.outer {
  margin: 50px;
}

.button {
  border: 1px solid red;
  border-radius: 3px;
  width: 100px;
  height: 30px;
  display: block;

  background: linear-gradient(to right, green 50%, white 50%);
  background-size: 200% 100%;
  background-position: right bottom;
  transition: all .5s ease-out;
}

.button:hover {
  background-position: left bottom;
}

.text {
  text-align: center;
  font-size: 16px;
  line-height: 30px; 
  color: black;
  transition: all .6s ease-out;
  display: block;
}

.text:hover {
  color: white;  
}