Hello student,

For Ti3+, metal ions usually form hexa coordinated  complexes with water molecules. According to crystal field theory, Ti has outer shell electron which is free to jump between energy levels. Thus Ti3+ ions selectively absorbs certain colors which makes it colorful.

Sc3+ on the other hand has empty d orbital i.e. complex has no electron in t2g or eg orbital. Hence solution remains colorless in aqueous form.

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