Publisher Summary This chapter explores that the term “four-electron donor,” which is used to describe the alkyne ligand in circumstances where alkyne π ⊥ donation supplements classic metal–olefin bonding. The utility of this scheme lies in its simplicity, and with some reluctance is relied on the “four-electron donor” terminology to suggest global properties of metal alkyne monomers. The general implications and specific hazards characterizing these descriptors are typical of broad classification schemes in chemistry—they are often conceptually helpful but seldom specifically correct. Criteria for recognizing four-electron alkyne donation encompass stoichiometry, structure, spectra, and reactivity. The chapter reviews that the chemistry that has been developed for molybdenum (II) and tungsten (II) alkyne monomers encompasses syntheses, structures, spectra, molecular orbital descriptions, and reactions. The Mo (II) and W (II) complexes addressed in the chapter are not unique in terms of alkyne π ⊥ donation. Related alkyne chemistry is appearing for d4 metals other than molybdenum and tungsten, as well as for d 2 complexes in general. The chapter also examines that chromium alkyne chemistry and reflects the importance of π ⊥ donation, but the stoichiometries differ from those of heavier Group VI monomers.

Four-electron alkyne ligands in molybdenum(II) and tungsten(II) complexes

New Yttrium Complexes Bearing Diamidoamine Ligands as Efficient and Diastereoselective Catalysts for the Intramolecular Hydroamination of Alkenes and Alkynes

Early transition metal hydride complexes: synthesis and reactivity

Photochemical reactivity associated with metal–hydrogen bonds is widespread among metal hydride complexes and has played a critical part in opening up C–H bond activation. It has been exploited to design different types of photocatalytic reactions and to obtain NMR spectra of dilute solutions with a single pulse of an NMR spectrometer. Because photolysis can be performed on fast time scales and at low temperature, metal-hydride photochemistry has enabled determination of the molecular structure and rates of reaction of highly reactive intermediates. We identify five characteristic photoprocesses of metal monohydride complexes associated with the M–H bond, of which the most widespread are M–H homolysis and R–H reductive elimination. For metal dihydride complexes, the dominant photoprocess is reductive elimination of H2. Dihydrogen complexes typically lose H2 photochemically. The majority of photochemical reactions are likely to be dissociative, but hydride complexes may be designed with equilibrated excite...

Photochemistry of transition metal hydrides

Metallorganische verbindungen der lanthanoide

The formation and molecular structure of di-η5-cyclopentadienyl{2-1(dimethylamino)methyl]phenyl-C,N}yttrium

The reaction of (eta/sup 5/-C/sub 5/H/sub 5/)/sub 2/TaCl/sub 2/ with NaBH/sub 4/ followed by the addition of triethylamine and carbon monoxide yields (eta/sup 5/-C/sub 5/H/sub 5/)/sub 2/Ta(CO)H. The molecular structure of (eta/sup 5/-C/sub 5/H/sub 5/)/sub 2/Ta(CO)H has been established by single-crystal X-ray diffraction methods. The compound crystallizes in the monoclinic space group P2/sub 1//n with a = 6.147 (3) angstrom, b = 13.864 (6) angstrom, c = 11.801 (5) angstrom, ..beta.. = 100.53 (3)/sup 0/, and rho/sub calcd/ = 2.28 g cm/sup -3/ for Z = 4. The structure was refined to an R value of 0.089 based on 1358 observed reflections. The tantalum-bound hydrogen atom was located on a difference electron density map and fixed Ta-H length of 1.45 angstrom (eta/sup 5/-C/sub 5/H/sub 5/)/sub 2/Ta(CO)H is not isostructural with the earlier reported niobium analogue. Photolyses of (eta/sup 5/-C/sub 5/H/sub 5/)/sub 2/MH/sub 3/ or (eta/sup 5/-C/sub 5/H/sub 5/)/sub 2/M(CO)H (M = Nb, Ta) in aromatic solvents lead to a common intermediate, (eta/sup 5/-C/sub 5/H/sub 5/)/sub 2/MH, that can react with monoxide, hydrogen, or triethylphosphine. These photogenerated species are also capable of inserting into the carbon-hydrogen bonds of benzene and catalyzing the photoassisted H/D exchange between molecular hydrogen and benzene.

Photoinduced reactions of (.eta.5-C5H5)2MH3 and (.eta.5-C5H5)2M(CO)H (M = Nb, Ta) and the molecular structure of (.eta.5-C5H5)2Ta(CO)H

Photo-degradation studies on Di-η5-Cyclopentadienyldimethylvanadium, Di-η5-Cyclopentadienylmethylvanadium, Di-η5-Cyclopentadienyldimethylniobium, and some deuterated analogs

Donald F. Foust

Papers

The formation and molecular structure of di-η5-cyclopentadienyl{2-1(dimethylamino)methyl]phenyl-C,N}yttrium

Photoinduced reactions of (.eta.5-C5H5)2MH3 and (.eta.5-C5H5)2M(CO)H (M = Nb, Ta) and the molecular structure of (.eta.5-C5H5)2Ta(CO)H

Photo-degradation studies on Di-η5-Cyclopentadienyldimethylvanadium, Di-η5-Cyclopentadienylmethylvanadium, Di-η5-Cyclopentadienyldimethylniobium, and some deuterated analogs

The formation and molecular structure of bis(η5-cyclopentadienyl)bis(pentafluorophenyl)-vinylenevanadium: an acetylene derivative of vanadocene

Photochemical and thermal reactions of η5-cyclopentadienyltetracarbonylvanadium and bis(pentafluorophenyl)acetylene