Ligands, multiply bonded

Organoimido complexes are currently of interest in studies of ligands multiply bonded to transition metal centers. It is now possible to prepare a variety of monoimido complexes for a diversity of the early transition metals,but bis(organoimido) complexes are less well known, inasmuch as routes for their synthesis have been less well developed. The bis(organoimido) moiety is isoelectronic with the cis-dioxo group that is commonly found for higher oxidation states, and it shows potential utility as a stable source of nitrenes in organic syntheses. ... 

Electrochemistry of ligands multiply bonded to molybdenum and tungsten... 

Stereochemical aspects of reactions of complexes of d° transition metals with multiply bonded ligands. Y. V. Kokunov and Y. A. Buslaev, Coord. Chem. Rev., 1982, 47,15-40 (120). 

Phosphorus, arsenic, antimony and bismuth multiply bonded systems with low coordination number — their role as complex ligands. O. J. Scherer, Angew. Chem., Int. Ed. Engl., 1984, 24, 924 (85). 

Fig. 2. Significant molecular orbitals of terminal transition metal-oxo units in a six-coordinate 4 ligand environment. The d° configuration is a formal triple bond. The highest occupied molecular orbital in the d configuration is formally nonbonding (8 symmetry) so the metal-oxo bond order remains 3.0. However, d-electron counts above d populate orbitals that are antibonding between the metal and the terminal multiply bonded ligand (0x0 in this case, but alternatively, alkyl-imido, nitrido, sulfido, etc.). Note that all the equatorial ligand orbitals and the metal dx2 y2 orbital (hi in 4 symmetry) are ignored for simplicity.

Coordination compounds of rhenium are known with the metal in the oxidation states +1 up to +7. The oxidation state with the by far most representatives is that of +5. The cF configuration is readily stabilized by multiply bonded ligands such as oxo, nitrido, or imido. 

Syn-syn bridging of the carbamate ligand is well established and gives rise to metal-metal multiply bonded systems. This is exemplified by the paddlewheel structure of the complexes141,143... 

Table 5 lists parameters for a selection of halogen-bridged species with this type of asymmetry. The effect of multiply bonded oxygen is larger than the other ligands listed, and the comparison of oxide as part of the ligand, in the pentane-2,4-dionate complex,27 where the asymmetric Mo—Cl distances only differ by 0.091 A, with the oxide trichloride, where the difference is 0.44 A, is particularly striking. 

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