TY - JOUR
T1 - The Problem of Metal-Metal (MM) Bond Alternation in [MX2(μ-X)4/2]∞ Chain Polymers
AU - Shaik, Sason S.
AU - Bar, Rony
PY - 1983/3
Y1 - 1983/3
N2 - The factors that influence MM bond alternation in trans edge-sharing polyoctahedra, [MX2(μ-X)4/2]∞, are investigated with use of the oligomer M3X14 as a model. The energies of the various oligomers are investigated as a function of the two independent X-M-X equatorial angles. It is shown that the oligomer with the bare core metals, i.e., the d°-d°-d° case, prefers a structure with two symmetric bridges (equal MM distances) while the d-block electrons tend to prefer asymmetric bridges (unequal MM distances). The analysis leads to one basic principle: polymers will exhibit MM bond alternation whenever the d-block orbitals are able to take advantage of relatively low energy exit routes in the [d°]∞, core. In trans edge-sharing [MX2(μ-X)4/2]∞ polymers, where X is a donor like a halogen atom, the d-block electrons are stabilized enough by bond alternation and they can take advantage of the shallow walls of the core in this direction. Therefore, all the d counts d1-d5 are predicted to exhibit bond alternation. An alternative structure with a uniform but long MM distance is also available for the d4 and d5 cases. In d6 polymers the d-block electrons show no tendency for bond alternation and as a result one expects to find only the structure with a uniform MM distance. Bond alternation is not a necessary feature of one-dimensional polymers. It can be circumvented by appropriate design. Thus, substituting the axial ligands by acceptors is expected to generate [M(CO)2(μ-X)4/2]∞ polymers with uniform, though long, MM distances for the d counts d1-d4, d6, and d7 or at least to reduce the extent of bond alternation. It is suggested that utilizing bridge ligands with high ligand-ligand overlap repulsion should generate polymers with uniform and short MM distances.
AB - The factors that influence MM bond alternation in trans edge-sharing polyoctahedra, [MX2(μ-X)4/2]∞, are investigated with use of the oligomer M3X14 as a model. The energies of the various oligomers are investigated as a function of the two independent X-M-X equatorial angles. It is shown that the oligomer with the bare core metals, i.e., the d°-d°-d° case, prefers a structure with two symmetric bridges (equal MM distances) while the d-block electrons tend to prefer asymmetric bridges (unequal MM distances). The analysis leads to one basic principle: polymers will exhibit MM bond alternation whenever the d-block orbitals are able to take advantage of relatively low energy exit routes in the [d°]∞, core. In trans edge-sharing [MX2(μ-X)4/2]∞ polymers, where X is a donor like a halogen atom, the d-block electrons are stabilized enough by bond alternation and they can take advantage of the shallow walls of the core in this direction. Therefore, all the d counts d1-d5 are predicted to exhibit bond alternation. An alternative structure with a uniform but long MM distance is also available for the d4 and d5 cases. In d6 polymers the d-block electrons show no tendency for bond alternation and as a result one expects to find only the structure with a uniform MM distance. Bond alternation is not a necessary feature of one-dimensional polymers. It can be circumvented by appropriate design. Thus, substituting the axial ligands by acceptors is expected to generate [M(CO)2(μ-X)4/2]∞ polymers with uniform, though long, MM distances for the d counts d1-d4, d6, and d7 or at least to reduce the extent of bond alternation. It is suggested that utilizing bridge ligands with high ligand-ligand overlap repulsion should generate polymers with uniform and short MM distances.
UR - http://www.scopus.com/inward/record.url?scp=25944473552&partnerID=8YFLogxK
U2 - 10.1021/ic00147a007
DO - 10.1021/ic00147a007
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AN - SCOPUS:25944473552
SN - 0020-1669
VL - 22
SP - 735
EP - 743
JO - Inorganic Chemistry
JF - Inorganic Chemistry
IS - 5
ER -