Neutron structure and inelastic-neutron-scattering and theoretical studies of Mo(CO)(H[sub 2])[(C[sub 6]D[sub 5])[sub 2]PC[sub 2]H[sub 4]P(C[sub 6]D[sub 5])[sub 2]][sub 2][center dot]4. 5C[sub 6]D[sub 6], a complex with an extremely low barrier to H[sub 2] rotation. Implications on the reaction coordinate for H-H cleavage to dihydride

Journal Article · Wed Jan 27 00:00:00 EST 1993 · Journal of the American Chemical Society; (United States)

DOI:https://doi.org/10.1021/ja00055a029· OSTI ID:6566944

Kubas, G J; Burns, C J; Eckert, J; Johnson, S W; Larson, A C; Vergamini, P J; Unkefer, C J ^[1]; Khalsa, G R.K. ^[2]; Jackson, S A; Eisenstein, O ^[3]

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1. Los Alamos National Lab., NM (United States)
2. Thiel College, Greenville, PA (United States)
3. Universite de Paris-Sud, Orsay (France)

The synthesis and characterization of derivatives of Mo(CO)(R[sub 2]PC[sub 2]H[sub 4]PR[sub 2])[sub 2] (R = Et, i-Bu, Ph, Et-Ph) and their reactions with H[sub 2], N[sub 2], and SO[sub 2] are reported. For R = Et and i-Bu, the H[sub 2] oxidatively adds to give dihydrides, but for R = Ph, a [eta][sup 2]-H[sub 2] complex is formed. Single-crystal neutron diffraction of Mo(CO)(H[sub 2])(Ph[sub 2]PC[sub 2]H[sub 4]PPh[sub 2])[sub 2] (as a 4.5-benzene solvate with all Ph groups deuterated) at 12 K showed the H-H bond to be oriented trans to the CO and parallel to a P-Mo-P axis, with a length close to that of free H[sub 2] (0.74 [Angstrom]). However, the thermal ellipsoids were very large, and inelastic neutron scattering showed that the barrier to rotation of the H[sub 2] is the lowest yet measured, ca. 0.7 kcal/mol. These observations indicate that librational motion of the H[sub 2] is artificially foreshortening the H-H bond length. Application of a correction procedure gave a distance of 0.80-0.85 [Angstrom] as being more likely. Extended Huckel calculations successfully modeled the H[sub 2] coordination and also showed a low rotational barrier (1.4 kcal/mol). Theoretical considerations suggest that the degree of distortion of the MP[sub 4] skeleton is largely responsible for the ability of the complex to bind molecular hydrogen and controls the amount of back-bonding from the metal d-orbital to H[sub 2] [sigma][sup *]. The lack of an elongated H-H bond length or equilibrium with a dihydride tautomer, despite the apparent nearness of the H[sub 2] to cleavage, leads to the conclusion that the reaction coordinate for oxidative addition of H[sub 2] is rather flat until relatively precipitous cleavage of the H[sub 2]. Mo(CO)(H[sub 2])[(C[sub 6]D[sub 5])[sub 2]PC[sub 2]H[sub 4]P(C[sub 6]D[sub 5])[sub 2]][sub 2][center dot]4.5C[sub 6]D[sub 6] crystallizes in the space group P[bar 1]. 45 refs., 9 figs., 4 tabs.

• OSTI ID: 6566944
• Journal Information: Journal of the American Chemical Society; (United States), Vol. 115:2; ISSN 0002-7863
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
HYDROGEN
ROTATION
HYDROGEN COMPOUNDS
MOLECULAR STRUCTURE
MOLYBDENUM COMPOUNDS
BOND LENGTHS
BONDING
CARBONYLS
CHEMICAL PREPARATION
CLEAVAGE
CRYSTAL STRUCTURE
DEUTERIUM COMPOUNDS
INELASTIC SCATTERING
MATHEMATICAL MODELS
NEUTRON DIFFRACTION
NITROGEN
ORGANOMETALLIC COMPOUNDS
PHOSPHINES
SULFUR DIOXIDE
CHALCOGENIDES
COHERENT SCATTERING
DIFFRACTION
DIMENSIONS
ELEMENTS
FABRICATION
JOINING
LENGTH
MICROSTRUCTURE
MOTION
NONMETALS
ORGANIC COMPOUNDS
OXIDES
OXYGEN COMPOUNDS
PHOSPHORUS COMPOUNDS
REFRACTORY METAL COMPOUNDS
SCATTERING
SULFUR COMPOUNDS
SULFUR OXIDES
SYNTHESIS
TRANSITION ELEMENT COMPOUNDS
400201* - Chemical & Physicochemical Properties
400101 - Activation
Nuclear Reaction
Radiometric & Radiochemical Procedures