Abstract
A useful measure of the total MoMo bonding in diverse phases containing Mo 6 Y 8-type clusters ( Y = S, Se, Cl, Br) is given by the Pauling bond order sum per electron ( PBO e ). These fall into two classes: (a) strongly bonded examples with PBO e values near 1.00 which contain either discrete clusters with extra outer (exo) atoms or infinite confacial clusters, and (b) the rhombohedral Mo 6 Ch 8 and M x Mo 6 Ch 8 Chevrel phases with reduced PBO e values of 0.72–0.84 in which face-capping chalcogenide ( Ch) must also fill exo positions. The matrix effect in the latter which is responsible for the reduced bond orders arises from a combination of particularly strong Mo Ch intercluster bonds and closed-shell Ch Ch repulsions [3.31 å (S), 3.38 å (Se)], which force an elongation of the Mo 6 trigonal antiprism and reduce the MoMo bonding. There is no distinction between sulfide and selenide Chevrel phases in the degree of total MoMo bonding as expressed in bond orders. Changes in the structure on reduction are analyzed; the major effects come from loosening of the intercluster Mo Ch bonding and thence a decreased distortion (matrix effect) together with a flexing of the Mo 6 Ch 8 host according to the size and charge of M. Distance considerations indicate substantial covalency between some M and Ch2, especially Pb and Ag, while cell volumes and Mo Ch 2 distances suggest significant constriction occurs in phases with higher charged M, possibly owing to compression by the host lattice and coulombic contributions to binding. Evidence for Mo M bonding with M = Fe, Co, Ni is also noted. Intercluster interactions and MoMo bonding in Mo 6S 6Br 2 and in the mixed Mo 6 Ch 8Mo n Ch m ( n = 9,12) cluster phases are quite consistent with those in the Chevrel phases, the total MoMo bonding per electron increasing with cluster condensation owing principally to reduce Ch Ch repulsions.
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have