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I am interested in understanding how carbenes and carbodiphosphoranes interact differently with a transition metals. I would like to use qualitative molecular orbital theory arguments (with an emphasis on qualitative) in order to highlight key differences.

A carbene is a di-valent carbon, with a long pair (in some structures it may be a diradical). It is a very strong $\sigma$-donor, in addition to being a $\pi$-acceptor: It can form a nice bond with an empty $d_{z^2}$ orbital from a metal, and it can accept two pi electrons into its empty p orbital ($p_x$ or $p_y$ depending on our coordinate system). The geometry of the carbene, including the lone pair, is trigonal planar.

Carbene

A carbodiphosphorane, on the other hand, is zero-valent, and has two lone pairs. These may be used in various different bonding schemes (see for example introduction of this paper). The geometry here, including the lone pairs, is tetrahedral.

Carbodiphosphorane

Assuming it binds to a metal in the following manner:

Carbodiphosphorane binds to metal

then I expect that the $d_{z^2}$ orbial is not involved in the bonding, but rather that for example $d_{xz}$ and $d_yz$ interact with the two lone pairs from the carbon.

But I find it difficult to summarize the bonding for a carbene and carbodiphosphorane into (over)simplistic MO diagrams.

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  • $\begingroup$ "The geometry here, including the lone pairs, is tetrahedral." - I doubt that. Carbon may well be sp2. $\endgroup$ – Mithoron Oct 29 '20 at 23:43
  • $\begingroup$ It is a bit far from my expertise but I thought this papers may be of help: Stable Carbenes and Persistent Triplet Carbenes $\endgroup$ – PAEP Nov 2 '20 at 20:29
  • $\begingroup$ Links take me to spanish log-in page.. $\endgroup$ – Yoda Nov 3 '20 at 8:55

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