Quimica

Chapter 1: Introduction—Structure and Bonding Types of Ligand Coordination
Three generic classes of ligands: L: a neutral electron pair donor (i.e., CO, PR3) X: an anionic electron pair donor (i.e., X , H ) M-M: neutral 1 electron donor Distinguishing L-type ligands from X-type ligands: To distinguish between X and L type ligands, consider removing ligand from metal center with the ligand takingthe electron pair in the M-L bond. If the ligand in it's free state is neutral, it is an L-type ligand. If it would be anionic, it is X-type.
CH3 X-type

CH3 L-type N N Pd PMe3 Cl PMe3 L-type

Cl—

X-type

Ligands that can donate more than one pair of electrons can be classified using L and X designations:
CH3 O L2 4 e donor N R LX 4 e donor H3C H3C CH3 CH3 L2X 6 e donor

Types ofligand coordination: Terminal: Ligand is bound to only one metal center (L-M or X-M) Bridging (µ): Ligand is attached to two different metal centers (M-L-Mʼ or M-X-Mʼ). For L-type ligands, the lone pair is usually shared between the two metals (count 1 electron for each metal). For X ligands, the lone pair can also be shared. If the X ligand has additional lone pairs (i.e., halide, alkoxide), thenthe additional lone pairs can be used to coordinate to the second metal center. Hapticity (η): Ligand attached to a metal center through more than one atom. Generally used to describe ligands with conjugated π-systems

M !4

M !5

M !6

Prepared by Dr. Kevin Shaughnessy, Spring 2011 The University of Alabama, Tuscaloosa, AL

Chapter 1: Structure and Bonding: 2

Hapticity for ligandssuch as Cp can be variable depending on how it is coordinated. Usually Cp 5 3 1 is a η -L2X ligand, but it can also coordinate as an η -LX, or even an η -X. The process of going 5 3 from η to η is often called ring slippage. Chelation: Ligand attached through more than one atom usually separated by one or more atoms. Chelating ligands are sometimes classified as being bidentate (2 points ofattachment), tridentate (three points of attachment), or tetradentate (4 points of attachment).
Cl Me2 N Pt Cl N Me2 !2N-coordination bidentate ligand Cl S P Fe S S !P, !3S-coordination tetradentate ligand

Kappa convention (κ): The kappa convention is sometimes used to indicate the coordinating atoms of a polydentate ligand. If only some of the possible coordination sites are bonded to the metal, thecoordinating atoms are indicated with a kappa. Some authors use kappa to indicate how many of the coordinating atoms are attached in a polydentate ligand.
!3-[N,N'-di(2-aminoethyl)ethane 1,2diamine]chloroplatinum ion or [N-(2-amino-!N-ethyl)-N'-(2aminoethyl)ethane 1,2-diamine!2N,N']chloroplatinum ion

NH Pt N NH H2 Cl NH2

The 18-Electron Rule Recall: First row elements have 4 valence orbitals(1 s + 3 p) so they can accommodate up to 8 valence electrons--the octet rule. Transition metals have 9 valence orbitals (1 s + 3 p + 5 d). Upon bonding to a ligand set, there will be a total of 9 low lying orbitals (see MO theory discussion below). Therefore, we can expect that the low lying MOs can accommodate up to 18 valence electrons--The 18-Electron Rule. Organometallic complexes with 18electrons are predicted to be particularly stable because they will have a closed shell of electrons. Complexes with 18 electrons are often referred to as being coordinatively saturated. Counting electrons: There are two models for counting electrons. Both give the same answer, but offer different advantages and disadvantages. Example: CH4 Covalent model: Since C-H bonds are covalent, assume thatthe electrons are shared equally between carbon and hydrogen. To count the electrons, we dissect the molecule giving each atom 1 electron of the bonding pair.

Prepared by Dr. Kevin Shaughnessy, Spring 2011 The University of Alabama, Tuscaloosa, AL

Chapter 1: Structure and Bonding: 3

H H H C H H H C H
Ionic model: Alternatively, we can treat the bonds as being ionic. This allows us to...