Translation:I have five kilograms of copper and five grams of silver.
Native English speaker here: no, we need the plural. Also the word 'of' is required in speech. It might be dropped in writing, but only as an abbreviation.
What you might be thinking of is phrases like "I have a five-kilogram lump of copper", where yes, it becomes singular because it is adjectival. Other examples: a two-man race, a 60-ton boat, a 2-hectare field.
There is also a funny archaic way of saying for example "5 foot of rope" instead of 'feet' - but to say 'feet' is much more common - and I'm not sure what other phrases like that.