BONDING AND STRUCTURE IN L4M2(MU-XRN)2 DIAMONDS OF TETRAHEDRAL D10 IONS - EFFECT OF SUBSTITUENTS ON THE M- -M INTERACTION

The large range of cross-ring M- -M distances found in the L4M2(mu-XR(n))2 diamonds of tetrahedrally coordinated d10 ions (M = Cu(I), Zn(II), Cd(II), Hg(II); n = 1-3) can be related to the number of electrons involved in the sigma-framework bonding. Framework electron counts (FEC) of 4 or 6 give rise to cross-ring bonding, whereas an FEC of 8 results in approximately square, cyclobutane-like structures. The bonding in diamonds with an FEC of 4 or 6 is electron-deficient, with an increasing contribution of d10-d10 interactions at short Cu- -Cu distances. The nature of the bridging XR(n) ligand and its orientation are crucial in determining the number of framework electrons, hence the existence or not of M- -M or even X- -X bonding. The unsubstituted bridges (n = 0), on the other hand, present a soft potential for ring squeezing and an electron count-independent variability of the M- -M distances. The framework electron count (FEC) is based on orbital topology and can be applied to all diamonds [(ML(n))k(XR(m))4-k], where M is a d10 metal ion, X is a main group element atom, and k = 0-4.