Closure properties of undecidable languages
Let $K$ be some undecidable language. Define languages $A_0,A_1,B_0,B_1$ as follows:
$$
A_0 = \{ 0 w : w \in K \}, A_1 = \{ 1 w : w \in K \}, \\
B_0 = A_0 \cup \{ 1 w : w \in \{0,1\}^* \}, B_1 = A_1 \cup \{ 1 w : w \in \{0,1\}^* \}.
$$
See if you can use these languages to show that the collection of undecidable languages is not closed under concatenation, union or intersection.

As for complementation, try to use the fact that if you can decide a language $L$ then you can decide its complement $\overline{L}$ (why?).