Synthesis and Physicochemical Studies of Nickel(II) Complexes of 2-substituted-1,3-diphenyl-1,3-propanedione, their 2,2 -Bipyridine and 1,10-Phenanthroline Adducts and X-Ray Structure of (2,2 -Bipyridine)Bis(1,3-diphenyl-1,3-propanedionato) (Nichel(II)