[ericajohnsonus]

My homework problem is if a molecule [NiCl2(NH3)4] reacts with double amount of ethylenediamine to yield products. Draw all possible isomers.
I wonder if the ethylenediamine can replace the ligands in [NiCl2(NH3)4] randomly or the replacement depends on the strength of the ligands. If the ligands substitution depends on the strength of ligands, how would I know which ligands are more favored?  Please help. Thank you very much 
Erica

[shelanachium]

I imagine it is the ammonia which is replaced, as only two moles of ethylenediamine react. Ethylenediamine is H2NCH2CH2NH2 and so two molecules can replace four of ammonia, attaching through the NH2 groups. Molecules like this that can attach by two or more groups are called chelators, and bind more strongly than others, in part because to detach them you must break more than one bond.

Make models and all will be clear (if you have no models, draw the structures - the ligands are attached octahedrally). In one isomer, called trans, the Cl atoms are opposite each other. They can also be adjacent to each other. How many isomers are there of this form?

[Alpha-Omega]

OK...I have been looking at this problem for awhile....are you sure your notation is correct? I am just checking because :

[NiCl₂(NH3)₄]  tetraaminedichloronickel (II)

The coordination sphere of a coordination compound or complex consists of the central metal atom/ion plus its attached ligands. The coordination sphere is usually enclosed in brackets when written in a formula.

According to your notation, [NiCl₂(NH₃)₄] is the coordination complex (the complex of the metal with its inner sphere ligands).  Inner sphere ligands are directly bonded to the metal (they share electrons).  The (NH₃)₄ and Cl₂ ligands are” inner sphere ligands.”  This coordination complex is neutral (not charged) and requires no counter ions to maintain charge neutrality.

I just want to make sure that the notaion is [NiCl₂(NH₃)₄] and NOT [Ni(NH₃)₄]Cl₂.  Because if this is the case then the complex (the metal with its inner sphere ligands is called a coordination complex.  Coordination complexes can be neutral, cationic, or anionic).

[Ni(NH3)4]2+ is the coordination complex (the complex of the metal with its inner sphere ligands).  Inner sphere ligands are directly bonded to the metal (they share electrons).  The (NH₃)₄ ligands are” inner sphere ligands.”

The “outer sphere ligands” (Cl₂) are not directly attached to the metal.  They are weakly bonded to the first coordination shell.  They affect the inner sphere in subtle ways.

So [NiCl₂(NH3)₄] is not charged and is a coordination compound.  This is the coordination compound.  Alternatively, this coordination compound is the coordination complex ([Ni(NH₃)4]²⁺ ) and its counter ions (Cl₂).

I agree with the statement above...the 4 NH₃ monodentate neutral ammine ligands will be replaced by 2 of the ethylenediammine neutral bidentate ligands.

"A counter ion is an ion, the presence of which allows the formation of an overall neutrally charged species. For example, in the (neutral) species NaCl the sodium ion is countered by the chloride ion and vice versa.  The counter ions are associated to the coordination complex." (ref Wiki).

Your reaction as written:

[NiCl₂(NH₃)₄] (aq) + 2 en (aq) ------> dichlorobis(ethylenediamine)nickel(II)

"Many ligands are capable of binding metal ions through multiple sites, usually because the ligands have lone pairs on more than one atom. Ligands that bind via more than one atom are often termed chelating. A ligand that binds through two sites is classified as bidentate, and three sites as tridentate. The bite angle refers to the angle between the two bonds of a bidentate chelate. Chelating ligands are commonly formed by linking donor groups via organic linkers. The classic bidentate ligand is ethylenediamine, which is derived by the linking of two ammonia groups with an ethylene (-CH2CH2-) linker." (ref:  Wiki).

Ethylenediammine is a neutral molecule containing two N atoms that can each donate a pair of electrons to a metal atom.  Ethylenediamine is a bidentate ligand, which means it has two different atoms that can donate electron pairs to a metal ion. Ethylenediamine does this through its nitrogen atoms (see attached structure for ethylenediamine).

Bidentate ligands are often referred to as chelating ligands ("chelate" is derived from the Greek word for "claw") because they can "grab" a metal atom in two places.

A partial spectrochemical series listing of ligands from small Δ to large Δ:

I⁻ < Br⁻ < S₂⁻ < SCN⁻ < Cl₋ < NO₃⁻ < N₃⁻ < F⁻ < OH⁻ < C₂O₄²⁻ ≈ H₂O < NCS⁻ < CH₃CN < py (pyridine) < NH₃ < en (ethylenediamine) < bipy (2,2'-bipyridine) < phen (1,10-phenanthroline) < NO₂⁻ < PPh₃ < CN⁻ ≈ CO

Increasing strength--------------------------------->

According to the spectrochemical series en (a neutral ligand) is a stronger ligand than Cl^- and NH₃ and will replace both species. Ethylenediammine is a bidentate ligand and will occupy two sites.

See the attached diagram which depicts how the en(aq) will bind to the Ni²⁺ metal center.

Chelate Effect-usual stability of coordination complexes involving a chelating, multidentate ligand as compared with equivalent compounds involving monodentate ligands. Entropy is responsible.

Additionally, these are known reactions for Ni²⁺:

Ni²⁺(aq) + 6 NH₃(aq) ---------> [Ni(NH3)₆]²⁺

Ni²⁺(aq) + 3 en(aq) ---------> [Ni(en)₃]²⁺

According to your notation the complex [NiCl₂(NH₃)₄] the metal ion is directly bonded to six other atoms in what is called an octahedral geometry. If you connected the six atoms, the resulting solid would be an octahedron, and hence the name of this geometry). Although it may not be obvious, there may be many ways in which the same six atoms can be arranged around a central atom in an octahedral geometry, and each of these different arrangements may give rise to compounds with the same chemical formula, but have different arrangements of the atoms (isomers).

If the bidentate en ligands replace the 4 (NH₃) monodentate ligands your complex will be [NiCl₂(en)₂].

Because en is a bidentate ligand, [NiCl₂(en)₂] exists as three geometric isomers; one pair of enantiomers and their diastereomer. The isomer where the chlorides are situated on either side of the Ni²+ (180° from each other) is called the trans isomer (Figure 1-attached word document) while the isomer where the chlorides are next to each other in the octahedron is the cis isomer. In addition, there are two different ways in which we can put two Cl atoms cis to one another, and these are enantiomers (see Figure 2-attached Word document). (Ref:  TransNi-CisNi1-CisNi2.doc)

In order to determine your isomers you will have to draw out the product species.  I have attached a document that shows the differnt kinds of isomerization possible of transition metal complexes.

I am attaching a link to a Coordination Chemistry site provided by Purdue, Eric Martz, and Elsavier.  Symyx MDL Home is where you will download the CHIME interface.  You have to register to download the Chime interface.  You can use the tools on this site to model you reaction and products (see the menu on the left). You may also want to download Jmol.  It is also useful.

http://www.chem.purdue.edu/gchelp/cchem/not2.html

Eric Martz's Chime Resource Page:  http://www.chem.purdue.edu/gchelp/cchem/not2.html