Question

Q3 (5 points) Discuss the bonding between CO, the prototypical ft-acid ligand, and d-metals in octahedral transition metal carbonyl complexes?

Answer 1

AN IMPORTANT FACTOR THAT CONTRIBUTES TO THE HIGH LIGAND FIELD STRENGTH OF LIGANDS SUCH AS CO,CN-,AND PHOSPHINES IS $\pi$ BONDING BETWEEN METAL AND THE LIGAND.THERE ARE THREE TYPE $\pi$ BONDINF IN METAL CO COMPLEX.

THE MOST COMMON SITUATION IS WHEN A LIGAND SUCH AS CARBON MONOXIDE OR CYANIDE DONATES ITS SIGMA (NONBONDING) ELECTRONS TO THE METAQL , WHILE ACCEPTING ELECTRON DENSITY FROM THE METAL THROUGH OVERLAP OF AMEGTAL T2g orbital AND A LIGAND $\pi$ * THIS SITUATIO N IS CALLED "BACK BONDING" BECAUSE THE LIGAND DONATES $\alpha$ ELECTRON DENSITY TO THE METAL AND THE METAL DONATES $\pi$ ELECTRON DENSITY TO THE LIGAND.THE LIGAND IS THUS ACTING AS A $\pi$ ACCEPTOR   AND $\alpha$ DONOR. IN $\pi$ BACK BONDING , THE METAL DONATES $\pi$ ELECTRON TO THIS LIGAND $\pi$ *  ORBITAL ADDING ELECTRON DENSITY TO AN ANTIBONDING MOLECULAR ORBITAL. THIS RESULTS IN WEAKENING OF THE CO BOND,WHICH IS EXPERIMENTALLY OBSERVED AS LENGTHENINGT OF THE BOND.

D-D BONDING OCCURS WHEN AN ELEMENT SUCH PHOSPHORUS ,WHICH HAS A$\alpha$ SYMMETRY LONE PAIR AND AN EMPTY METAL 3D ORBITAL,BINDS TO A METAL THAT HAS ELECTRONS IN A T2g   ORBITAL.THIS IS A COMMON SITUATION FOR PHOSPHINE COMPLEXES ( EG: TRIPHENYLPHOSPHINE) BOUND TO LOW - VALENT ,LATE TRANSITION METAL.CO EXAMPLE THE ACCEPTOR ORBITAL IS A PHOSPHORUS 3D ORBITAL RATHER THAN A LIGAND $\pi$ * ORBITAL.