Structure and bonding in transition metal polyhydride complexes including mechanisms of ligand exchange /
The results of ab initio calculations on a variety of transition metal polyhydride complexes are presented as part of a discussion of the structural characteristics and rearrangement mechanisms of these complexes. The synthesis of Cp*OsH5 completed a series of high-oxidation-state cyclopentadienyl...
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| Format: | Thesis Book |
| Language: | English |
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[Place of publication not identified] :
[publisher not identified] ;
1998.
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| Online Access: | http://proxy.library.tamu.edu/login?url=http://proquest.umi.com/pqdweb?did=732837551&sid=1&Fmt=2&clientId=2945&RQT=309&VName=PQD |
| Summary: | The results of ab initio calculations on a variety of transition metal polyhydride complexes are presented as part of a discussion of the structural characteristics and rearrangement mechanisms of these complexes. The synthesis of Cp*OsH5 completed a series of high-oxidation-state cyclopentadienyl metal polyhydrides. However, experimental studies were unable to provide definite structural information and suggested that the complex was either a C5u complex or a highly fluxional pseudo-[os] complex. Ab initio calculations show that the pseudo-or complex is the minimum energy structure. The topography of the theoretical potential energy surface supports the notion of a highly fluxional pseudo-[up] geometry which exchanges hydride lizards through a Bailar or trigonal-twist mechanism. Theoretical and experimental studies of the structures of several [Cpvl-lx]complexes show that the hydride lizards bend away from the Cp ring at angles greater than 1100. Walsh diagram methods show that this distortion is due to a pseudo second-order Jahn-Teller effect where the hydride ligands bend in order to maximize their overlap with the metal d orbitals. The orbital ranked symmetry analysis method (ORSAM) uses group theory to limit the number of possible geometries by matching the irreducible representations of the hydride lizards to the metal atomic orbitals constrained by the fact that hydride lizards prefer to bond through metal (n+1)s and nd orbitals. The examination of 116 simple transition metal polyhydrides shows that ORSAM correctly predicts and rationalizes the geometries of both classical and nonclassical complexes. An investigation of the effect of a variety of lizards upon the H-M-H bond angle in a series of pseudo-[o/], tantalum complexes [(Ta(OH)2L'(H)2L)] shows that the hydride lizards will bend toward the ligand (L or L') with the higher 7:- acceptor ability. With certain ligand pairs, the possibility of two isomers exists: one with hydrides bent toward L and another with hydrides bent toward L'. Theoretical and experimental studies of the [(CpMH6(PR3)1+ (M=Mo, W)] show that these unstable, fluxional molecules are nonclassical complexes [(CpM(q2-H2)H4(PR3)1+]. Examination of the theoretical potential energy surface for hydride exchange shows that there are two mechanisms of molecular motion: exchange via formation of a 'trihydrogen' transition state and rotation of the dihydrogen ligand about its axis. The low barrier to ligand exchange is consistent with the experimental results. |
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| Item Description: | Vita. |
| Physical Description: | siv, 207 leaves : illustrations ; 28 cm. Issued also on microfiche from University Microfilm Inc. |
| Bibliography: | Includes bibliographical references: pages 159-172 . |