Considerable interest in the chemistry of metal and mixed metal carbonyl clusters lies in their catalytic activities [1-2]. Tetranuclear mixed metal carbonyl clusters of group VIII (B) have a structure similar to Co4(CO)12in which three and nine carbonyl groups are placed in bridging and terminal sites, respectively. The experimental results show that the substitution of carbonyl group with p-donor ligandsoccurs only at the terminal carbonyl groups [3-4]. Since the change of ligand position and replacement of metal centers in apical or basal positions produce various structures with different stability energies, the exploring of the most stable structure based on theoretical calculations can be valuable in the chemistry of carbonyl clusters. In this regard we have performed the systematic theoretical studies on the 28 isomeric structures for each of [Co2 Rh2(CO)10(P(OMe)3)2] (I) and [Co2 Rh2(CO)10(PF3)2] (II) compounds. These calculations have been done by using density functional theory (DFT) with B3LYP/LanL2DZ method. In this study investigation of the geometric structure, stability and IR spectroscopic properties of titled compounds revealed that the stability order of the structures is the same in two compounds (Fig.1). Natural bond orbital (NBO) analysis indicated that the natural charges of oxygen atoms in (I) are comparable with(II) and basicity of the oxygen sites in both cases are the same with the following order: Ob(Co-Rh)>Ob(Rh -Rh)>Ot(eq)>Ot(ax) .

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