The role of saturated transition metal carbonyl complexes as π-acid ligands; a theoretical investigation of the bonding in [Cr(µ-CO)<sub>2</sub>(CO)(η-C<sub>6</sub>H<sub>6</sub>)Rh(CO)(η-C<sub>5</sub>Me<sub>5</sub>)] and (Co)(µ-CO)<sub>2</sub>(η-C<sub>5</sub>H<sub>5</sub>)Rh(PH<sub>3</sub>)(η-C<sub>5</sub>H<sub>5</sub>)]