The first ab-initio calculations of phosphido-bridged binuclear organo
metallic compounds are reported. By analysis of the influence of elect
ron correlation on the complex molecular structure, conclusions can be
drawn about the nature of the direct bonding interaction between the
two metal centers. The formal M-M bond order overestimates the direct
M-M bonding interaction in the V case but underestimates it in the Cr
and Mn cases.