Selenium oxydichloride bond angle

According to me , the $\ce{Cl—Se—Cl}$ bond angle should be greater than $\ce{Cl —Se—O}$
bond angle because since oxygen is more electronegative than $\ce{Cl}$ , the orbital directed towards $\ce{O}$ will have more p character hence, the bond angle should be less than $\ce{Cl—Se—Cl}$ bond angle. 


But in my book its written that the $\ce{Cl—Se—O}$ bond angle is greater. Why?




My suspicion is that the info in the book relies on single-crystal x-ray diffraction experiment ([1](http://pubs.rsc.org/en/Content/ArticleLanding/1980/DT/DT9800000115)), where geometry of selenium oxydichloride cannot be determined solely from MO principles. So, here is the structure of dioxane-seleninyl-dichloride:


[![enter image description here](https://i.stack.imgur.com/wEc5u.png)](https://i.stack.imgur.com/wEc5u.png)


As far as you can see, `Se1` atom has three more neighbors: `O1` from another $\ce{[SeOCl2]}$ unit, and `O2` + `O3` from dioxanes. The bond angles correspond to those you discovered in the book, $\angle \ce{Cl1-Se-Cl2} = 95.6(3)$, and $\angle \ce{Cl1-Se-O} = 101.6(7)$:


[![enter image description here](https://i.stack.imgur.com/mbcg2.png)](https://i.stack.imgur.com/mbcg2.png) 


The reason for that is given in the paper as well:



> 
> APPLICATION of the Gillespie-Nyholm theory to the molecules of
>  seleninyl dichloride, $\ce{SeOCl2}$, ... leads to the same prediction
>  for each: pyramidal geometry with the fourth position of a
>  tetrahedron occupied by a lone pair. However, more detailed study of
>  many non-metal compounds containing lone pairs has shown that the
>  Gillespie-Nyholm approach gives an incomplete description of the
>  bonding. The central atoms very frequently form additional bonds, of 
>  greater length than the primary covalent links. In the case of
>  pyramidal molecules, it has been found that there are often three of
>  these secondary bonds, completing a distorted octahedron around the
>  central atom. Striking conformation of this has come from complexes
>  (or solvates) of $\ce{SeOCl2}$. All show $\ce{Se-O}$ and $\ce{Se-Cl}$
>  secondary bonds, with distorted octahedral geometry, although in some
>  cases the third secondary bond opposite the $\ce{Se-O}$ bond may be
>  significantly longer than the other two secondary bonds. Furthermore,
>  there are rare examples where a secondary bond has been replaced by 
>  two longer contacts, and it may be inferred from these results that
>  the steric importance of the lone pair of electrons on $\ce{SeOCl2}$,
>  varies. In those cases where an octahedron can be observed, the angles
>  $\ce{X-Se-Y}$ ($\ce{X, Y}$ = $\ce{O}$ or $\ce{Cl}$) have been found to
>  lie in the range 142 -- 173, and there appears to be no clear way of
>  deciding which atoms, chlorine or oxygen, are going to form the
>  secondary bonds. The present structure of $\ce{SeOCl2 \* C4H8O}$, is,
>  however, the first in which $\ce{SeOCl2}$, is involved in three
>  secondary bonds to oxygen with the $\ce{SeOCl2}$, being classed as
>  amphoteric.
> 
> 
> 


[Original](https://i.stack.imgur.com/Id4nU.png)


Long story short, your assumption would probably be correct for the "idealized" molecule in gas phase, whereas in reality there always will be a distortion as $\ce{As}$ strives to complete unsaturated coordination environment.


(1) Alcock, N. W.; Sawyer, J. F. J. Chem. Soc., Dalton Trans. 1980, No. 1, 115–120. [DOI 10.1039/DT9800000115](http://pubs.rsc.org/en/Content/ArticleLanding/1980/DT/DT9800000115)


 3