UNBRIDGED M-M MULTIPLE BONDS IN THE CR2(TMTAA)2 AND MO2(TMTAA)2 MOLECULES (TMTAA = DIANION OF 7,16-DIHYDRO-6,8,15,17-TETRAMETHYLDIBENZO[B,I][1,4,8,11]TETRAAZACYCLOTETRADECINE) - EXPERIMENTAL AND THEORETICAL INVESTIGATIONS

The title compounds have been prepared, their molecular structures determined by X-ray crystallography, and their electronic structures calculated by the SCF-Xα-SW method. The chromium compound (I) was obtained by two methods that are different from any previously reported while the molybdenum compound was made by the reaction of Mo2(02CCH3)4and Li2tmtaa, previously mentioned but not described in detail. The two compounds are isomorphous (P1, Z = 2) with the following unit cell dimensions (I followed by II in each case): a = 11.707 (4), 11.743 (6) Å; b = 16.436 (5), 16.716 (9) Å; c = 1 1.571 (3), 1 1.680 (8) Å; α = 90.39 (3), 90.87 (6)°; β = 107.13 (3), 106.81 (5)°; γ = 108.85 (3), 108.36 (5)°. The molecules are very similar, each having the two saddle-shaped ligands rotated 90° to each other. For I, the Cr atoms are ca. 0.50 Å above the N4planes with Cr-Cr = 2.101 (1) Å; for II, the corresponding numbers are 0.55 and 2.175 (1) Å. Both M-M distances are greater than those previously found for M24+units in an M2X8framework without axial interactions. The SCF-Xα-SW calculations show that little if any Cr-Cr δ bonding exists whereas there is a significant amount of δ bonding in the molybdenum compound. The relatively small (0.074 Å) difference between the Cr-Cr and Mo-Mo distances (0.11-0.22 Å being the previously known range) is discussed. © 1990, American Chemical Society. All rights reserved.