how do we know if a complex compd follows EAN rule or not?

[Cr(NH₃)₆]^{3+ y doesnt this compd follow it?}

Here's the link to the concept. Though I would advise you not follow this rule because it is not fixed for all compounds and there are a lot of exceptions to it

http://books.google.co.in/books?id=RCm43YjyTP0C&pg=PA13&lpg=PA13&dq=effective+atomic+number+coordination+compounds&source=web&ots=14BvAhfMWw&sig=NqSYrVcPhnE-RIXfIWqAavD2r8U&hl=en

THE EAN RULE SAYS....count the total electrons and subtract the no of electrons for +ve charge and add for negative charge...

1>no of electrons for central metal ion here = atomic no. Z=24

2>treat every ligand to be donating 2 electrons...that means for 6 ligands....6*2 =12 electrons

3>now u gotta subtract the +3 charge

4>so total electrons = 24+12-(+3) = 33, which doesnt match any inert gas configuration [the magic number configuration]. It possesses 2/8/18/5....so doesnt follow EAN rule for stability of complex!!!!

EAN rule- If the calculated EAN comes out to be equal to atomic number of a noble gas, the compound is said to obey EAN rule.

E.A.N.= z - x + ny

where, z-Atomic no. of central metal atom

x-Oxidation state of Central metal atom

n- no of ligands(for eg, 2 in case of a bidentate, 3 for tridentate, 1 for monodentate)

y-no of electrons given by ligand

Metals that obey EAN rule are ALWAYS diamagnetic in nature. If it doesnot obey, it is generally paramagnetic although it can be diamagnetic also.

For [Cr(NH₃)₆]³⁺ ,

z=24, x=3, n=6 , y=2(as each NH₃ donates one pair of electrons)

So, EAN=24-3+12=33

Now,33 is not an atomic number of any noble gas, so it doesnt follow EAN rule and rightly also, it is PARAMAGNETIC.

EXCEPTIONS- Complexes which do not obey EAN but are dimagnetic-

[Pt(NH₃)₂Cl₂] and [Ni(dmg)₂]²⁺

Hope you find this information useful.