Why is chlordiazepoxides, diazepam, clofibrate, barbiturates contraindicated in renal failure?

Impaired elimination: All drugs that are taken are eventually metabolized and eliminated from the body. Although liver enzymes perform the bulk of metabolizing of these agents, elimination of the drug and their metabolites relies, to varying degrees, on the kidney. If the kidneys are failing, elimination is impaired, and a toxic buildup of the drug and/or its metabolites is possible.