Determining number of ligands

[Kelvy Mansfield 1H]

I have a question about one of the Quiz 3 prep problems. The question asks how many en ligands (NH2CH2CH2NH2) would be found in a square planar complex with Pt2+ as the central metal ion.

I'm a bit confused as to where to even start on this kind of question. If someone could show me how to approach a problem like this I would appreciate it greatly.

[Eduardo Torres 3G]

In this type of problems, first you look at the number of areas that are bonded around the central atom. Since it wants a square planar complex, it will have four bonding areas (compared to octahedral which has six). Next, we learned that en ligands bond to an atom at two sites (it is bidentate because each N atom bonds to a cite). This means that only two en ligands can bind to Pt2+ to make a square planar shape because all four bonding sites would be used. I hope this helps.

[Hannah Owens- Lecture 1 section 1E]

^How do you know that en binds to two sites (bidentate)? Why can only the nitrogens bind to sites?

[Saira Purewal 3E]

I find this confusing too but I think the Nitrogen can bind to the metal center because it has a lone pair of electrons that can form that bond with the metal. Hope that helps a little!

[GinaYoung1L]

I reposted my answer from another question regarding the same problem.

Information about the geometry of the complex allows us to determine the number of bonds that the central metal will form. In this case, platinum ion is the center of a square planar complex. A square planar complex has four corners and therefore four bonds to the ligand(s). This particular ligand is bidentate, so it can form two bonds with the central metal. Hence, we need two of these bidentate ligands to form a total of four bonds which will fulfill the square planar shape.

Remember: in the octahedral complex, the tetrahedral complex, and the square-planar complex, the ligand(s) bonds with the central metal at the corners of the particular geometry. Look back at page 99 of the course reader.

[Kelvy Mansfield 1H]

Thanks to all of you for the replies! My only question now is how would you know that Pt2+ is involved in a square planar complex, rather than a tetrahedral or octahedral complex?

[Niharika Reddy 1D]

The question states that Pt²⁺ is forming a square planar complex, so that information was given to us.

[Kelvy Mansfield 1H]

Oops. Don't know how I missed that when I typed it in my original post...

Anyways, thanks everyone for the help! I now understand that because a square planar complex has 4 binding sites, and because ethylenediammine is bidentate, two total en ligands can bind to Pt2+.