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In my textbook, these reasons are given as of why transition metals form a large number of complex compounds:-
What is the use of the first reason, i.e small size? Shouldn't it be opposite? In interhalogen compounds too, in XYn, X should be of greater size than Y. So why opposite in case of complex compounds?
The 3d electrons are attracted strongly to the nucleus (positive charge) and this overalls 'pulls' the electron cloud closer to nucleus making the atomic radii much smaller, which is why there is a general trend of decreasing atomic radii across the period.
Since in transition elements series the 4s orbitals are filled first before 3d orbit, this is turn makes the transition element very reactive and readily form compound as the 4s electrons are released easily, (in addition 3d orbitals provide a screening effect) and excited to a higher energy level during typical reaction to form complex salts.
