std::vector emplace and std::vector emplace back with pair

I have this code:

std::vector<std::pair<const std::string, int>> vec;

vec.emplace_back("a", 1); //success
vec.emplace(vec.end(), "b", 2); //compile error

vec.emplace_back(std::make_pair<const std::string, int>("c", 3));  //success
vec.emplace(vec.end(),
     std::make_pair<const std::string, int>("d", 4)); //compile error

Can you explain me why?