Chevrel phases: An analysis of their metal-metal bonding and crystal chemistry

Abstract

A useful measure of the total MoMo 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 MoMo bonding. There is no distinction between sulfide and selenide Chevrel phases in the degree of total MoMo 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 MoMo bonding in Mo 6S 6Br 2 and in the mixed Mo 6 Ch 8Mo n Ch m ( n = 9,12) cluster phases are quite consistent with those in the Chevrel phases, the total MoMo bonding per electron increasing with cluster condensation owing principally to reduce Ch Ch repulsions.