, NC5H5, to determine the response of the Cr-Cr bond to the intriduction and variation of axial ligands.The structures reported here are for the following compounds, where the numbers in parantheses are the lengths in Angstroem of the Cr-Cr bond: Cr2<(4-Me2N-C6H4)NC(O)CH3>4*THF (1) (2.006(2)); Cr2<(2,6-xylyl)NC(O)CH3>4*3THF (2) (2.221(3)); Cr2<2,6-xylyl)NC(O)CH3>4*THF*C6H5CH3 (3) (2.023(1)); Cr24*2THF*C6H14 (4) (2.246(2)); Cr2<(2,6-xylyl)NC(O)CH3>4*3NC5H5 (5) (2.354(5)).The Cr-Cr bond length is shown to be a sensitive function of both the number of axial ligands present and the donor ability of the axial ligands.Each THF molecules in the axial porition is shown to cause an ca.0.18-Angstroem increase in the length of the Cr-Cr bond.The Cr-Cr bond length is also seen to increase by about 0.12 Angstroem upon replacing two axial THF ligands by two pyridine ligands.The basic crystallographic data on the five compounds are as follows. 1: space group, C2/c; a = 21.254(4) Angstroem, b = 10.990(2) Angstroem, c = 20.733(4) Angstroem, β = 110.64(2) deg, Z = 4. 2: Pbcn; a = 16.775(2) Angstroem, b = 19.736(3) Angstroem, c = 19.739(2) Angstroem, Z = 4. 3: Pbcn; a = 16.806(2) Angstroem, b = 19.311(3) Angstroem, c = 14.887(3) Angstroem, Z = 4. 4: Pccn; a = 24.813(5), Angstroem, b = 13.408(3) Angstroem, c = 18.631(3) Angstroem, Z = 4. 5: P2₁/n; a = 17.374(4) Angstroem, b = 12.836(4) Angstroem, c = 23.603(6) Angstroem, Z = 4.These structures are then compared with those of some 30 other quadruply bonded dichromium compounds with bridging ligand chains of the types O-C-O, N-C-O, N-C-N, N-N-N, C-C-O, and C-P-C, and certain trends in the Cr-Cr distance as these types are changed are noted.