I am curious why people use Pd Complexes bonded with EWG O-donor ligand AcO-, CF3CO2- or OxSO-. Less people use Pd complexes with EDG alkoxy ligands?? It seems to me d(pi)-p(pi) repulsion may destabilze the Pd complexes. People use EWG O-ligand just for reduce the extend of  d(pi)-p(pi) repulsion? Or other reasons? Stability of the Pd complexes? Welcome for your commnets or EXP!

the main reason from my point of view is that
the EWGs connected to the phosphine ligand decrease the electronic density of the Pd center.

Sorry. Thx for your comment! I am concerned about M-OR complexes such as acetates. Not Pd-P(OR)3.

People seldom use e.g. Pd(OMe)2 as a catalyst, but quite many use Pd(OAc)2 or other EWG oxygen donor ligands. I guess too unstable of Pd(OMe)2????

In essence, I believe you guys are agreeing with each other.
In order to increase the stability of low-vanlent (electron rich) Pd complexes, The Pd center like Pi-acid ligands such as CO, phosphine, which may accept the electron density fom the Pd center. One of the examples is the zero valent Pd(0) cluster stabilized by carbonyl ligands. Compared with EDG-O ligand, EWG group in EWG-O ligand will benefit EWG-O ligand accept electron density from the metal center. Thus, EWG-O ligand complexed Pd is more popular.

However, for CO or PR3 ligand, Pd(0) complexes is prefered. In my question, Pd(II) complex!

In my opinion, niuniu123's comment is reasonable. For Pd(0), OR has almost no change to form Pd-O bond due to the high electron-rich metal centre. Pd(II) provids a possibility althgh the case is very rare. I only found one example in which the Pd-O bond is formed when O donates 2e to Pd to reduce the d-p repulsion(For more details, see the attachments). For pure OMe (O does not donate 2e), It may be impossible.

Thx niuniu123 & M=C.
I am concerning only Pd(II) complexes with O-donor ligands. I found many people use weakly-coordinated O-ligand e.g. CH3CO2-, but seldom see Pd(OMe)2. I would like to know why?

The paper M=C gave is the Pd complex with one EWG O-donoe ligand and one EDG O-ligand(OMe). Definitely, it can occur beta-H elimination for Pd-(OEt)2 complexes.

To M=C, "d-O bond is formed when O donates 2e to Pd to reduce the d-p repulsion" should be wrong!
when O donates 2 pi e to Pd to increase the d-p repulsion.
That's why I guess people use weak-bound & EWG O-ligand (OAc, OTf) to stabilze Pd(OR)2 complexes?!?!

引用:

原帖由 Metalcarbene 于 2006-7-9 16:37 发表
In my opinion, niuniu123's comment is reasonable. For Pd(0), OR has almost no change to form Pd-O bond due to the high electron-rich metal centre. Pd(II) provids a possibility althgh the case is very rare. I only found one example in which the Pd-O bond is formed when O donates 2e to Pd to reduce the d-p repulsion(For more details, see the attachments). For pure OMe (O does not donate 2e), It may be impossible.

[ 本帖最后由 choscar 于 2006-7-9 17:09 编辑 ]

The favorate ligands for these low-valent Pd (0, I, or II) complexes are those that act as the electron acceptors while they donate electrons to the metal center. Since the Pd (II) center are electron rich, the high electron density in metal has the donation of electrons from O-ligands unfavorable. However, this may happen only if the electron density in center Pd metal decrease through back-donation of it to the empty orbital of the surrounding ligands. For Pd(0) complex, its metal center's electrons are so rich that very strong pi-acid ligand such as CO is necessary for stabilizing it. For Pt(II), it is still electron rich, although it is not so electron rich as Pd(0) center. Thus, Pd(II) complex also favors acid ligand that can decrease its metal center's electron density although the acid ligand is not necessary to be so strong like CO (EWG-O ligand is OK). As for EDG-O ligand, it cannot decrease the electron density in metal center by accepting electron density from Pd(II), which is why it is not favored by comparison to EWG-O ligand.

In short, I think my comment is fit for all the Low-valent Pd complexes. In other words, it is fit for all the electron rich Pd complexes including Pd(0), Pd(I) and Pd(II), etcs.

引用:

原帖由 choscar 于 2006-7-9 17:07 发表
Thx niuniu123 & M=C.
I am concerning only Pd(II) complexes with O-donor ligands. I found many people use weakly-coordinated O-ligand e.g. CH3CO2-, but seldom see Pd(OMe)2. I would like t ...

[ 本帖最后由 niuniu123 于 2006-7-9 22:34 编辑 ]

Thx a lot for MC's ref and structure. I just browse them. For this case, MeO-Pd is survived cause of the presence of very strong CF3-containing EWG-O ligands that benefit the decrease of electron density in Pd(II).

Thx! I guess and understand what you mean, not exactly.
I ask this Q, because we can discuss some topic seriously. Also I saw 3 recent JACS commun. papers using late transition metals (Ir Pd, or Pt) containing OAc- lligand for C-H activation.

(1)  Davies, J. Am. Chem. Soc.; 2006; 128(13); 4210-4211. (2) Fagnou J. Am. Chem. Soc.;  2006; 128(27); 8754-8756. (3) Periana et al J. Am. Chem. Soc.; 2006.128(27); 7404-7405.

  They all use computaional method and propose  a 6mr metathesis TS. I also study non-d0 complexes containing alkoxy ligand for activation of small molecules. I expect that using alkoxy ligands for very late transition metals should be more reactive (M-O is weakened by d(pi)-p(pi) repulsion. Thus, I don't why they do not use?? Instability of the Pd(OMe)2 or 6mr metathesis pathway for aetate ligand is more superior to 4mr metathesis for alkoxy ligand???