The present study deals with the small number of structurally characterized 64 → 60e- tetranuclear clusters containing metals of the groups 7 → 10 and having in common the primary feature of planarity. The M4 skeleton has a variety of shapes: square, rhombus, or more generally quadrilateral. Empirical electron counting rules do not always provide a sufficient description of the bonding in these two-dimensional compounds nor is the viewpoint unique for different cases. In order to determine the electronic distribution, hence the trends of the M4 skeleton to deform, extended Hückel calculations have been carried out for a number of models. The nature of the outer M-M σ interactions and the distribution of the metal lone pairs are illustrated for 64e- square clusters. Then, it is shown that 62e- M4(CO)16 rhombuses correlate ideally with 64e- squares through the formation of one diagonal bond. Five M-M bonds are also ascertained in the 62e- quadrilateral Os4(CO)15 that is short one ligand. In any case, the fifth bond is shown to involve "t2g" orbitals in an unforeseen way. The distribution of the M-M bonds in 64e- clusters, containing π-donor coplanar phosphido bridges, is irregular. In Ru4(CO)13(μ-PPh2)2, formed by two condensed triangles sharing one side, only two outer consecutive Ru-Ru linkages have bond order 1, whereas the other three (including the quadrilateral's diagonal) are assigned bond order 2/3. In Ru4(CO)10(μPPh2)4 the M4 bonding network is something intermediate between two limiting viewpoints. The first one assigns a total of three bonds to the four sides of the Ru4 rhombus. The fourth linkage, coinciding with one diagonal, is best described as an uncommon four electron/three orbitals bond. The other description indicates that a fourth cyclic M-M bond is formed at expenses of part of the M-P bridge-bonding network. The theoretical result has also an experimental confirmation. Finally, there are planar tetranuclear clusters having all of the metal atoms square planarly coordinated by ligands and a reduced number of M-M connectivities. Among the latter, Ni4(μ-SR)8 (64 electrons) has a square Ni4 skeleton but no M-M bond, whereas Pd4(μ-CO)4(μ-CH3CO2) 4, with 60 electrons, is a parallelogram with two M-M bonds at those sides doubly bridged by the carbonyl ligands.
|Titolo:||INTERMETAL BONDING NETWORK IN 2-DIMENSIONAL TETRANUCLEAR CLUSTERS|
PROSERPIO, DAVIDE MARIA (Ultimo)
|Settore Scientifico Disciplinare:||Settore CHIM/03 - Chimica Generale e Inorganica|
|Data di pubblicazione:||1990|
|Digital Object Identifier (DOI):||10.1021/ja00170a012|
|Appare nelle tipologie:||01 - Articolo su periodico|