CARNEGIE MELLON UNIVERSITY

09-105 Introduction to Modern Chemistry

Quiz #9

December 1, 1998

Name

A hexadentate ligand is sold by a chemical manufacturer under the tradename "Bettis." Co²⁺ forms a hexadentate chelate complex with this ligand and a solution containing this complex ion appears colorless. Can you tell from this information whether "Bettis" is a weak-field ligand or a strong-field ligand? Explain briefly.

Co²⁺ has 25 electrons and a configuration given by [Ar]3d⁷. The two possible configurations in an octahedral environment correspond to weak field and strong field ligands shown here.

The left figure is for a weak field ligand and shows three unpaired spins. The right figure is for a strong field ligand and shows one spin. The transitions corresponding to absorbing light could take place in the infrared on the right and in the ultraviolet on the left, each of which would result in a colorless appearance. You cannot determine the crystal field strength from the information given. ( 4 points )

The magnetic properties of the complex ion mentioned above are found to be identical to those of the complex ion of V²⁺ with carbon monoxide ligands, but it is not known if that complex is octahedral or tetrahedral. Explain briefly what I can be learned from this information, if anything.

V²⁺ has 21 electrons and a configuration given by [Ar]3d³ . Since carbon monoxide is a strong field ligand, the configuration of the vanadium ion in an tetrahedral field and a octahedral field would be as shown below.

The conclusion you should draw is that if this vanadium complex is tetrahedral, then the "Bettis" ligand is a strong field ligand, whereas if the vanadium complex should be determined to be octahedral, then this would imply that the "Bettis" ligand is a weak field ligand according to the possibilities in the first part. ( 6 points )