Exceedingly short metal-to-metal multiple bonds

Tetrakis[μ-(6-methyl-2-pyridinolato)]dirhodium

“…In [Rh2(mhp) 41 the Rh-Rh length is 2.359 (1)/k, which is the shortest found so far for Rh. Similar shortenings of M-M bonds in [M2(mhp) 4] and similar complexes compared with carboxylato-bridged species have already been noted for Cr, Mo and W (Cotton, Koch & Millar, 1977;Cotton, Fanwick, Niswander & Sekutowski, 1978, and references therein), and can be attributed to the electronic structure of the four mhp ligands, which reduce the net positive charge on the metal atoms, encouraging M-M bonding, while simultaneously sterically blocking the axial positions for possible ligands.…”

Tetracarbanilanilidodichromium(II): A Clue to the Understanding of the CrCr Quadruple Bond

“…This finding would indicate a further more energy-demanding rearrangement process, e. g. following 3 80" rotation about the P P axis in (5a)I'I. An unequivocal assignment of the diastereomers and simultaneous elucidation of the arrangement of the (E,Z)-substituents at the C --P bond is expected from an X-ray structure analysis of a crystal fraction which, according to low-temperature 3iP-NMR analysis, contains only the (5a)-form.…”

“…Since its diphosphane (A)-and methylenephosphane (X)-signal groupings coalesce centrally and reversibly as a function of the recording temperature, we assume that (5b) experiences an identical rearrangement-independently of (5u)I3l. Table 1 The equilibrium (d) would represent the first sigmatropic [3,3]-shift with P P linkage and migration of phosphorus-carbon pn-pm bonds; thus it would fully correspond to the degenerate Cope rearrangement in the carba-analogous 1,5-hexadiene system'41.…”

“…18, 469 (1979). [3] The activation energy of cu. 10 kcal/mol, as determined by the coalescence method, is much lower than that found for other (hetero) Cope rearrangements.…”

“…Other important bond lengths and angles are: Cr 0 1.993 (6), Cr N 2.093(7) A; Cr-Cr--O'(THF) 178.2(2), Cr Cr 0 93.8(2), Cr--Cr--N 86.5(2)". Comparison with (2)l' l indicates weaker bonding to the bridging ligands in (3).…”