Non-local density functional calculations have been carried out on the electronic and molecular structures of (C5H5)M(L) (L=CO,PH3;M=Rh,Ir) (a) and M(CO)4 (M=Ru,Os) (b).All systems are found to have a singlet ground state.Optimized geometries are reported for each system on the singlet ground state as well as the first excited triplet state.The coordinatively unsaturated 16 electron species XnM = a,b are usually generated from the 18-electron systems XnMY by photolytic (or in some cases thermal) dissociation of Y.Calculated dissociation energies are presented for Y=CO, PH3, and H2 in the case of XnM = a and for Y=CO,H2 in the case of XnM = b. Complete reaction profiles have been calculated for the oxidative addition of H2 and CH4 to a and b.The addition reactions are found to be more facile for a than for b .It is argued that a is unique as a C-H activating agent in having only empty s-type d-based orbitals interacting with the incoming C-H bond.Most other mono-nuclear d8-systems ,such as b ,have empty as well as occupied s-type metal-based orbitals and the latter will impede the addition reaction.It is futher argued that the high energy of the HOMO on a aids in the addition of H-H and H-CH3 bonds to Cp(L)M.Calculations are presented on the reaction enthalpies of the H-H and C-H addition processes along with the M-H and M-CH3 bond energies .The 5d-elements are found to form stronger bonds than their 4d-congeners as a result of relativistic effects as well as better bonding overlaps.Geometry optimizations were carried out on the di-hydride and hydrido-alkyl complexes .Approximate transition state structures are presented for the C-H addition reactions.