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What is the coordination number for BeCl2?

I am confused, should it be 2 or 4?

If I consider as a chain structure the coordination number is 4, and in the gas phase itis 2.

BeCl2

inorganic-chemistry coordination-compounds

edited Aug 31 at 12:37

jonsca ♦1,115 4 11 38

asked Jun 5 at 13:16

DSinghvi490 16

2 Answers

Coordination number is determined by compound. It is normal for an element to have severaldifferent CN. Preferable CN can change for an element depending on the atom's neighbors. Forexample, Fe has preferrable CN=6 for fluorine anions, but CN=4 for chloride anions. When heatedor under extreme pressure preferable CN may change as well. For example, has high-pressure modification with 6-coordinated silicon and 3-coordinated oxygen, while at normalpressure they are 4- and 2-coordinated respectively.

SiO2

In case of it prefers, just like all elements of the 2nd row, to have CN=4, but in many cases isforced to live with smaller numbers. BTW, in gas phase, exists in equilibrium betweenmonomer with Be's CN=2 and dimer with Be's CN=3.

BeBeCl2

In your case CN for Be is 4, while in gas phase it is 3 and 2 for dimere andmonomer respectively. There is no 'one CN for Be atom right for all Be compounds.'

in cristalline BeCl2

answered Jun 5 at 15:28

permeakra5,848 1 6 25

– same compound can have different CN .I mean is there any explanation to it DSinghvi Jun 5 at 15:40

–

@DSinghvi Of course there is! The actual free energy of a system is a very complex function ofcoordinates of the atoms and the environment. Joy! It does not necessarily have exactly one minimum! Even morejoy! There are no simple rules to find how many and where! meaning that we have to do it hard way, using X-ray andquantum chemistry to find ones. For some systems there are well-working empirical rules to find wich position maybe close to local minimum like for CHNOSF compounds, but even they may fail. Being simple: it is so because thereis no reason to be other way.

being sarcastic

permeakra Jun 5 at 17:25

– I WANTED TO KNOW why it happen qualitatively @permeakra DSinghvi Jun 5 at 17:27

–

Again, the general answer is the same. CN is not something to work to much around it. Normally, Be has CN 4. Ifyou use violence, you can force it to have another CN, and it so happens, that and BeCl2 are localminimums, so in extreme circumstances Be will adopt these forms as trade-off between having too much kineticenergy to stay in crystal structure and attempt to follow octet rule. In said structure octet rule is followed throwadditional pi-donation from Cl to Be, which is not as energy-efficient as sigma-bonds

B Ce2 l4

permeakra Jun 5 at 17:35

– Do you know the answer of my second question that you may find in newest question DSinghvi Jun 5 at 17:39

Beryllium is the most electronegative of the Group 1 and Group 2 elements (excluding hydrogen)and so rather than lose 2 electrons (and become - a lot of charge on such a small atom) itcan often bond covalently. Just like with carbon, the covalent bonding will involve mixing ofberyllium's and orbitals. The coordination number for Beryllium can vary depending uponhow the orbitals mix and the what hybridization results. In the gas phase the monomer hasa linear structure and the coordination number is 2. This is because the molecule is hybridizedwith two empty orbitals. hybridization with two empty orbitals requires bonding to two groups(coordination number = 2) in a linear arrangement. In the solid phase, where other molecules are close, the chlorine atom from one molecule can also form a dative bond toan adjacent beryllium atom. In this case the molecules can rehybridize to and form 4 bonds(coordination number = 4) arranged along the vertices of a tetrahedron. In this case the formerlyempty orbitals have become orbitals and share the lone pair of electrons contributed from achlorine atom on an adjacent molecule.

Be+2

2 s 2 pBeCl2

spp sp p

BeCl2BeCl2

sp3

p sp3

BeCl2

edited Jun 5 at 20:23 answered Jun 5 at 14:40

ron19.6k 1 11 50

– i know the hybridisation but I want coordination number\ @ron DSinghvi Jun 5 at 14:52