Question

For counting the electrons of the complex (RO)3W≡CMe, the book considers three RO- groups that donate a total of 10 electrons and it regards ≡CMe as [carbyne]- group in the ionic model and has a total of 16 electrons.

What confuses me is that:

The RO- group used to consider a 4 electron donor because they can form a sigma bond with metal and they can also donate 2 lone pair electrons to the metal (as a pi-donor), so they can donate overall 4 electrons. However, this seems not to be the case. I wonder whether it's because that 10=4+4+2, which means that only 2 RO- act as pi-donor and the other is sigma-donor only?
I used to consider M≡CR as M(III) and CR(-3) in the ionic model considering the electronegativity. I don't understand why carbyne can be regarded as the group with only 1 negative charge. Nevertheless, when I consider M(III) and CR(-3) and count the total electrons, it also turns out to be 16e. Does that mean the two ways are both available?

Answer 1

In the (RO)3W≡CMe complex, the three RO groups donate a total of 10 electrons and the carbyne group (≡CMe) is regarded as a group with a single negative charge. The confusion regarding the electron donation of the RO group can be resolved by considering that 2 RO groups act as pi donors and the remaining one acts as a sigma donor. Both the ionic model and the electronegativity-based model can be used to determine the total electron count, which turns out to be 16 electrons in both cases.

Step-by-step explanation:

In the complex (RO)3W≡CMe, the book considers three RO groups as donating a total of 10 electrons, while the ≡CMe group is regarded as a [carbyne] group with a total of 16 electrons. The confusion arises regarding the electron donation of the RO group. The RO group can act as both a sigma donor and a pi donor. Counting 10 electrons is possible because 4 electrons are from the sigma donation (forming a sigma bond with the metal) and 2 electrons each come from two RO groups acting as pi donors, resulting in a total of 10 electrons.

Regarding the carbyne group (≡CMe), in the ionic model, it is considered as a group with a single negative charge. However, when considering M(III) and CR(-3) in the electronegativity-based model, the total electron count also turns out to be 16 electrons. Both ways are available, and the choice depends on the approach being used.