Why are pentahalides more covalent than trihalides
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Why are pentahalides more covalent than trihalides?
The group 15 elements (nitrogen, phosphorus,arsenic,antimony and bismuth) when reacted with halogen atom ,tend to form halides. The halides formed are of two types:
1) Trihalides (MX3)
2) Pentahalides (MX5) where M -15
group element X- halogen atom
The trihalides are formed by all the elements of group 15 while pentahalides are formed by all the elements except nitrogen because there is absence of vacant d- orbital in its outermost shell.
The oxidation state of +5 in pentahalides is more as compared to +3 oxidation state in trihalides. Due to the higher positive oxidation state of central atom in pentahalide state, these atoms will have larger polarizing power than the halogen atom attached to them. The central atom in pentahalide state will tend to polarize more the halide ion to which it is attached.
But In case of trihalides due to +3 oxidation state the central atom will polarize the halogen atom to a lesser extent as compared to pentahalide state. Therefore, more the polarization, larger will be the covalent character of the bond.
Hence due larger polarization of bond in pentahalide state as compared to trihalide state, the pentahalides are more covalent than trihalides.
I hope you find it useful.
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Originally Answered: What makes pentahalides more covalent than trihalide?
What makes pentahalides more covalent than trihalide?
The answer is the small size of cation of pentahalides and large size of halide ions.
as according to Fajan’s Rule …..
cations with high positive charge and with small size will have higher attractions of shared pair which increases the pull on shared pair and thus produces the large covelent character.
let me explain…………………
When a cation approaches an anion, the electron cloud of the anion is attracted towards a cation and hence gets distorted. The effect is called polarisation of the anion.
The power of cation to polarise anion is called its polarising power and tendency of the anion to get polarised is called polarisability. The greater is the polarisation produced, more is the neutralisation of the charges and hence the ionic character decreases or the covalent character increases.
The polarising power of the cation and the polarisability of the anion and hence the formation of covalent bond depends on:
1)Small size of the cation
Smaller the cation, greater is its polarising power.
2)large size of the anion
larger the anion, greater is its polarisability. Covalent character of lithium halides is in the order:
LiI > LiBr > LiCl> LiF
3)large charge on the cation or anion
Larger the charge on the cation, greater is its polarising power. Hence covalent character increases .
The covalent character of the chlorides is in the order:
NaCl < MgCl2 < AlCl3
Greater the charge on the anion, more easily it gets polarized .
The covalency of pentahalides and trihalides is directly dependent on the polarizing power.
The oxidation state of trihalides is +3 and that of pentahalides is +5 and that is the reason behind the more polarizing power of the pentahalides.
As known, covalency or covalent bond is sharing of electrons between the two elements, especially metals.
Therefore, when three electrons are shared between the elements it produces trihalides.
When there is sharing of five electrons between the metals it forms pentahalides.
The more the oxidation state, the more is the polarising power and the more is the polarising power, the more will be the covalency.