A single-crystal X-ray diffraction study on bromotricarbonylbis(methy1 isocyanide)manganese, Mn( CO)a( CNCH3)2Br, has been completed. In addition, a nonparameterized approximate molecular orbital calculation has been carried out on the compound for the purpose of aiding in the interpretation of the bonding. The crystal structure is triclinic, space group Pi with two discrete molecules per unit cell, a = 8.900 (2) 8, b = 10.907 (3) 8, c = 6.155 (2) A, CY = 97.93 (l)', p = 108.45 (l)', y = 81.86 (l)', V = 558.3 Aa, pm = 1.77Diffractometer data were collected using Mo Kor radiation and the structure was refined by full-matrix least-squares methods with anisotropic thermal parameters for all but the hydrogen atoms to a final conventional R = 0.040 for 831 independent reflections. The molecule has a nearly octahedral configuration with cis methyl isocyanide ligands. The three Mn-CO distances are 1.854 (12), 1.815 ( l l ) , and 1.789 (11) 8, with theoshortest of these being the carbonyl trans to the bromine. The Mn-CNCH3 bond distances are 1.992 (11) an$ 1.950 (12) A, and the C-N-C angle does not deviate from linearity by more than 3'. The Mn-Br distance of 2.537 (2) A corresponds to a single-bond distance. Comparisons are made between the present structure and other reported methyl isocyanide complexes, as well as with the electronically related compounds, Mn(CO)bX, X = C1 and Br.Bond length consideratjons and the molecular orbital results confirm that back-bonding to the methyl isocyanide ligand is taking place, even in the presence of carbonyl, a much stronger r-electron acceptor. For the methyl isocyanide complexes studied to date comparisons of the metal to isocyanide bond length show a striking similarity to the metal to cyanide distance in related molecules. 0.02 g/cma, and p o = 1.79 g/cma.Introduction A primary source of evidence for the strong n-acceptor ability of the methyl isocyanide ligand on bonding with transition metals has been a shortening of the metal to carbon bond distance relative to the expected single-bond length in such complexes as [Co-(CNCH3)5]+,l trans-Fe(CN)z(CNCHs)4,2 and other species in which the methyl isocyanide ligand is competing for electron density on the metal either with itself or with cyanide. However, prior to the present study, structural data have not been presented in which the methyl isocyanide ligand is present in a complex which
