Hydrides of the Main-Group Metals: New Variations on an Old Theme

Carbon-hydrogen-transition metal bonds

Carbon-hydrogen bonds, especially those of saturated (s$) carbon centres, are normally regarded as being chemically inert. Generally, the C-H group is not thought of as a potential ligand which can have a structural role or play an energetically significant part in ground states or in reaction intermediates. In this article we review recent observations which show that there are in fact many circumstances in which a carbon-hydrogen group will interact with a transition metal centre with formation of a two-electron three-centre bond and that the extent of the interaction is such as to have a marked effect on the molecular and electronic structure and hence reactivity of the molecule. We propose the term “agostic” which will be used to discuss the various manifestations of covalent interactions between carbon-hydrogen groups and transition metal centres in organometallic compounds l . The word agostic will be used to refer specifically to situations in which a hydrogen atom is covalently bonded simultaneously to both a carbon atom and to a transition metal atom. The agostic C-H-M bond is similar to the familiar and long known bridging hydrogen systems which occur in B-H-B, M-H-M, and B-H-M groups. The C-H-M bonds are a recent observation but as will be discussed below, they are probably very much more common than hitherto suspected. Since carbon-hydrogen bonds are a ubiquitous feature of organometallic compounds we have found it useful to define

Carbon-Hydrogen-Transition Metal Bonds

The ground state structures of disilane, methyl silane and the silyl halides, and an SiH bond length correlation with stretching frequency

A detailed analysis of the vibrational spectra of CH3, CH2D, and CD3 adsorbed on Ni(111) and the products of their reactions is presented. The synthesis of adsorbed methyl radicals from CH4, CH3D, or CD4 is effected by molecular beam techniques. The ability to measure these spectra by high‐resolution electron energy loss spectroscopy (HREELS) at higher resolution (35 cm−1) and higher sensitivity (5×106 counts/s) has allowed new features to be observed and a symmetry analysis to be carried out. It is concluded that the CH3 radical is adsorbed with C3v symmetry on a threefold hollow site. The symmetric C–H stretch mode of CH3 and the overtone of the antisymmetric deformation mode are observed to be in Fermi resonance. At temperatures above 150 K, CH3 dissociates to form adsorbed CH. Confirmation for the assignment to a CH species is found in the observation that the spectrum measured after thermal decomposition of CH2D is a superposition of those from the decomposition of CH3 and CD3. The adsorption site of...

The structure and chemistry of ch3 and ch radicals adsorbed on ni(111)

Vibrational spectra of cis and trans but-2-enes: assignments, isolated CH stretching frequencies and CH bond lengths

Fourier transform infrared spectroscopic analyses of the v12 fundamentals of C2H4 and C2D4

SiH stretching anharmonicity, local-mode behaviour and dissociation energies in silane and methyl silanes

The infrared spectra of /sup 16/CH/sub 3/, /sup 13/CH/sub 3/, /sup 12/CD/sub 3/, and /sup 13/CD/sub 3/ species of MeMn(CO)/sub 5/ and MeRe(CO)/sub 5/ have been studied in the gas phase to 450 cm/sup -1/ and in the solid at 78 K down to 200 cm/sup -1/. Solid-phase Raman spectra (600-300 cm/sup -1/) for MeRe(CO)/sub 5/ are also reported. Nearly all of the delta(MCO) and nu(MC) modes are assigned with confidence in both compounds, and two reassignments are proposed for MeMn(CO)/sub 5/. An energy-factored force field is calculated for the CO stretching vibrations by using gas-phase data which include the B/sub 1/ fundamental. An A/sub 1/ force field for all vibrations shows that all the metal-carbon bonds increase in strength from Mn to Re, while the methyl CH bond is weakened. The axial and equatorial M-CO bonds in MeRe(CO)/sub 5/ are equal in strength, suggesting a negligible trans effect on the part of the methyl ligand. This is interpreted in terms of equal and opposite ..pi..- and sigma-trans effects, the former strengthening the axial Re-CO bond and the latter weakening it. The gas-phase contour of the absorption complex assigned to rho(CH/sub 3/) in MeRe(CO)/sub 5/ resembles those due to nu/submore » as/(CH/sub 3/) and nu/sub as/(CD/sub 3/) and is attributed to a variation in the rocking force constant with internal rotation angle, together with free internal rotation. The corresponding band in MeMn(CO)/sub 5/ may also reflect the same effect. 32 references, 5 figures, 9 tables.« less

Vibrational spectra and assignments of MeMn(CO)5 and MeRe(CO)5 species, energy-factored and A1 force fields, and a further effect of free internal rotation

La constante de force de la vibration CH varie avec l'angle de rotation interne d'un groupe methyle. La barriere de rotation est probablement inferieure a 10 cm −1 . Les principales longueurs de liaison CH sont de 1,095 (pour Mn) et 1,098 (pour Re) avec une variation de 0,004 a 0,006 A lors de la rotation

A.R. Morrisson

Papers

Vibrational spectra of cis and trans but-2-enes: assignments, isolated CH stretching frequencies and CH bond lengths

Fourier transform infrared spectroscopic analyses of the v12 fundamentals of C2H4 and C2D4

SiH stretching anharmonicity, local-mode behaviour and dissociation energies in silane and methyl silanes

Vibrational spectra and assignments of MeMn(CO)5 and MeRe(CO)5 species, energy-factored and A1 force fields, and a further effect of free internal rotation

Infrared spectra of methylmanganese pentacarbonyl and methylrhenium pentacarbonyl species: methyl group geometry and the effects of internal rotation