PP

P P

P+

+

d-orbitals * orbitals acceptor MOs

P

CoI

Et3P PEt3

CoII

Et3P PEt3

- e-

Phosphines:PR3 - very important ligands

- -donors - -acceptors

For years, it was assumed that -backdonation occurred from the metal into empty d‑orbitals on phosphorus.

Actually: -backdonation occurs into MOs formed by combination of two d‑orbitals on phosphorus and the * orbitals involved in P-R bonding.

Each acceptor MO has 2 lobes (similar to a d-orbital) but is antibonding with respect to the P-R bond

As the amount of -backdonation increases, the length of P-R increases. This can be observed in the crystal structures of phosphine complexes

Average Co-P 221.8 pm 223.0 pmAverage P-C 184.6 pm 182.9 pm

A huge variety of phosphines have been prepared, many of which are commercially available. One can just choose a phosphine with the desired steric (size) and electronic properties (‑donation/‑acceptance).

PM

228 pm

The size of ligands (not just phosphines) can be measured using the concept of a cone angle (Chadwick A. Tolman, Chem. Rev., 1977, 313).

- TM-P distance fixed at 228 pm to standardize the cone angle ().- The cone encompases the van der Waals radii of the outermost atoms of the ligand.

Cone angles measured by crystallography (although TM-P bond distance fixed at 228 pm)

Electronic properties measured by IR and electrochemistry – (CO) and E1/2 for oxidation or reduction measured for a large series of carbonyl

phosphine complexes with different PR3 (e.g. [Ni(CO)3(PR3)] or

[CpFeMe(CO)(PR3)]) [ = old fashioned abbreviation for phenyl]

x-axis = cone angle () y-axis = (CO) for [Ni(CO)3L]

Typical Ligands – 2 (Phosphines) - Bite angles for bidentate ligands

P

M

Pn

n = bite angle = ligand-metal-

ligand angle for bidentate ligands

O

t-But-Bu

Ph2P PPh2Rh

PPh3

H

OC

Ph2P PPh2

Rh

PPh3

H

OC

DPPM Xantphos

small bite angle = large bite angle

= 72 o

= 111 o

Bite angle can have a profound effect on the rate and selectivity of metal catalyzed reactions

Reviews: (1) P. C. J. Kamer, P. W. N. M. van Leeuwen, Wide Bite Angle Diphosphines: Xantphos Ligands in Transition Metal Complexes and Catalysis, Acc. Chem. Res., 2001, 895-904; (2) Z. Freixa, P. W. N. M. van Leeuwen, Bite angle effects in diphosphine metal catalysis: steric or electronic?, Dalton Trans., 2003, 1890-1901; (3) P. W. N. M. van Leeuwen, Ligand Bite Angle Effects in Metal-catalyzed C-C bond formation, Chem. Rev., 2000, 2741-2769.

Phosphines/Arsines in early TM and lanthanide chemistry

• PR3 excellent ligands for soft late transition metals

• PR3 typically NOT v. suitable to form complexes with hard early TMs (PR3 > PAr3)

• AsR3 even worse, and early TM SbR3 or BiR3 complexes unknown

• NPh3 useless as a ligand for early or late TMs (terrible donor), NMe3 OK for early TMs

R. G. Cavell (U of A), OM, 1991, 539

•Route to early/late heterobimetallics

M. D. Fryzuk (UBC), OM, 2005, 1112

2. xs TMS-Cl

•Hard/Soft donor combination

G. Reid (U Southampton),DT, 2004, 3005 & EJIC, 2001, 2927

•Rare early TM (Ti, Zr, Hf) arsine complexes