TY - JOUR
T1 - Density-functional study of the ground- and excited-spin states of [M2Cl9]3- (M = Mo or W) face-shared dimers
T2 - Consequences for structural variation in A3M2Cl9 complexes
AU - Stranger, Robert
AU - Macgregor, Stuart A.
AU - Lovell, Timothy
AU - McGrady, John E.
AU - Heath, Graham A.
PY - 1996/12/7
Y1 - 1996/12/7
N2 - The optimized geometries and relative energies for the ground- and excited-spin states (Smax = 0-3) of [M2Cl9]3- (M = Mo or W) have been determined from density-functional calculations. For both systems the calculations predict a dramatic increase in metal-metal distance, with a corresponding increase in M-Clb-M bridge angle, as the dimer spin state (Smax) increases. The terminal MCl3 groups on the other hand are relatively insensitive to changes in the M-M separation. For both [Mo2Cl9]3- and [W2Cl9]3- the spin-singlet structure (Smax = 0) is predicted to be the most stable species when using the local-density approximation (LDA), in agreement with experiment. In contrast, when non-local gradient corrections to the total energy are incorporated, both the spin-quintet (Smax = 2) and -septet (Smax = 3) species are predicted to be more stable than the spin singlet for [Mo2Cl9]3-. The calculated (LDA) singlet geometry for [W2Cl9]3- is in very good agreement with the observed structure whereas for [Mo2Cl9]3- the geometry of the spin-triplet species is closer to experiment. Incorporation of relativistic effects is more significant for [W2Cl9]3- resulting in a further destabilization of the higher-spin states, particularly the spin-quintet and -septet species, relative to the singlet configuration. Fragment analysis showed that the metal-metal and metal-bridge contributions to the total bonding in the higher-spin species counteract each other. The destabilization due to loss of metal-metal bonding in the higher-spin states is greater than the stabilization gained from the enhanced metal-bridge interaction. However, the reduction in the M-M interaction is more pronounced for [W2Cl9]3- and thus its higher-spin states are less accessible than for [Mo2Cl9]3-, accounting for the more dramatic variation in M-M distances observed in A3Mo2Cl9 complexes.
AB - The optimized geometries and relative energies for the ground- and excited-spin states (Smax = 0-3) of [M2Cl9]3- (M = Mo or W) have been determined from density-functional calculations. For both systems the calculations predict a dramatic increase in metal-metal distance, with a corresponding increase in M-Clb-M bridge angle, as the dimer spin state (Smax) increases. The terminal MCl3 groups on the other hand are relatively insensitive to changes in the M-M separation. For both [Mo2Cl9]3- and [W2Cl9]3- the spin-singlet structure (Smax = 0) is predicted to be the most stable species when using the local-density approximation (LDA), in agreement with experiment. In contrast, when non-local gradient corrections to the total energy are incorporated, both the spin-quintet (Smax = 2) and -septet (Smax = 3) species are predicted to be more stable than the spin singlet for [Mo2Cl9]3-. The calculated (LDA) singlet geometry for [W2Cl9]3- is in very good agreement with the observed structure whereas for [Mo2Cl9]3- the geometry of the spin-triplet species is closer to experiment. Incorporation of relativistic effects is more significant for [W2Cl9]3- resulting in a further destabilization of the higher-spin states, particularly the spin-quintet and -septet species, relative to the singlet configuration. Fragment analysis showed that the metal-metal and metal-bridge contributions to the total bonding in the higher-spin species counteract each other. The destabilization due to loss of metal-metal bonding in the higher-spin states is greater than the stabilization gained from the enhanced metal-bridge interaction. However, the reduction in the M-M interaction is more pronounced for [W2Cl9]3- and thus its higher-spin states are less accessible than for [Mo2Cl9]3-, accounting for the more dramatic variation in M-M distances observed in A3Mo2Cl9 complexes.
UR - http://www.scopus.com/inward/record.url?scp=33748389435&partnerID=8YFLogxK
U2 - 10.1039/DT9960004485
DO - 10.1039/DT9960004485
M3 - Article
AN - SCOPUS:33748389435
SN - 0300-9246
SP - 4485
EP - 4491
JO - Journal of the Chemical Society - Dalton Transactions
JF - Journal of the Chemical Society - Dalton Transactions
IS - 23
ER -