Why some compounds are undergoes dimerization ?

Compound are dimerized due to following reasons:

(1) If there is no steric crowding and back bonding in a molecules then bridge bond formed and molecules dimerised and stabilized and dimerisation are more stable than back bonding.

(2) Most of the electron deficient compound attains stability by performing back bonding or they undergo dimerisation provided certain conditions are fulfilled

(3) BCl_3 ,BBr₃ BI₃ and B(Me)₃ although  they are electron deficient compound but do not undergo dimerisation  because  of steric factor in demmeric formed

How do you calculate the number of all possible structural isomers for alkanes?

Yes we can find all possible structural isomers Of a given hydrocarbon.

Number of total structural isomers = 2^(N-4)+1

Where N is number of carbons in hydrocarbon

Example: (1)
Number of structural somers of butane (C4H10) is 

SOLUTION:

Where , n=4
=2^(4-4)+1
=2^(0)+1
=1+1
=2

Example: (2)
Number of structural somers of butane (C5H12) is.

SOLUTION:

Where , n=5
=2^(5-4)+1
=2^(1)+1
=2+1
=3

Example: (3)
Number of structural somers of butane (C6H14) is.

SOLUTION:

Where , n=6
=2^(6-4)+1
=2^(2)+1
=4+1
=5

Example: (4)
Number of structural somers of butane (C7H16) is.

SOLUTION:

Where , n=7
=2^(7-4)+1
=2^(3)+1
=8+1
=9

Example: (5)
Number of structural somers of butane (C8H18) is.

SOLUTION:

Where , n=8
=2^(8-4)+1
=2^(4)+1
=16+1
=17

Example: (5)
Number of structural somers of butane (C8H18) is.

SOLUTION:

Where , n=9
=2^(9-4)+1
=2^(5)+1
=32+1
=33

How to find number of stereoisomers in a large compound (polyenes) containing double bonds?

Number of stereoisomer of polyenes can calculated by following formulas .

How to find number of stereoisomers in a large compound containing chiral centres ?.

What is denticity of NO and NO+ ligands ?

"NO" is a special ligand which can donate three electrons (3e) to the central metal atom in neutral state but in charge state (NO+) it can donate two electrons only to the central atoms.