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No, even true copper pennies (those dated before 1982) are not going to decompose fast enough to cause problems. I'm not sure today's pennies will even work, because they're mostly zinc. There are lots of other slug deterrents you can use, though. Read on: https://www.gardeningknowhow.com/plant-problems/pests/facts-about-slugs-and-how-to-kill-garden-slugs.htm

Here's an article with several ideas for you: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/improving-compacted-soil.htm

You will need to till it up, add some soils amendments such as one called Clay Buster soils amendment and some good compost, then till it all up again. If the area for the garden does not have good drainage, determine which way it does drain. Then till out a strip about 10 feet from that garden edge. Put a bunch of play sand or landscaping sand in there and some of the clay buster soils amendment and till again. Lay some landscaping fabric over that drainage area and cover loosely with dirt.

This article may help: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/copper-for-the-garden.htm

The first problem is that I don't know what kind of plant you're talking about. And do you mean the stems turn black, and then the leaves? If you can send a little more information, we'll try to offer some suggestions.

In all probability they are nematodes. These articles will give you more information:

https://www.gardeningknowhow.com/garden-how-to/beneficial/nematodes-as-pest-control.htm

http://www.fao.org/docrep/009/a0100e/a0100e0d.htm