What is the ruling of Islamic law regarding a wife's refusal to engage in sexual intercourse for two years, despite the husband's various attempts to initiate it, and is the husband considered negligent in this case?

There is nothing to indicate negligence on the husband's part. He should strive to understand the reasons for his wife's reluctance to engage in intercourse and help her overcome them, either through ruqyah (definition) or by consulting doctors. The wife should be reminded of the husband's right to her in bed, and that her refusal without a legitimate religious excuse incurs Allah's displeasure, as evidenced by the hadith (definition): "If a man calls his wife to his bed and she refuses, and he spends the night angry with her, the angels curse her until morning." If she persists, she is considered rebellious (nashiz), and the husband may marry another or divorce her.